DB: Browsing Object-Oriented Databases over the Web

C. Varela, D. Nekhayev, P. Chandrasekharan, C. Krishnan, V. Govindan, D. Modgil, S. Siddiqui, O. Nickolayev, D. Lebedenko, M. Winslett
Department of Computer Science
1304 W. Springfield Ave
University of Illinois
Urbana, IL 61801. U.S.A.
{ cvarela, d-nekha, p-chand, charuki, govindan, modgil, siddiqui, nickolay, dlebeden, winslett}@uiuc.edu

Abstract:
In this paper, we present some critical issues involving the browsing of object-oriented databases over the World-Wide Web, as well as performance results using our Database Browser (DB) implementation.
First, given the statelessness of HTTP, we introduced a dispatcher script architecture. In this architecture, a CGI script communicates with an intermediate data server connected to the database application, which keeps the database open for faster future transactions. Second, we used ODL, a standard object definition language and we defined a simple intermediate data format for moving database objects across the network. And lastly, we defined five generic hypertext interfaces for: a database schema, a class definition, a class query form, a class extent, and a set of class instances.
We implemented these ideas using ObjectStore with two database applications, one containing university registration information and another one containing electronic mailboxes. An important result was the dramatic performance improvement gained by introducing our Database Browser (DB) architecture.
Keywords:
World-Wide Web, Object-Oriented Databases, Dispatcher Scripts, ObjectStore, Object Definition Language, Common Gateway Interface.
Contents:
  1. Introduction
  2. Background and Related Work
  3. Database Browser (DB) Architecture
  4. ObjectStore Examples and Performance Evaluation
  5. Conclusions

1. Introduction

Six years after its conception in 1989, the World-Wide Web is arguably the most popular and powerful networked information system to date. Its growth in the past years has been exponential and it has started an information revolution that will probably continue to take place until the beginning of the next century. The databases world with barely 25 years as a basic science research field [Silberschatz et al. 91] is by no means less interesting. Needless to say, these two worlds' encounter brings us many new opportunities for creating advanced information management applications.

In this paper, we present some critical issues involving the browsing of object-oriented databases over the web. In particular, we present the following topics:

We implemented two exemplary applications: a university course registration system and an electronic mailbox manager. On the WWW server side, we used C/Lex/Yacc for the dispatcher script and NCSA HTTPd [NCSA 93a], a high-performance HTTP server. On the database server side, we used C++ and ObjectStore [Lamb et al. 91], an object-oriented database management system, with a page-server architecture. A remarkable result was the performance improvement gained by incorporating the dispatcher script architecture, as opposed to opening and closing the database server in every transaction.

The structure of this paper is as follows: in Section 2 we will give some background information and introduce related work. In Section 3, we will describe our Database Browser (DB) architecture. Section 4 will show two object-oriented web database applications in action and will present some performance results. This paper concludes highlighting some results and discussing further research issues.

2. Background and Related Work

2.1. The World-Wide Web

The World-Wide Web [Berners-Lee et al. 92, 94] offers easy access to a universe of information by providing links to documents stored on a world wide network of computers in a very simple and understandable fashion. Much of its success is due to the simplicity with which it allows users to provide, use and refer to information distributed geographically around the globe.

Another important feature is its compatibility with other existing protocols, such as gopher, ftp, netnews and telnet. Furthermore, it provides users with the ability to browse multimedia documents independently of the computer hardware being used.

The World-Wide Web consists of a network of computers which can act in two roles: as servers, providing information; or as clients, requesting for information. Examples of server software are NCSA HTTPd [NCSA 93a] and Netscape Communication Server [Netscape 95a], while examples of client software are NCSA Mosaic [NCSA 93b] and Netscape Navigator [Netscape 95b].

The World-Wide Web is based on the HyperText Transfer Protocol (HTTP,) the HyperText Markup Language (HTML,) and Universal Resource Locators (URLs.)

2.2. Databases in the Web

When the documents to be published are dynamic, like those resulting from queries to databases, the hypertext needs to be generated by the servers. For this purpose, there are scripts, which are programs that perform conversions from different data formats into HTML on-the-fly. These programs also need to understand the queries performed by clients through HTML forms and the results generated by the applications owning the data (for example, a DataBase Manager System.)

2.3. Object Definition Language

For defining the database schemas, we used the Object Definition Language(ODL) developed by the Object Database Management Group (ODMG) and specified in [Cattell et al. 94]. The ODL specification conforms to the Object Data model, characterized by the following basic features:

As an example, the Professor class in this figure would be represented in ODL as follows: 
interface Professor: Employee {
        extent professors;
        keys name, id;

        attribute string name;
        attribute int id;
        attribute string rank;

        relationship Set<Section> teaches inverse Section::is_taught_by;
        grant_tenure() raises (ineligible_for_tenure);
}
We refer the interested reader to the complete University schema with all classes as defined in Cattell's book. We have also annexed our ODL specification for the Electronic Mailbox schema.

3. Database Browser (DB) Architecture

In the first version of our hypertext interface to the object databases, the database was accessed through CGI scripts which would open the database, process the request and close the database everytime. We found that this sequence of operations was extremely time-consuming and the response was very slow.

