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A message-based remote database access facility Koorland, Neil Karl 1985

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A MESSAGE-BASED REMOTE DATABASE ACCESS FACILITY by NEIL KARL KO OR LAND B.Sc. University of Cape Town, 1982. A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF COMPUTER SCIENCE We accept this thesis as conforming tome required standard THE UNIVERSITY OF BRITISH COLUMBIA August, 1985 ©Neil Koorland, 1985 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of C o ^ p ^ e r S c ^ c a The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date . ^ l°i / DE-6 (3/81) Abstract We present a design for a remote database access facility, which uses a message sys-tem as its communication medium. Adopting a message-based design offers a number of advantages over more conventional connection-oriented architectures for those remote database applications where the interaction between user and DBMS involves a single query followed by a single result. By complying with the C C I T T X.400 Recommendations on Message Handling Sys-tems, the design allows for networkwide access to remote DBMSs that is independent of the nature of DBMSs being accessed, the systems on which they reside and the network over which they are accessed. A n initial implementation using the E A N distributed message system, developed at the University of British Columbia, is described as a means of demonstrating the design's feasibility. ii Acknowledgement Both my supervisor, Dr. Paul Gilmore, and the architect of EAN, Gerry Neufeld, have given valuable advice and guidance in the formulation of a message-based design. Ed Sadowski's M.Sc. thesis work on a message-based File Transfer Facility also proved a useful source of ideas for both the design and the implementation. I am also grateful to the backroom EAN programmers John Demco, Brent Hilpert and Rick Sample for their help in using and debugging the EAN software, and to Barry Brachman for help in debugging earlier drafts of this thesis. The financial assistance of the National Science and Engineering Research Council of Canada is gratefully acknowledged. I i i Table of Contents Abstract » Acknowledgement in List of Figures vi Chapter 1 - Introduction 1 1.1 Motivation 1 1.2 Proposed Design 3 1.3 Implementation 4 1.4 Thesis Organisation 4 Chapter 2 - Survey and Design Motivation 5 2.1 Currently Available Remote Database Facilities 5 2.1.1 Virtual Terminal Access 5 2.1.2 Value-added Network Services 5 2.1.3 Integrated Network Access 7 2.2 The Connection-based Model 8 2.3 The Connectionless or Message-based Model 10 2.3.1 Suitable Applications for Connectionless Transmission 11 2.3.2 Relevance to Remote Database Applications 12 Chapter 3 - A Message-based Design 15 3.1 Design Issues 15 3.2 The C C I T T X.400 Recommendations on Message Handling Systems 17 3.3 Facilitating Remote Database Access 20 3.4 A Protocol for Remote Database Applications 22 Chapter 4 - Implementation 26 4.1 Implementation Objective 26 4.2 Use of the E A N Distributed Message System 26 4.3 The Remote Database User Agent 28 4.3.1 A Hybrid User Agent 28 4.3.2 Use of Existing Software for the User Agent 29 4.3.3 Implementation Overview of the Remote Database UA 29 lv 4.3.4 Formulating a Query 30 4.3.5 Processing a Result 33 4.4 The Database User Agent 34 4.4.1 Invoking the DBUA 34 4.4.2 Query Validation 35 4.4.3 Returning a Result 36 Chapter 5 - Results and Evaluation 37 4.5 Results of a Typical Session 37 4.6 Response Time 37 4.7 Drawbacks of a Hybrid UA 37 4.8 Evaluating the Design 38 4.9 A More Extensive Implementation 41 4.10 Conclusions 41 Appendix A - CCITT X.420 Specification 42 Appendix B - User and Database User Agent Installation 49 Appendix C - Test Examples 51 v List of Figures 2.1 - Systems Currently in Use 6 2.2 - The Connection-based Model 9 2.3 - The Connectionless Model 11 2.4 - Message-based Remote Database Access 13 3.1 - CCITT X.400 Model for Message Handling Systems 17 3.2 - CCITT X.400 Message Structure 19 3.3 - Using X.400 as a Remote Database Access Facility 21 4.1 - Message Structure of a Query 31 5.1 Dimensions of variation in DBMSs 38 vl CHAPTER 1 Introduction 1.1. Motivation Electronic message systems1 have undergone extensive development over the past decade. They have matured to the extent where their use is pervasive and the emphasis in their design has progressed from the provision of rudimentary services to the addition of sophisticated functionality. This maturation is evidenced by the emergence of interna-tional standards for their design; most notably the CCITT X.400 Recommendations on Message Handling Systems [CCITT83a]. Although by far their most common application, the use of message systems has not been restricted to the domain of interpersonal messaging (electronic mail). The mes-sage paradigm has much to recommend its use in other applications and to date, the broader potential of message systems has been exploited to achieve, inter alia, File Transfer [SADOW84], Videotex Transmission and Forms Processing [VALLE84]. In recognition of this wider applicability, the CCITT has ensured that the X.400 recom-mendations provide a design framework for message systems in general and not only for interpersonal messaging. This thesis is motivated, in part, by the success enjoyed by this investigation into the generality of message systems and extends their application to the accessing of ^ h e terms Electronic Message System, Computer-based Message System, Message Handling System and Message System are used interchangeably in the literature to refer to any system which supports the electronic transmission of a message from source to destination in an asynchronous store and forward fashion. 1 Introduction 2 Introduction remote databases2. It proposes a design for a remote database access facility which uses a message system as its communication medium; i.e. a system in which queries are sub-mitted to remote databases, and their results returned as the contents of messages. The aim is to present a design framework which can provide convenient access to multiple remote databases, possibly residing on different host systems, and managed by hetero-geneous Database Management Systems (DBMSs). The thesis has also been motivated by the existence of an operational implementa-tion of the X.400 model with which to work. The implementation, called EAN, has been developed by the Distributed Systems Research Group (DSRG) at the University of Brit-ish Columbia and is currently being used between several sites in Canada and Europe primarily for interpersonal messaging, and to a lesser extent, for file transfer. EAN has been chosen as the message system of the CDN network (CanaDiaN research network) [NEUF83] [CILM084]. It is envisaged that the CDN network will constitute a primary communication and file transfer medium for the Canadian research community. A natural outgrowth of this work has been to use EAN as a vehicle for an examina-tion of the wider applicability of message systems. The ability to access remote data-bases is of particular use to the research community and incorporating such a facility into the EAN system would be a significant enhancement. This is especially true in light of the fact that the design proposed here is well suited to those situations, typical of research organisations, where databases reside on host systems not normally accessible to external users. 2 A database application is said to be remote if a database is accessed over a network. 3 Introduction 1 . 2 . Proposed Design Because of the store-and-forward nature of a message system, accessing a remote database in this manner is inherently connectionless; meaning that no direct connection (or virtual circuit) is established between the user and the database. As such, it differs fundamentally from the conventional connection-based or "online" approach to remote database applications where a network connection is esta-blished between user and database and maintained for the duration of the user/database session. The various implications of this difference are discussed fully in the thesis. At the outset it should be stated that this message-based approach is not intended as an alternative to all connection-based remote database applications. A connectionless facility could not satisfy the real-time or fast-response requirements of many applications since a message system cannot guarantee a sufficiently small delay for messages, contain-ing queries and results, to propagate through the system. However, there are a number of applications that involve single or batch queries which do not require real-time responses and for which a connectionless design would not only suffice, but would in fact be more suitable and economical. It is for these applica-tions that the connectionless design, presented in this thesis, is intended. As stated above, a key design objective is to make the design as system indepen-dent as possible. Stated another way, the design should not be dependent on the nature of the DBMS's being accessed, the hosts on which they reside or even the networks over which they are accessed. This objective is attained by framing the design in compliance with the C C I T T X.400 recommendations which, as part of the ISO Reference Model for Open Systems Interconnection [CCITT82], is intended for use in a heterogeneous net-work environment. 4 Introduction 1.3. Implementation The proposed design has been implemented using much of the existing E A N mes-sage system software to access an INGRES relational DBMS [STONE76] running under UNIX 4.2Bsd3. This implementation was carried out both as a means of demonstrating the feasibility of the design, and as a way to illustrate the ease with which it can be implemented on top of an existing X.400-based message system. 1.4. Thesis Organisation Chapter 2 motivates a message-based design by first surveying recent work in the areas of message systems and remote database applications and then discussing the features of a message system which make it a desirable alternative to certain types of connection-based remote database applications. Chapter 3 presents a design for a message-based remote database access facility which complies with the C C I T T X.400 recommendations. The design allows for network-wide access to heterogeneous database management systems (DBMSs) and does not entail the modification of DBMSs being accessed. Chapter 4 is an overview of a first implementation of the design using the E A N distributed message system. Chapter 5 presents a number of examples of the implementation in operation and also evaluates the design with respect to various criteria. The chapter concludes with a discussion of issues which are not addressed in the thesis and which could be the subject of further study. 3 Unix is a trademark of A T & T Bell Laboratories. CHAPTER 2 Survey and Design Motivation 2.1. Currently Available Remote Database Facilities Accessing remote databases1 is one of the most commonplace network applications and takes place within a wide variety of operational settings. Without describing all them in detail it is useful to identify three classes of systems that are currently available. Figure 2.1 illustrates the examples discussed below. 2.1.1. Virtual Terminal Access At the most rudimentary level there are virtual terminal facilities, which allow users at a local site to access the remote site, on which the database resides, via a remote login. By furnishing appropriate authorisation (usually a userid/password) over the net-work, users interact with the remote database in the same way as if they were signed on directly (see Figure 2.1a). The fact that the database is being accessed over a network, is transparent to the user. In a recent survey it was estimated that over 2400 databases containing information on a wide range of topics are available to the general public in this fashion [LISAN80]. The same type of direct virtual terminal access is also in com-mon use by organisations accessing private databases. 2.1.2. Value-added Network Services A major problem with the virtual terminal service is the necessity to negotiate a different login procedure for each different host. This is a major inconvenience to users of ^ ' h e r e its context does not lead to any ambiguity, the term database will be used to refer to a database itself as well as the database management system (DBMS) which controls it. 5 Survey and Design Motivation 6 Survey and Design Motivation host A host B S \ ( a ) V i r t u a l T e r m i n a l A c c e s s value-added network service host A host A B ( b ) V a l u e - a d d e d N e t w o r k S e r v i c e ~7_ canonic8l-to-local translation local-to-cenonical translation ( c ) I n t e g r a t e d N e t w o r k A c c e s s Figure 2.1 S y s t e m s C u r r e n t l y i n U s e multiple databases residing on heterogeneous hosts. This problem has led to the development of value-added network services such as 7 Survey and Design Motivation that offered by Telecom Canada's iNet 20002 [CUNNI83] [SOLOS84]. Value-added ser-vices act as a gateway between the user and multiple remote databases. Users login with the value-added service which, on receiving a request for access to a particular database, performs the requisite access procedure on behalf of, and transparent to, the user. In this way, users only have to go through one access procedure regardless of the number of DBMSs accessed. This configuration is shown in Figure 2.1b. 2.1.3. Integrated Network Access A completely separate and more complex issue, of course, is the problem of con-tending with heterogeneous data models and query languages. iNet avoids this problem altogether by requiring users to be familiar with each different data model and query language. It only assumes responsibility for suppressing differences in system access pro-cedures. Recently two experimental systems, the Network Virtual Data Manager [WANG83] and M U L T I B A S E [SMITH81], have gone a step further in attempting to provide integrated access to heterogeneous remote databases that accounts for differences in data models and query languages as well as DBMS invocation procedures. Both systems employ the notion of an intermediate canonical database system with a unified global (networkwide) schema and a single high-level query language. Using a Data Transfer Protocol (DTP), queries in canonical form are mapped into the represen-tation appropriate to the target DBMS with the inverse mapping done to the result of the query (see Figure 2.1c). To the extent that these systems are able to accommodate a number of different data models and query languages they constitute a useful contribution to the problem of ^Net 2000 is a trademark of Telecom Canada. 8 Survey and Design Motivation integrated networkwide database access. However, not all data models and query languages can be mapped into canonical forms of the kind specified by N V D M and M U L T I B A S E and the problem of finding a truly universal canonical query language, into which all others can be mapped, is still under investigation. 2 . 2 . The Connection-based Model Regardless of the degree of transparency with which access to remote databases is facilitated by any of the systems described above, they all share a common feature, namely they are connection-based or message-based. As will be seen, the design presented in this thesis is connectionless. To appreciate the implications of this difference, the nature and inherent limitations of connection-based systems first needs to be examined. A connection-based application is characterised by three distinct phases : connec-tion establishment, data transfer, and connection release. These are shown schematically in Figure 2.2. The connection-based approach is an integral part of the the International Stan-dards Organisation (ISO) Reference Model for Open Systems Interconnections (OSI)3 [CCITT82], at least insofar as its original formulation is concerned, and is well suited to stream-oriented applications where a series of related data units must be transferred between communicating entities. This includes many remote database applications which are interactive in nature and which require fast-response. The fact that a connection must first be established, before the transfer of data can take place, implies that there has to be some form of prior negotiation between the 3 A t h o r o u g h d iscussion of issues r e l a t i n g t o t h e w i d e l y embraced O S I mode l is b e y o n d t h e scope of t h i s thesis. T h e mode l is, however , re levant t o t h e m a t e r i a l p resented in t h e thesis a n d there fo re t he reader shou ld have a reasonable u n d e r s t a n d i n g of t he var ious e lements c o n s t i t u t i n g t h e m o d e l ; p a r t i c u l a r l y layer 7, t h e a p p l i c a t i o n layer . g Survey and Design Motivation C O N N E C T I O N E S T A B L I S H M E N T D A T A T R A N S F E R c o n n e c t r e q u e s t c o n n e c t c o n f i r m C O M M U N I C A T I O N NETWORK c o n n e c t i n d i c a t i o n c o n n e c t r e s p o n s e d a t a r e q u e s t d a t a i n d i c a t i o n C O M M U N I C A T I O N NETWORK C O N N E C T I O N R E L E A S E d i s c o n n e c t r e q u e s t C O M M U N I C A T I O N NETWORK d i s c o n n e c t i n d i c a t i o n d a t a i n d i c a t i o n d a t a r e q u e s t Figure 2.2 The Connection-based Model communicating entities. In the case of connection-based remote database applications this is achieved by means of a remote login procedure. Because of this, the connection-based model has the following drawbacks: Prior to accessing a remote database, users have to have registered as users of the remote site in order to obtain access authorisation and this can involve a substan-tial delay if the database is being accessed for the first time. - While issuing userid/passwords to every remote user might suit the needs of com-mercial database vendors whose sole function is to facilitate access to their data-10 Survey and Design Motivation bases, it is clearly unacceptable for most other organisations because of the admin-istrative and security problems it would create. Such is the case, for example, when a research organisation wishes to make available certain information from a data-base in their system to external users. In order to do this, the system administrator has to maintain a record of external users permitted access to the system, a task that might be prohibitively time-consuming. Although this problem is often solved by allowing access to the system by all remote users through a single "guest" account, one is still faced with serious security problems if, as is most likely, it is impossible to prevent the guest user from using other facilities on the system. - The connection-based model dictates that the connection be maintained for the entire duration of the user/database interaction even during times when no data is transferred. This can be costly over long distances. It might not be possible to establish a direct connection, between user and remote DBMS, because of physical constraints imposed by the network. For example, either the local or the remote system might not be part of a switched network or they might belong to different networks and the gateway between them cannot sup-port a virtual circuit. 2.3. The Connectionless or Message-based Model Having discussed the problems associated with adopting the connection-orientated approach to remote database applications we now describe the connectionless or message-based model of data communication and how it might be exploited in remote database applications. Unlike the three-phase connection-orientated approach, the connectionless data model involves the transmission of a single independent data unit from source to 11 Survey and Design Motivation destination without prior negotiation and subsequent connection release. This is illus-trated in Figure 2.3 . At the application level these data units are called messages and the application processes, which cooperate to achieve connectionless data transmission, are collectively known as a Message Handling System. Intimately associated with the concept of connectionless data transmission is the notion of a store-and-forward network; i.e. the notion that a message is built as a com-plete data unit at its source and propagated through intermediary nodes until it reaches its ultimate destination. Each node along the way assumes responsibility for the message for the duration of its possession and is not obliged to pass it on within any time limit. 2.3.1. Suitable Applications for Connectionless Transmission Chapin [CHAPI82] [CHAPI83] has identified a number of applications for which connectionless data transmission is better suited than connection-based transmission. These include : data request data confirm C O M M U N I C A T I O N N E T W O R K data indication Figure 2.3 The Connectionless Model 12 Survey and Design Motivation Inward data collection - the periodic sampling of a number of remote data sources. Instead of having to poll each remote site for data, the sampler is sent data as the content of a message. Broadcast and multicast communication - the dissemination of a single mes-sage to a number remote destinations. Office information exchange - store-and-forward transmission of multimedia documents. Request-Response application - here a server process associated with a remote resource is responsible for processing requests for the resource, which are submitted to it as messages by request sources, and returning response messages to each request source. In these applications, the typical interaction between source and server involves a single request followed by a single response. 2.3.2. Relevance to Remote Database Applications It is the last example which is of particular interest here, since many remote data-base applications are of the request-response type in that they only involve a single query followed by a single result. Directory services, transaction-based systems and batch query applications are all good examples of request-response type remote database appli-cations. Figure 2.4 shows remote database access modelled as a request-response application. At present, even those remote database applications which are more suited to this message-based approach, are implemented in a connection-based environment. As such, they are susceptible to the problems, discussed earlier, that are associated with connection-based systems. We suggest these problems could largely be avoided if, 13 Survey and Design Motivation Figure 2.4 Message-based Remote Database Acces s instead, they were implemented in accordance with the message-based approach. Specifically, implementing them in a message-based environment would have the follow-ing advantages : Since all queries are submitted to a remote DBMS indirectly through a server pro-cess operating at the remote site, users only have to direct their queries to the server process without the need to login at the remote system. There is no need for them to register as users at the remote site prior to accessing the DBMS. Since external users only have access to the services offered by the DBMS server process, and not any other resources on the remote system, administrative and security considerations are greatly simplified. Naturally, the server process is free to screen and reject any queries it considers to be unauthorised. 14 Survey and Design Motivation Data transmission costs are reduced. Since there is no need for maintaining a con-nection between users and remote sites, only the cost of transferring messages, con-taining queries or results, is incurred. - Results returned to the user as messages, can be stored, retrieved, edited, forwarded to other users, and combined with interpersonal messages. Potential accessibility to remote databases is greatly increased as queries, destined for sites unreachable via a direct connection, can be relayed by the message system through intermediary sites which lie on a path to the target site. It should be emphasised, however, that because of the inherent store-and-forward nature of message systems, a connectionless design is clearly only appropriate for remote database applications which are request-response oriented. Those applications which involve a protracted interaction between user and DBMS and which require fast response times are not appropriate for the message-based design presented here. While this cer-tainly limits the applicability of the message-based design presented in the next chapter, the fact that there are many request-response type remote database applications, war-rants an investigation into the feasibility of a message-based design. CHAPTER 3 A Message-based Design 3.1. Design Issues Having discussed the desirability of a message-based remote database access facility for request-response applications, we now examine the issues which have to be addressed in designing such a system. It is useful to divide these issues into two main categories which can then be dealt with separately. Firstly, there are those issues which concern the message system itself, regardless of its application. These include : Message submission and extraction. This concerns the nature of the interface between the sender and the message system, when a message is submitted for delivery or when a reply has been received. In most message systems this interface embodies some notion of a "mailbox" or a "mailing slot". Message transfer. The manner in which messages are transferred reliably from originator to recipient in a store-and-forward fashion. Message format. All messages contain not only data but also control information such as a description of the data, the source and destination address, and the qual-ity of service to be applied to the message. A message must therefore conform to an agreed upon structure in order for all entities handling the message in the system to interpret this information correctly. Secondly, there are those issues which relate to how the message system can be used to facilitate remote database access in particular. The issues that have to be dealt 15 A Message-based Design IB A Message-based Design with here are : The user interface. How does the user formulate and dispatch a query or receive its result as a message, and what kind of functionality should be provided in assist-ing the user in doing this. DBMS interface. How are queries submitted to the DBMS for processing once they have been delivered and how results are submitted to the message system for return to the user. Security and administration. While all users of the message system have the ability to send queries to a remote DBMS, there should be a mechanism that ensures that only messages sent from authorised users are actually processed. It should also be possible to carry out various administrative tasks such as the collec-tion of usage statistics and accounting information. Because of the complexity of these issues, designing a message-based remote data-base access facility from scratch would be a formidable problem. Fortunately, a design for a message system which adequately addresses most of them is already in existence in the form of the CCITT X.400 recommendations on Message Handling Systems proposed in 1983 as an international standard1. The fact that the X.400 recommendations are intended as an international standard for message systems, makes their adoption in this thesis, as a basis for a message-based remote database facility, a logical design decision and one which reduces the design task to manageable proportions. *In actual fact, the C C I T T recommendations consist of a set of eight related but separate proposals, each dealing with a specific aspect of message systems. Where context does not lead to any ambiguity, they are referred to collectively here by the first recommendation, X.400. 17 A Message-based Design 3.2. The CCITT X.400 Recommendations on Message Handling Systems The X.400 recommendations define a distributed architecture for a message han-dling system. As shown in a simplified form in Figure 3.1, users send messages to each other via their respective User Agents (UAs). As the name implies, a UA acts as a server process on behalf of the user and is responsible for the proper formulation, management, submission and reception of mes-sages. The use of the term "mailbox" has been avoided in the CCITT recommendations because it would lead to various ambiguities. But for the sake of clarification, the UA can be loosely thought of as the user's mailbox. An important point to note here is that although a "user" is a person in an interpersonal messaging environment, there is noth-ing implicitly or explicitly contained in the X.400 specifications which restricts the ^ u s e r ^ u s e r a g e n t u s e r a g e n t m e s s a g e h a n d l i n g s y s t e m m e s s a g e t r a n s f e r s y s t e m message transfer agent) message transfer agent message transfer agent u s e r a g e n t u s e r a g e n t ^ u s e ^ Figure 3 1 CCITT X.400 Model for Message Handling Systems 18 A Message-based Design nature of the user. A user can equally be, for example, an automated process. UAs communicate with each other through the Message Transfer System (MTS) which consists of nodes called Message Transfer Agents (MTAs). There is a many-to-one relationship between UAs and MTAs; i.e. each UA is associated with only one MTA but each MTA can be associated with more than one UA. Messages propagate through the MTS, from the MTA associated with the originating UA to the MTA associated with the recipient UA, in a store-and-forward fashion. As is the case with all store-and-forward systems, no guarantee is given as to the speed with which messages are delivered, although it is possible to specify that a message should not be delivered beyond a certain time. In the terminology of the OSI reference model, UAs are said to reside at the User Agent Layer (UAL) which exists above the Message Transfer Layer (MTL) where com-munication between MTAs take place. Both the UAL and the MTL reside at the Appli-cation layer of the OSI model. Messages are submitted to, and extracted from, the MTS at the interface between the UAL and the MTL. The clear delineation of responsibilities between the MTS and UAs is an important feature of the X.400 model because it allows for both their logical and physical separation. As far as naming and addressing are concerned, X.400 specifies a complete scheme for all entities in the message system. Briefly, each UA must be associated with at least one Originator/Recipient (O/R) Name which uniquely identifies the UA in the message handling system. An O/R Name which in some way implies the physical address of the UA (although not the route that should be taken to get there), is known as an Originator/Recipient (O/R) Address. An O/R name is hierarchical as opposed to flat, in order to allow for distributed control. 19 A Message-based Design The CCITT recommendations also specify the format of a message in detail. As shown in Figure 3.2, a message consists of an envelope and its content both encoded and structured according to a given protocol. In the case of the envelope, the protocol used is specified in recommendation X.411 [CCITT83c]. Information on the envelope, such as the recipient's O/R Name, is used exclusively by the MTS to deliver the message from its source to its destination. This information is of course common to all message-based applications and the protocol is therefore used for all messages regardless of the application being supported. The message content, on the other hand, is used exclusively by UAs and remains transparent to the MTS. The intention here is that a different content structure protocol can be defined for each different application that best reflects the nature of the applica-tion at hand. s t r u c t u r e d a c c o r d i n g t o p r o t o c o l s p e c i f i e d i n X.41 I (application independent) s t r u c t u r e d a c c o r d i n g t o an a p p l i c a t i o n d e p e n d e n t p r o t o c o l envelope content Figure 3.2 CCITT X.400 Message Structure 20 A Message-based Design Each UA is therefore thought of as belonging to a class of UAs where the UAs comprising a particular class communicate according to their own protocol. However, they all share the same envelope protocol and MTS. To date, only a protocol for interpersonal messaging (X.420) has been included in the CCITT recommendations [CCITT83d] and is given in Appendix A. 3.3. Facilitating Remote Database Access Accommodating remote database access, within the X.400 architectural framework is straightforward. Three components of the X.400 model must be exploited. Firstly, each user must have access to a UA capable of structuring a query as the contents of a message according to a given protocol and submitting it to the MTS for delivery to the remote DBMS. The UA is also responsible for receiving messages contain-ing results from the MTS on behalf of the user. It can provide an arbitrary degree of functionality in performing this task. For example it can provide sophisticated query/result storage facilities and an interface to UAs which provides interpersonal mes-saging. Secondly, UAs are required to act as server processes on behalf of remote DBMSs. Their task is to accept messages containing queries from the MTS and submit it to the DBMS for processing. Once the query has been processed, the UA must obtain the result of the query and submit it to the MTS for delivery back to the user's UA. For the sake of clarification, these UAs will be referred to as Database User Agents (DBUAs) in order to distinguish them from the UAs associated with each user. Strictly speaking, the X.400 model requires that each remote DBMS be associated with a different DBUA. However, there are certain advantages to having only one DBUA acting on behalf of a number of DBMSs, not the least of which is the decreased 21 A Message-based Design implementation overhead incurred in situations where a large number of DBMSs reside on one host. In this thesis, therefore, it is assumed that one DBUA can act on behalf of more than one DBMS. Figure 3.3 illustrates the relationship of UAs and DBUAs to each other and to the message system. In the terminology of X.400, UAs and DBUAs using the MTS in this way are said to constitute a class of UAs which cooperate to support remote database access. Thirdly a protocol appropriate for remote database applications must be defined. The protocol should specify the structure of messages containing queries as well as mes-sages containing results. The protocol elements which define the structure of queries must make it possible for the remote DBUA to find and extract the information it needs in order to invoke the appropriate DBMS correctly as well as locating that part of the user D B M S user agent database user aqent message handling system message transfer system message message transfer transfer . agent / agent message' transfer agent database user agent user agent D B M S u s e r Figure 3 . 3 Using X.400 as a Remote Database Access Facility 22 A Message-based Design content which constitutes the actual query to be passed onto the DBMS. Similarly, the protocol must define the structure of a result so that the user's UA is able to find and extract a result from a message on its arrival. The next section discusses in greater detail the protocol chosen in this design. 3.4. A Protocol for Remote Database Applications The query-result protocol proposed here is shown below using the notation specified in CCITT recommendation X.409 [CCITT83b]. The protocol is hereafter referred to as the Database Access Protocol. M O D U L E D B A P D U ::= C H O I C E { 101 I M P L I C I T Q u e r y , 111 I M P L I C I T R e s u l t } Q u e r y :: = S E Q U E N C E { 101 I M P L I C I T Q u e r y H e a d , Ml I M P L I C I T Q u e r y B o d y } Q u e r y H e a d ::= S E T { Recipient DBUA 10] I M P L I C I T O R N a m e Recipient DBMS 111 I M P L I C I T O R N a m e Originating User 121 I M P L I C I T O R N a m e QuerylD [31 I M P L I C I T S T R I N G Usend [41 I M P L I C I T S T R I N G O P T I O N A L Password [51 I M P L I C I T E n c r y p t e d O P T I O N A L InvokeCommand [61 I M P L I C I T S T R I N G O P T I O N A L } Q u e r y B o d y ::= [01 I M P L I C I T I A 5 T e x t - - plaintext R e s u l t ::= S E Q U E N C E { 10) I M P L I C I T R e s u l t H e a d Ml I M P L I C I T R e s u l t B o d u } R e s u l t H e a d : : = S E T { Originating DBUA f 0 ] I M P L I C I T O R N a m e Originating DBMS Ml I M P L I C I T O R N a m e Recipient User [21 I M P L I C I T O R N a m e QuerglD [31 I M P L I C I T S T R I N G Error Condition [41 I M P L I C I T I N T E G E R (NoError (0), NoDBMSO), UnavailableDBMS (2), NoAuthonzation (3), Badinvocation (4) ... } } R e s u l t B o d y : : - [01 IMPLICIT I A 5 T e x t - - Plaintext E N D 23 A Message-based Design In the terminology of CCITT X.400, a message is made up of data structures called protocol data units (PDUs) each of which may consist of other PDUs. Each PDU in a message is identified by an integer code, the interpretation of which is dependent on the data unit's context in the message (denoted by the keyword IMPLI-CIT). A message is therefore hierarchical in structure. Using integer codes instead of character strings to identify protocol elements makes the protocol language indepen-dent2. According to the notation above, the content of a message in a database applica-tion is known as a Database Application Protocol Data Unit (DBAPDU). A DBAPDU is either a Query or a Result. This alternation is denoted by the keyword CHOICE. Queries consist of a QueryHead and a QueryBody. The keyword SEQUENCE specifies that the QueryHead must precede the QueryBody. A QueryHead consists of a number of fields or elements some of them mandatory and some of them optional (denoted by OPTIONAL). The keyword SET specifies that the ordering of elements within the QueryHead is immaterial and can differ from mes-sage to message. Included here are the O/R Name of the DBUA that must receive the query, the DBMS for which the Query is intended, and the user which originated the query. Each QueryHead also has a QuerylD which is used to uniquely identify the query. This is only used for reference and administrative purposes. The three optional QueryHead fields are used by the DBUA to invoke the appropri-ate DBMS. Their inclusion in the Database Access Protocol requires a word of explana-tion. In general, some combination of userid, password and user-supplied parameters must be passed to a DBMS when it is invoked. A DBMS uses this information both as a 2 The specifications O/RName, STRING, Encrypttd, IA5Text and INTEGER, are general purpose protocol ele-ments whose detailed composition is specified in X.411. 24 A Message-based Design means of determining the user/DBMS interaction environment (e.g. which database(s) the user requires access to), and as a user-authorisation mechanism. But since different DBMSs differ with respect to precisely which of these it requires, it is necessary to pro-vide sufficient flexibility to include some, all or none of these within the message struc-ture. This flexibility is achieved by allowing the user to determine which of the three optional fields are included in the query message. It was felt that these three fields were probably sufficient for most DBMSs and host operating systems. However, it is a simple matter to extend the protocol with additional fields should a need for them arise as a result of the peculiarities of certain DBMSs not anticipated here. A QueryBody contains the actual query. Results consist of a ResultHead, used by the UA in referencing a previously submitted query, and a ResultBody, which contains the actual result. According to this protocol, neither the UA which submits a query, nor the DBUA, which receives it, is expected to have any knowledge of the data models or query languages of the DBMSs on whose behalf it operates. Both the UA and the DBUA treat the QueryBody and the ResultBody as a byte-stream without any associated semantics. It is up to the DBMS to interpret the query and the user to interpret the result. Only the control information contained in the QueryHead and ResultHead is interpreted by the UA and DBUA. In this way, neither the UA nor the DBUA is dependent on the nature of individual DBMSs thereby making the system extendible and easy to implement. Under this proto-col, it is entirely up to the user to ensure that queries submitted by their UA are valid for the target DBMS. This is the same policy adopted in connection-oriented systems such as iNet 2000. 2 5 A Message-based Design Using the X.400 architecture in this way has a threefold advantage. Firstly, it pro-vides generality in the sense that X.400 conforms to the OSI reference model thereby facilitating its use in network environments comprising heterogeneous systems. Secondly, it is an extendible design in that it is easy to add DBMSs to the system. All that is required is the establishment of a DBUA at the site on which the DBMS resides or, if a DBUA already exists at the site, the updating of that DBUA. Thirdly, it is com-patible in that no change is required to the software comprising remote DBMSs, since a DBMS perceives the DBUA as a conventional user of the DBMS. CHAPTER 4 Implementation 4.1. Implementation Objective This chapter describes an implementation of the design discussed in the previous chapter. The design has been implemented more as a means of demonstrating its feasibil-ity in an operational setting, than to provide a fully functional and reliable remote data-base access facility of the type that would be required by the user community. Specifically, it was felt that the ability to access at least one remote DBMS using a message-based system would be sufficient to demonstrate its feasibility. The setting of a modest objective was mainly a result of limited time and comput-ing resources. At the outset of the implementation access was only available to two host machines (a DEC VAX 11/750 and a SUN Workstation1) both running under the same operating system (UNIX 4.2Bsd) and one DBMS (an INGRES relational DBMS). 4.2. Use of the E A N Distributed Message System The task of implementing the design was greatly simplified by the existence of the EAN Distributed Message System. As mentioned earlier, the EAN message system is currently in use at several test sites in Canada and Europe for interpersonal messaging. EAN was developed by the Distributed Systems Research Group (DSRG) at the Univer-sity of British Columbia over a period of approximately four years. X S U N Workstation is a trademark of S U N Microsystems Inc. 26 Implementation 27 Implementation The entire system is written in C and has been designed as a portable application system with all host operating system dependencies identified and isolated in the code. To date EAN runs under MTS, UNIX 4.2Bsd and VMS on a variety of machines. As an implementation of the X.400 recommendations for interpersonal messaging, those elements of the EAN message system, which specifically support interpersonal mes-saging (i.e. the User Agent Layer) have been separated from the those elements which provide generalised message transfer (i.e. the Message Transfer Layer)2. This means that EAN's Message Transfer System can be used in its entirety, and without any modification, to support other message-based applications. As mentioned in the previous chapter, this generality is one of the primary design principles underpinning the X.400 model. In fact, EAN's MTS has already been used to provide a store-and-forward file transfer facility [SADOW84]. Needless to say, the implementation of the underlying MTS constitutes the major software engineering component of any message-based system. Most of the design and implementation efforts of the DSRG, in developing EAN, have been focussed on the pro-vision of an extensive and reliable MTS with a relatively minor effort being required for the implementation of the User Agent Layer. Therefore, the ability to utilise an already existing MTS, in providing remote database access, represents a substantial reduction in implementation effort. All that is required, then, is the provision of a class of UAs and DBUAs capable of transferring queries and results according to the Database Access Protocol described in the previous chapter. Each user must have access to a UA and each DBMS must be 2 A t present, this separation is achieved through program module linkage. However, it could be achieved by struc-turing the UAs of the U A L and the M T A s of the M T L as separate processes (possibly residing on different machines), and having them communicate via some appropriate interprocess communication mechanism. It is intended that E A N will eventually employ this structure. 28 Implementation associated with a DBUA. Considering the widespread usage enjoyed by message systems for interpersonal messaging, this simplification serves to strengthen the case for a message-based remote database access facility as a desirable enhancement of the capabil-ities of existing message systems. Those aspects of EAN's MTS which bear directly on the implementation of the remote database access facility will be discussed here, where appropriate. However, the MTS can, for the most part, be regarded as a reliable transfer service for transferring messages between UAs and DBUAs with the exact nature of its operation being of no concern for the purposes of this thesis. While UAs and DBUAs are considered as belonging to the same class, by virtue of the fact that they use a mutually understood protocol to support a particular applica-tion, their responsibilities differ fundamentally. This in turn, means that their respective implementations bear little resemblance to each other, apart from the manner in which they use EAN's MTS to send and receive query/result messages. Their implementation will therefore be described independently in the following discussion. 4.3. The Remote Database User Agent 4.3.1. A Hybrid User Agent Implicit in the X.400 recommendations is the requirement that a UA should only support one type of application, i.e. if a number of message-based applications are sup-ported in compliance with the X.400 model, then the user has to access a separate UA for each of those applications. Strictly speaking, the remote database UA should be unre-lated to EAN's interpersonal UA and the remote database UA is presented as a separate entity in the previous chapter. 29 Implementation Notwithstanding this, it was decided to incorporate the functions of the remote database U A as an enhancement to the existing interpersonal U A ; i.e. to merge the two UAs into one and to allow a user to send queries and receive results along with interper-sonal messages. This violation of the X.400 model was considered to be acceptable because of the greatly simplified implementation that it allowed for. 4.3.2. Use of Existing Software for the User Agent As in the case of the M T S , much of the existing E A N software was used to imple-ment the remote database access UA. This was possible because the existing interper-sonal U A contains many of the functions required by the remote database U A . Although a remote database U A differs from a U A in an interpersonal messaging applica-tion in the nature of messages that are sent and received as well as the type of services offered to the user, they both have the task of assisting the user in the preparation (con-struction) of messages as well as providing message storage capabilities. 4.3.3. Implementation Overview of the Remote Database U A The existing E A N interpersonal U A is a large application program (approximately 12000 lines of C code) and provides the user with all the interpersonal messaging services stipulated in the X.400 model (recommendation X.420 [CCITT83d]). In addition it also provides sophisticated message storage and retrieval facility and user profile definition3. Users interact with the interpersonal U A using the User Command Interface (UCI) which consists of 28 commands, each an implementation of a specific interpersonal mes-^The X.420 recommendations only specify the composition of an interpersonal message and not the nature of the user interface. The features offered by the user interface can incorporate an arbitrary degree of sophistication. In the case of EAN ' s UA, the large program size is a result of the inclusion of extensive functionality such as user profile definition and a message folder abstraction. 30 Implementation saging service4. Incorporating a remote database UA into the EAN UA was a simple matter of adding a new command to the UCI which implements the remote database UA. This command has been called the QUERY command. 4.3.4. Formulating a Query The content of an interpersonal message consists of a heading and a body. The heading, like the envelope, contains various delivery parameters which are interpreted in the appropriate manner by interpersonal UAs. The body of the message contains the actual text of the interpersonal message. Since the QUERY command is merely part of the interpersonal UA, its task is to structure the body of an interpersonal message according to the Database Access Proto-col. If the remote database UA had been implemented as a seperate program form the interpersonal UA, the entire message content would be structured according to the Data-base Access Protocol (as shown in Figure 4.1a). This complication is the result of com-bining the two UAs. Figure 4.2b shows the structure of an interpersonal message which contains a query. The Network User Address of the recipient DBMS as well as the three optional pro-tocol elements of the QueryHead are taken from arguments supplied in the QUERY command5. The user has the option of giving the NUA of the recipient DBMS either as a full NUA or as an alias which is then interpreted from the user's profile definition. Optional fields are only included in message structure if they are given as arguments to the QUERY command. 4 F o r a more detailed description of the E A N User Agent see [DADOU84]. 5 In E A N O / R Names are in fact O /R Addresses and are known as Network User Addresses (NUAs). 31 Implementation structured according to protocol specified in . X.41 1 envelope structured according to protocol specified in X.41 1 QueryHead interpersonal message heading. content interpersonal message QueryBody Doay QueryHead QueryBody e n v e l o p e c o n t e n t ' j (a) Query structured according to Database Access Protocol (b) Query as structured in implementation Figure 4.1 Message structure of a query The rest of the protocol elements are generated automatically. The NUA of the recipient DBUA is formed by substituting the user name part of the recipient DBMS's NUA with the reserved user name dbagent. The hierarchical naming and addressing scheme used by the message system guarantees that the resultant NUA can be unambi-guously determined as the NUA of the DBUA responsible for that DBMS. The QuerylD field is made unique by concatenating the NUA of the originating UA with a timestamp. The intended use of the QuerylD is for accounting and query process-ing status reporting which have not been included in this first implementation. The QuerylD has been included for the sake of completeness. The QueryBody is prompted for in the same way as the text of an interpersonal message and the user can include an already existing text file instead of having to type the entire query. This is useful for queries submitted periodically with little or no 32 Implementation alteration or for sending the same query to a number of DBMSs. A useful feature to provide in a remote database access facility is the ability to keep a record of a query submitted to the M T S for later reference. For example, if a result is received at some later stage stating that there was a syntax error in the original query, it would be useful for the user to have the ability to access the original query for examination. The E A N U A already includes a sophisticated interpersonal message storage/retrieval facility and this implementation has extended it to handle queries (and results) as well. The heading part of the message content is also generated automatically and con-sists of a TO field (the N U A of the recipient DBUA), a FROM field (the N U A of the ori-ginating UA). Many of the fields in the message envelope, the content .heading and the query heading are therefore the same. This redundancy is more apparent than real, since the fields are all interpreted at logically distinct levels in the message system. This is a feature of layered systems in general, but is considered more as an asset than a lia-bility because of the increased flexibility it allows. Once the entire message content is constructed it is added to an envelope and sub-mitted to the M T S for delivery to the recipient DBUA. Once the MTS accepts responsi-bility for the message, the Q U E R Y command exits and the user is free to use any other service offered by the U A . Incorporating the Q U E R Y command into the E A N U A resulted in an additional 400 lines of C code, a minor addition relative to the overall size of the UA. Keeping the Q U E R Y command small was possible because all the message construction and M T S interface routines are already included in the UA. 3 3 Implementation In E A N , messages are constructed by linking together basic data structures called ENODEs. Each E N O D E corresponds to a X.400 protocol element with each E N O D E containing the context sensitive integer code identifying which protocol element it is. Routines for constructing specific ENODEs such as an O / R Name or a character STRING as well as general purpose routines for linking ENODEs together or locating a particular E N O D E in the message structure were already part of the E A N interpersonal UA. 4 . 3 . 5 . Processing a Result A result of a previously submitted query is delivered to the U A of the originating user6. The U A is responsible for allowing the user to manipulate the result in the same way that it provides various services to the user for processing incoming interpersonal messages. As mentioned above, the result will also contain the original query. Since remote database access service is built into the interpersonal UA, it is necessary to treat results in exactly the same manner as interpersonal messages; so as to avoid having the user confronted with two different types of messages which require differential treatment. Treating results and interpersonal messages identically, allows the user to combine results with interpersonal messages for forwarding to other users. It also means that existing U A software can be used to store, retrieve, view, edit and print results. However, if the result is returned to the U A in the format specified by the Database Access Protocol, it will be misinterpreted by the U A because it would contain protocol elements, in the body part of a message, which are not defined in the X.420 interpersonal protocol. 6In actual fact, the M T S retains the result (message) until the U A explicitly requests the M T S for any messages that might have arrived for the UA. Once a message is delivered to the UA, the M T S is no longer responsible for it. 34 Implementation Therefore, in the interest of uniform treatment of results and interpersonal mes-sages, it was decided not to encode results according to the Database Access Protocol but instead, to have results returned as normal interpersonal messages (i.e. containing only interpersonal protocol elements in the body part of the message content). Similarly queries and results stored for later reference in the UA message file system have to be stored in interpersonal message format and not in Database Access Protocol Format. If a separate UA was used to implement remote database access, then naturally those elements of the Database Access Protocol pertaining to a Result, would be used. This is the intention of the design and would be implemented were sufficient functional-ity required of the remote database UA to warrant a separate UA. 4.4. The Database User Agent The DBUA has been implemented aa a special EAN UA with the reserved NUA dbagent@/»oa( where host is a qualifier which uniquely determines the O/R Name of a particular DBUA. Here again, the use of existing EAN software contributed to a simple implementation the different aspects of which are discussed below. 4.4.1. Invoking the D B U A Unlike the UA, which is invoked manually by the user, the DBUA is a server pro-cess which has to be invoked automatically upon the arrival of a message (query). For-tunately, the EAN message system already provides a mechanism for doing this. It is possible to arrange for the MTA responsible for a DBUA to wake up a program on arrival of a message and this feature is used to invoke the DBUA automatically. To avoid possible problems with concurrency, the first thing done by a DBUA on being awaken, is to check that no other DBUA is already active. If another DBUA is 35 Implementation active then it exits immediately. This ensures that only one D B U A is active at a time for any given D B U A N U A . A DBUA remains awake as long as messages arrive; i.e. queries which arrive while a D B U A is already active, will be processed on a FIFO basis by the active D B U A . 4.4.2. Query Validation The D B U A has to ensure that incoming messages are valid with respect to two cri-teria. Firstly, all messages have to be checked to see that they are in fact queries. Since a DBUA has a N U A (a well-known one at that), there is nothing stopping someone mistak-enly sending an interpersonal, or any other kind of message, to the DBUA. To prevent this, the body part of the content of incoming messages is examined and if it does not contain a QueryHead, it is rejected and an appropriate error message is returned to the originating U A (the N U A of which is obtained from the TO field of the content head-ing)7. Secondly, the D B U A must check to see that the QueryHead contains sufficient information for the appropriate DBMS to be invoked correctly. To begin with the N U A of the recipient DBMS, which appears in the QueryHead, must be checked against a table containing all the NUAs of DBMSs on whose behalf the D B U A acts. If no match is found then a result is returned to the originating U A indicating that a DBMS with that N U A does not exist. If the N U A for that DBMS does appear in the file, then the DBUA has to invoke the DBMS using the information contained in the three optional QueryHead fields. This means that individual DBUAs, unlike UAs, will differ (in detail if not in overall struc-7 Another possibility here, is that if an incoming message is a status report on a message submitted previously by the D B U A but which could not be delivered by the M T S for some reason. In this case, the status report is ignored. 36 Implementation ture) from each other since different DBMS (or identical DBMSs residing on different host operating systems) have to be invoked differently. This system dependency is iso-lated in the D B U A implementation by confining the invocation to a separate program module. This allows for simple addition (or deletion) of DBMSs. If a DBMS has to be added to the DBUA, then all that is needed is the addition of a program module which can invoke the DBMS in the correct way. Needless to say, each DBMS is invoked by the DBUA as if the originating user had invoked it. This ensures that only authorised users access the DBMS. Once the DBMS is invoked, the D B U A is responsible for passing the QueryBody to the DBMS in such a way that the DBMS perceives the D B U A as a con-ventional user. 4.4.3. Returning a Result A D B U A has to construct a result containing both the original query and the result obtained from the interaction with the DBMS. This is done using the same message con-struction routines as are used in the UA. As has already been explained, the result does not conform to the Database Access Protocol in this implementation. Instead, a conven-tional interpersonal message is constructed with the query and the result placed as plain-text in the body of the message content. The content heading is built to contain the delivery parameters necessary to ensure that the result is returned to the originating U A . Once the message content has been built, it is added to an envelope and submitted to the M T S . CHAPTER 5 Results and Evaluation 4.5. Results of a Typical Session This implementation of the remote database access facility has been demonstrated to work in a number of test examples, the transcript of which is given in Appendix C . In these examples queries formulated at a U A at one E A N site dsrg.ubc.cdn were submit-ted to an INGRES relational database residing on another E A N site cs.ubc.cdn. Although these results realise the objectives of a first implementation, the functionality of both the U A and the D B U A would have to be improved before they could be used as reliably as the existing E A N U A . 4.6. Response Time The fact that the two E A N sites used in these examples are part of the same local network, meant that the time taken for queries to be processed was very short (30 - 50 seconds). However, since a message-based remote database access facility is primarily intended for use over long-haul networks where the delay would be considerably longer, this low response time is not considered to be a significant system performance measure. However, since a message-based design is not intended for fast-response applications, a longer response time could not be regarded as a drawback of the system. 4.7. Drawbacks of a Hybrid UA Deviating from the C C I T T X.400 model in combining the functions of a remote database UA and an interpersonal U A into one entity has resulted in a somewhat "quick 37 Results and Evaluation 38 Results and Evaluation and dirty" implementation. Results do not conform to the Database Access Protocol and queries and results are stored in interpersonal message format. If a fully functional remote database access facility had to be implemented, a cleaner implementation could only be achieved by ensuring that the remote database U A was kept as a separate entity. 4.8. Evaluating the Design As illustrated in Figure 5.1, the problem of networkwide database access can be characterised by identifying the three dimensions of heterogeneity in which DBMSs differ from each other. There are differences in the way a DBMS is invoked (dependent on the host operat-ing system), differences in the nature of the user/DBMS interaction (dependent on the nature of the data model and query language used), and differences in the location of relational C O D A S Y L hierarchical DBMS type UNIX Host System V M S MUlt1C3 partially fully partitioned redundant redundant j 1 1 • D i s t r i b u t i o n Strategy Figure 5.1 Dimensions of variations in DBMSs [WANG831 30 Results and Evaluation data in the DBMS (dependent on the data distribution strategy used by the DBMS). As was discussed in Chapter 2, these variations have given rise to a number of connection-based systems whose sophistication varies according to the degree with which these differences are overcome. Less sophisticated systems, such as iNet, only deal with differences in system dependent invocation procedures while more sophisticated systems, such as Multibase, accommodate different data models, query languages and distributed data. Similarly, the degree of sophistication with which a message-based facility pro-vides access to remote databases can be measured by its ability to accommodate the differences mentioned above. The design proposed here accommodates both differences in DBMS invocation as well as differences in Data Models and Query Languages. In providing this generalisation, however, a certain onus is placed on users to be aware of the peculiarities of the DBMS they wish to access. In the case of remote DBMS invocation, users are required to know which of the three optional fields must be included in the QueryHead. This is an inevitable result of the intractibility of designing a D B U A capable of generalised DBMS invocation, since for the most part a DBMS is invoked differently each time it is used (e.g. many DBMS require the name of a particular database as part of the system command which invokes it). As far as allowing different Data Models and Query languages is concerned, neither the U A nor the D B U A assume responsibility for ensuring the syntactic or semantic vali-dity of queries and so users have to be entirely familiar with the query language of DBMSs they access. Here it must be stated that a more ambitious design would relieve .users of this requirement. It would have been possible to allow users to submit queries in 40 Results and Evaluation a canonical query language, such as is used in systems like M U L T I B A S E . The Query-Body would then be structured according to this canonical form instead of plaintext and each D B U A would be responsible for translating the structured canonical query into the query language of the recipient DBMS. It would also, of course, be responsible for the inverse translation so that results are always returned to originating UAs in a uniform format. This approach was not adopted because it would have resulted in a far more exten-sive implementation (both the U A and the D B U A would have to enlarged substantially, while detracting from the primary goal of demonstrating the feasibility of a message-based design. In addition, the present design does not address the problem of providing access to distributed DBMSs (i.e. partitioned databases managed by multiple controllers and located at multiple sites). It was felt that the complexity of dealing with distributed DBMSs would unduly complicate the protocol design as well as the overall system archi-tecture. For instance, one would have to contend with the problem of deciding which controller (or controllers) should be sent queries, as well as collating partial results. These, and many other issues, relating to distributed databases are not fully understood and are still the subject of active research. Trying to deal with these issues within the context of this thesis would result in a design of far lesser simplicity and ease of imple-mentation. However the fact that a distributed message system is used as the communication medium suggests that it might be suitable for distributed database applications as well. This would be a subject for further investigation. 41 Results and Evaluation 4.9. A More Extensive Implementation It would have been desirable to implement the design over more host operating sys-tems and DBMSs as well as over a wider range of E A N sites, but this was not possible due to the lack of system resources. 4.10. Conclusions We have given the motivation and presented a design for a message-based remote database access facility which is well suited to request-response type applications. The design complies with the C C I T T X.400 international recommendations for message han-dling systems. A first implementation of the design has demonstrated that it is both feasible and simple to implement on top of an existing message system. Refining the Database Access Protocol to allow for data model/query language independence, and exploring the potential of a message-based design in distributed database applications are both areas for further study. R e f e r e n c e s [CCITT82] C C I T T Provisional Recommendation X.200, "Reference Model of Open Sys-tems Interconnection for C C I T T Applications", Sept 1982. [CCITT83a] C C I T T Final Draft Recommendation X.400, "Message Handling Systems:System Model-Service Elements", Dec 1983. [CCITT83b] C C I T T Final Draft Recommendation X.409, "Message Handling SystemsrPresentation Transfer Syntax and Notation", Dec 1983. [CCITT83c] C C I T T Final Draft Recommendation X.411, "Message Handling Systems:Message Transfer Layer", Dec 1983. [CCITT83d] C C I T T Final Draft Recommendation X.420, "Message Handling Systems interpersonal Messaging User Agent Layer", Dec 1983. [CHAPI82] A . L . Chapin, "Connectionless Data Transmission", Comp. Comm. Rev., Vol 12, no. 2, April 1982, pp 21-61. [CHAPI83] A . L . Chapin, "Connections and Connectionless Data Transmission", Proc. IEEE, Vol 71, no. 12, Dec 1983, pp 1365-1371. [CUNNI83] I. Cunningham, J. Raiswell, "Gateway to Online Information Services", Telesis, vol 1. 1983, pp 2-7. [DADOU84] N. Dadoun, " E A N User Agent Implementation Notes", CPSC 530 Term Paper, Univ. of Brit. Col., April 1984. [DAY83] J. Day, H. Zimmermann, "The OSI Reference Model", Proc. IEEE, Vol 71, no. 12, Dec 1983, pp 1334-1340. [GILM084] P.C. Gilmore, " E A N and CDNnet: A Progress Report", Information Pam-phlet, Dept. of Comp. Sc., Univ. of Brit. Col., Nov 1984. [LISAN80] S. Lisanti, "The Online Search", Byte, Dec 1980, pp 215-230. [NEUFE83] G.W. Neufeld, " E A N : A Distributed Message Handling System", Proc CIPS 1983 Conf., Ottawa, May 1983, pp. 144-149. [NEUFE84] G.W. Neufeld, "The E A N Distributed Message System User Manual", Dis-tributed Systems Research Group, Dept. of Comp. Sc., Univ. of Brit. Col., 1984. [SADOW84] E.R. Sadowski, "The Efficacy of a Store-and-Forward File Transfer Sys-tem", M.Sc. Thesis, Univ. of British Columbia, April 1984. [SMITH81] J .M. Smith et al., "Multibase - Integrating Heterogeneous Distributed Data-base Systems", AFIPS Conf. Proc, 1981 National Computer Conference, Vol 50, pp 487-99. 4-2 43 References [STONE76] M . Stonebraker et al., "The Design and Implementation of INGRES" , ACM Trans. Database Syst., Vol 1, no 3, Sept 1976, pp 189-222. [SOLOS84] A.G. Solosky, "iNet 2000 : Gateway to the Information Marketplace", CIPS Session 84 Proc, Calgary, May 1984, pp 477-78. [VALLE84] J. Vallee, Computer Message Systems, McGraw-Hill, (1984), 163 pp. [WANG83] P.S.-C. Wang, S.R. Kimbleton, " A n Application Protocol for Networkwide Database Access", J. of Telecommunication Networks, Vol 2, no 3, Fall 1983, pp 285-294. 44 A p p e n d i x A C C I T T X .420 Specif ication Appendix A C C I T T X.420 specifies the service elements and protocol structure of the Interper-sonal User Agent Layer. The protocol used to encode an interpersonal message is called P2 and is given here according to the notation specified by C C I T T X.409 [CCITT409]. The essential elements of P2 state that a message, in an interpersonal messaging application, is either a user message or a status report giving status information about a previously submitted message. As can be seen, X.420 provides substantial flexibility in the type of data that can be included in the body of a message. 45 . The converted EncodedlnformationTypes component conveys the new EncodedlnformationTypes if conversion took place on the IM-UAPDU. P2 DEFINITIONS :: a BEGIN — P2 makes use of types defined in the following, modules: PI: X.411, Section 3.4 P3: X.411,Section4JJ — SFD: this Recommendation, Section 5 — Sa: S.a, Section 4.1 UAPDU ~ IP-messageUAPDU IM-UAPDU — heading CHOICE { [0] IMPUOT IM-UAPDU, [1] IMPUOT SR-UAPOU} SEQUENCE [Heading, Body} Heading IPMessageld, originator authorizingUsers srr{ [0] IMPLICIT OROescriptor OPTIONAL, [1] IMPUOT SEQUENCE OF OROescriptor OPTIONAL, only if not the originator primaryRecipients [2] IMPUOT SEQUENCE OF Recipient OPTIONAL. copy Recipients [3] IMPUOT SEQUENCE OF Recipient OPTIONAL. blindCopyRecipients [4] IMPUOT SEQUENCE OF Recipient OPTIONAL, in Reply To [5] IMPUOT IPMessageld OPTIONAL, =- omitted if not in reply to a previous message obsolete* erossReferences subject expiryQate replyBy replyToUsers [6] IMPUOT SEQUENCE OF IPMessageld OPTIONAL. [7] IMPUOT SEQUENCE OF IPMessageld OPTIONAL, [8] CHOICE (S61 String} OPTIONAL, [9] IMPUOT Time OPTIONAL, « if omitted, expiry date is never [10] IMPUOT Time OPTIONAL, [11] IMPUOT SEQUENCE OF ORDescriptor OPTIONAL, =- each O/R descriptor must contain an O/R name importance [12] IMPUOT INTEGER (low(O), normal(1), high(2)} DEFAULT normal, sensitivity [13] IMPLIOTINTEGER (personal(1), prtvate{2). companyConfidential(3)} OPTIONAL, autoforwarded [14] IMPUOT BOOLEAN DEFAULT FALSE — indicates that the forwarded message body partis) were autoforwarded — } [APPUCA TION 11 ] IMPLICIT S ET { ORName OPTIONAL, PrintableString} IPMessageld ORName P1.ORName -- P2 definitions to be continued Figure 3/X.420. Formal Definition o f l M - U A P D U (Part 1 of 3) 46 -- P2 definitions continued ORDescriptor a SET {— at least one of th? first two members must be present ORName OPTIONAL, treef ormName [0] IMPLICIT S61 String OPTIONAL, telephoneNumb* * [1] IMPLICIT PrintableString OPTIONAL} Recipient :: • srr{ [0] IMPLICIT ORDeicriptor, reportRequest [1] IMPLICIT BIT STRINC { receipt NotificatiorKQ), nonReceiptNotificatiorr(1), returniPMessage<2)} DEFAULT Q. ~ if requested, the QIR descriptor must contain — an OIR name reply Request [2] IMPLICIT BOOLEAN DEFAULT FALSE ~ if true, the QIR descriptor must contain " an QIR name —} — body Body SEQUENCE OF BodyPart BodyPart :: a CHOICE { • [Ol IMPLICIT IA5Text [11 IMPLICIT TLX, PI IMPLICIT Voice, [31 IMPLICIT G3Fax. [41 IMPLICIT 71F0. [S3 IMPLICIT TTX, [61 IMPLICIT Videotex, [7J NationallyDefined, [81 IMPLIQT Encrypted, [91 IMPLICIT ForwardedlPMessage, [101 IMPLICIT SFD, [111 IMPLICIT T1F1} — body part types JASText » SEQUENCE { SET {repertoire [01 IMPLICIT INTEGER (iaS(S). ita2(2)} DEFAULT ia5 — additional members of this Set — are a possible future extension ••}, lASString} TLX :: • for further study Voice :: = SEQUENCE { SET, -- members are f o r f u r t h e r s t u d y BIT STRING} -- P2 definitions to be continued Figure 4/X. 420. Formal Definition o f l M - L ' A P D U (Part 2 of 3) 47 - P2 definitions continued G3Fax TtFO SaDocument TTX Videotex NationallyDefined Encrypted ForwardedlPMessage Delivery! nfermatibn SFD SEQUENCE{ SET{ numberOf Pages [0] IMPLICIT INTEGER OPTIONAL, [11 IMPLICIT PI .G3NonBasicParams OPTIONAL}. SEQUENCE OF BIT STRING} SaDocument SEQUENCE OF Sa.ProtocolElement SEQUENCE { S«TC numberOf Pages [0] IMPLICIT INTEGER OPTIONAL, telexCompatibie [1] IMPLICIT BOOLEAN DEFAULT FALSE, PI IMPLICIT PT.TeletexNonBasicParams OPTIONAL}, SEQUENCE OF S61 String} SEQUENCE{ SIT, — members are for further study SlOOString} ANY SEQUENCE{ SET, — members are for further study BIT STRING} SEQUENCE{ SET{ delivery [0] IMPLICIT Time OPTIONAL, [1] IMPLICIT Deliverylnformation OPTIONAL}, IM-UAPDU} P3. DeliverEnvelope — This merely reuses a data type definition, and does not dimply that the information was ever carried in P3. SFD.Document -- note that SFD and SaDocument use the same space of application-wide tags — which is different from that used for other MHS protocols TIF1 ::» SaDocument — P2 definitions to be continued Figure 5/X.420. Formal Definition of IM-LTAPDL7 (Part 3 of 3) 48 -- P2 definitions continued IP M-status-report UAPDU SR-UAPOU ::- SET{ [OJ CHOICE { nonReceipt [0] IMPUOT NonReceiptlnformation, receipt [1] IMPUOT Receiptlnformation}, reported IPMessageld, actualRecipient [1] IMPUOT OROescriptor OPTIONAL, intendedRecipient [2] IMPUOTOROesc r i p to roPT iONAL , — only present if not actual recipient — the O/R descriptor must contain an O/R name converted P1 .EncodedlnformationTypes OPTION AU} NonReceiptlnformation : :• SET{ reason [0] IMPUOT INTEGER {uaelnitiatedDiscard(O). autoForwarded(l)}, nonReceiptQualifier [1] IMPUOT INTEGER {exprred(O), obsoleted(l), subscriptionTerminated(2)} OPTIONAL, comments [2] IMPUOT PrintableString OPTIONAL, — on auto-forward returned [3] IMPUOT IM-UAPDU OPTIONAL} Receiptlnformation : : a SET{ receipt [0] IMPUOT Time, typeOfReceipt[1] IMPLICIT INTEGER { explicit(0>,automatic(1)} DEFAULT explicit, [2] IMPUOT P1 .Supplementarylnfcrmation OPTIONAL} ENO — ofP2 definitions Figure 67X.420. Formal Definition of SR-UAPDU • 49 Appendix B Appendix B User and Database User Agent Installation The following are instructions for the installation of the U A and the D B U A on UNIX 4.2Bsd systems. The User Agent The User Agent consists of existing E A N software routines (located in the E A N source directory - usually ~ ean/src) and a number of other routines used to provide the additional QUERY command. These routines are presently located in koorland/dbajdba on ubc-cs along with a Makefile. In order to compile the user agent, assuming pathnames in # include statements are all accurate, the command make myua should be issued. On successful compilation, the file myua will exist and be similar to the E A N U A except for the additional Q U E R Y command. The Database User Agent All the routines for the D B U A are located in the same directory and presently sup-port an INGRES DBMS. If additional DBMS are added to the D B U A then the code has to be modified to accommodate each new DBMS (the sections of the code that have to be modified are documented in the body of the code). The D B U A also uses a number of existing E A N routines which have to be present before it can be compiled. To compile the DBUA, make dbagent. 50 Appendix B Each D B U A must be associated with a O / R Name (Address) dbagent@site and its local M T A must know the name of the D B U A program. This can be arranged by the E A N system administrator. 51 Appendix C Test Examples Appendix C The following is an annotated transcript of some examples of the remote database access facility in use. In these examples a U A with the N U A koorland@darg.ubc.cdn is used to submit queries to an INGRES relational DBMS with the N U A ingrea@cs.ubc.cdn and under the control of the DBUA dbagent@ca.ubc.cdn. In the following, annotations are given in Italic, user input in Roman face, and sys-tem responses in Bold face. Familiarity with the use of the E A N interpersonal U A would be helpful in following these examples. EXAMPLE 1 - formulate a query to ingrea@cs.ubc.cdn and invoke it with command "/user etc ..." which specifies the "demo" database > query ingres@cs.ubc.cdn command="/user/ingres/bin/ingres -s demo" Enter query ("." to end — "break" to abort) print parts \go \quit **** query sent accept the result from the MTS > accept Accepting new messages: . inbox: 3 N U dbagent@cs.ubc.cdn A p r 14 85 query to : ingres@cs.ubc.cdn print the result > print 3 52 Appendix C Message inbox:3 - Unread From: <dbagent@cs.ubc.cdn> T o : <koorland@dsrg.ubc.cdn> Subject: query to : ingres@cs.ubc.cdn a copy of the original query is returned with the result Q U E R Y S U B M I T T E D print parts \go \quit Q U E R Y R E S U L T parts relation pnum pname color weight qoh 10 byte-soap clear 0 143 1 central processor pink 10 1 11 card reader gray 327 0 2 memory gray 20 32 12 card punch gray 427 0 3 disk drive black 685 2 13 paper tape reader black 107 0 4 tape drive black 450 4 14 paper tape punch black 147 0 5 tapes gray 1 250 6 line printer yellow 578 3 7 1-p paper white 15 95 8 terminals blue 19 15 9 terminal paper white 2 350 EXAMPLE 2 - use EAN alias facility and command abbreviation to avoid tedious typing and submit the same query > set alias ingres=ingres@cs.ubc.cdn > query ingres c=="/user/ingres/bin/ingres -s demo" Enter query ("." to end — "break" to abort) 5 3 Appendix C print parts \ g ° \quit * * * * query sent accept and print result > accept Accepting new messages: . inbox: 4 N U dbagent@cs.ubc.cdn A p r 14 85 query to : ingres@cs.ubc.cdn > print 4 Message inbox:4 - Unread From: <dbagent@cs.ubc.cdn> T o : <koorland@dsrg.ubc.cdn> Subject: query to : ingres@cs.ubc.cdn Q U E R Y S U B M I T T E D print parts \go \quit 54 Appendix C Q U E R Y R E S U L T parts relation pnum pname color weight qoh 10 byte-soap clear 0 143 1 central processor pink 10 1 11 card reader gray 327 0 2 memory gray 20 32 12 card punch gray 427 0 3 disk drive black 685 2 13 paper tape reader black 107 0 4 tape drive black 450 4 14 paper tape punch black 147 0 5 tapes gray 1 250 6 line printer yellow 578 3 7 1-p paper white 15 95 8 terminals blue 19 15 9 terminal paper white 2 350 EXAMPLE 8 - use batch query facility > query ingres c="/user/ingres/bin/ingres -s demo" Enter query ("." to end — "break" to abort) ^include batch_query * * * * query sent > accept Accepting new messages: . in box: 5 N U dbagent@cs.ubc.cdn A p r 14 85 query to : ingres@cs.ubc.cdn > print 5 Message inbox:5 - Unread From: <dbagent@cs.ubc.cdn> T o : <koorland@dsrg.ubc.cdn> Subject: query to : ingres@cs.ubc.cdn Q U E R Y S U B M I T T E D 55 Appendix C print parts \ g ° \quit Q U E R Y R E S U L T parts relation pnum pname color weight qoh 10 byte-soap clear 0 143 1 central processor pink 10 1 11 card reader gray 327 0 2 memory gray 20 32 12 card punch gray 427 0 3 disk drive black 685 2 13 paper tape reader black 107 0 4 tape drive black 450 4 14 paper tape punch black 147 0 5 tapes gray 1 250 6 line printer yellow 578 3 7 1-p paper white 15 95 8 terminals blue 19 15 9 terminal paper white 2 350 EXAMPLE 4 - query a non-ezistant DBMS > query spires@cs.ubc.cdn Enter query ("." to end — "break" to abort) * * * * query sent > accept Accepting new messages: . in box: 6 N U dbagent@cs.ubc.cdn A p r 14 85 query to : spires@cs.ubc.cdn 56 Appendix C > 6 Message inbox:8 - Unread From: <dbagent@cs.ubc.cdn> To: <koorIand@dsrg.ubc.cdn> Subject: query to : spires@cs.ubc.cdn D B M S spires@cs.ubc.cdn does not exist EXAMPLE 5 - Do not specify the correct optional QueryHead fields > query ingres Enter query ("." to end — "break" to abort) print parts \g \ q **** query sent > accept Accepting new messages: . in box: 7 N U dbagent@cs.ubc.cdn Apr 14 85 query to : ingres@cs.ubc.cdn > 7 Message inbox:7 - Unread From: <dbagent@cs.ubc.cdn> To: <koorland@dsrg.ubc.cdn> Subject: query to : ingres@cs.ubc.cdn Insufficient or Invalid authorisation given EXAMPLE 6 - specify the invocation command incorrectly > query ingres c="dfhdfhjdfshjkfdjfhj" Enter query ("." to end — "break" to abort) print parts \go \quit 57 Appendix C * * * * query sent > accept Accepting new messages: inbox: 8 N U dbagent@cs.ubc.cdn A p r 14 85 query to : ingres@cs.ubc.cdn > 8 Message inbox:8 - Unread From: <dbagent@cs.ubc.cdn> T o : <koorland@dsrg.ubc.cdn> Subject: query to : ingres@cs.ubc.cdn D B M S ingres@cs.ubc.cdn cannot be invoked EXAMPLE 7 - use incorrect query language in QueryBody > query ingres c="/user/ingres/bin/ingres -s demo" Enter query ("." to end — "break" to abort) this is not a valid query in EQUEL, the INGRES query language \go \quit ***+ query sent the UA doesn't know the query is incorrect > accept Accepting new messages: . inbox: 10 N U dbagent@cs.ubc.cdn A p r 14 85 query to : ingres@cs.ubc.cdn > 10 Message inbox:10 - Unread From: <dbagent@cs.ubc.cdn> T o : <koorland@dsrg.ubc.cdn> Subject: query to : ingres@cs.ubc.cdn Q U E R Y S U B M I T T E D this is not a valid query in E Q U E L , the I N G R E S query language \go 58 \quit according to the UA this result is correct and the user is responsible for interpreting the error message from INGRES QUERY RESULT 2600: syntax error on line 1 last symbol read was: this > quit 

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