We reasoned that this slow response was due to the fact that Objectstore has a page fault model with prefetch and thus, every time the database is opened, the pages containing the data are brought into memory and the pointers are swizzled. Under regular circumstances, this paging activity would only be a one-time startup overhead and the subsequent database operations would be very fast. But, since the CGI scripts are stateless, (in the sense that they cease to exist after the operation is performed), this overhead was being incurred on every database request. Each request entailed a separate "open database, perform operation, close database" sequence. We were thus completely losing the performance gains of the paging model adopted by Objectstore.

In our next and current version, we adopted the database browser (DB) architecture as illustrated in the figure below. The main components of this architecture are the dispatcher script, the intermediate data server and the simple intermediate exchange format each of which are discussed separately in the following subsections.

3.1. A Dispatcher Script

The dispatcher program is a CGI script, the role of which is to provide a user-friendly interface to query the intermediate data server and to return the database schema, extent and object data to the WWW browser. Separating this dispatcher from the intermediate data server is beneficial for several reasons. First, it allows us to keep the intermediate date server active even when the transaction is over, so the transaction performance increases significantly. Second, this architecture provides more flexibility to customize the whole application. The database side can be changed without causing any interference with the user interface. On the other hand, the interface itself can be customized without affecting the database server.

The dispatcher script consists of the following modules:

3.2. An Intermediate Data Server

The main characteritic of the intermediate data server is that it remains alive across multiple requests and thereby eliminates the earlier problem of the script ceasing to exist after every operation.

The data server was designed to use the standard BSD sockets and to listen on a pre-determined port for requests. Whenever the dispatcher script gets a request, it connects to the data server which is listening for requests and sends the request in our own simple intermediate data exchange format described in the following section. The server performs the database operation and sends back the results using the same connection to the dispatcher script. The dispatcher script in turn parses it and converts it to HTML. This is then forwarded by the web server to the browser.

Thus the high startup overhead occurs only on the first database operation (which we term as cold time) and subsequent operations are relatively very fast (termed as warm time).

The server also keeps a queue of requests and therefore, multiple simultaneous requests are handled serially by the server. The same server can also be used to handle operations on multiple databases. This is achieved through the liberal use of lookup tables to perform name-to-object and name-to-function translations.

3.3. A Simple Intermediate Data Exchange Format

The intermediate object exchange format is used for communication between the CGI dispatcher script and the intermediate data server. It has two parts: query and data format.

3.3.1. Query format

3.3.2. Data format

For many of the object oriented databases, the data format described here is sufficient. This format is based on attribute-value pairs. In principle, it can be augmented to fit multimedia data, but in our case, we have only used data in text or numerical format. In these cases, attribute-value pairs is a natural intermediate representation.

As a response to a query, the database server can return several objects. In our format we use the line 
%#%
as a delimiter between successive objects. The format for an object looks as follows: 
attribute_1_name: attribute_1_value
attribute_2_name: attribute_2_value
.
.
attribute_n_name: attribute_n_value
relationship_1_traversal_path: target_instance_1_keys
target_instance_2_keys
.
.
target_instance_n_keys
relationship_2_traversal_path: target_instance_1_keys
.
.
As we see, for attributes we give colon-separated name-value pairs. Attribute values can be complex text with newlines, special symbols, etc. Ambiguities may occur when the text of the attribute value at the beginning of a line coincides with the following attribute name. The intermediate data server resolves this special case by inserting a space immediately after the newline.

We assume that the key value is a simple string or number and can not have colons, commas, spaces or newlines. The value part of relationship-value pair is newline-separated list of keys of all target instances. Each element of this list is a comma-separated list of all the keys of a particular target instance.

In case of EXTENT queries, the intermediate data server returns the data in a brief format containing only the keys to instances.

3.4. Generic Hypertext Interfaces to OODBs

The dispatcher script has five different presentation screens to represent database schemas, query forms and actual data. We proceed to describe all of them.

4. ObjectStore Examples and Performance Evaluation

4.1. University Course Registration System

This database was built using the example ODL schema given in the ODMG-93 book. This demonstrates how an object database specifed in ODL can be browsed in a generic manner and how one could build a hypertext interface to navigate through the database. The university database exemplifies many of the concepts specific to object-oriented databases like inheritance, operation functions, inverse relationships, exceptions, multiple inheritance, etc.

The database objects are the typical entities in a university as illustrated in the schema diagram and ODL specifications given in the subsection on ODL. To run this example, please proceed to the University Schema presentation page.

4.2. Electronic Mailbox Manager

This database was created to hold regular email, with particular emphasis on huge mailboxes. The database mainly consists of two basic objects, one is the message object and the other the folder object. One can associate a multiple messages to a folder and also, a message to multiple folders. A parser was written to parse a regular mail folder and put it into the database. This database was created to evaluate the performance of the browser with huge databases and to provide a hypertext interface to a real-world application. To run this example, please proceed to the Mailbox Schema presentation page.

4.3. Performance Evaluation

We instrumented the server to obtain the time spent on the various operations. We measured the performance by taking the times to perform various operations in different scenarios. All the times are for a single user performing queries, we did not measure the performance with multiple, simultaneous requests.

The results of the performance measurements are summarized in the following graphs.

4.3.1. Message extent in the mailbox database : (Large database, large results)

In this case, the size of the result (about 400 KB) causes quite a big difference in the times, especially in the remote case. We feel that the network overhead (congestion, packetizing, etc) contributes to a major part of the time taken. The slow speed of the link (64 Kbps) in the remote case could also be a major cause for the delay, so a user over a phone line would get an even slower response.

We thus conclude that network delays are certainly a factor to be considered, especially for large query results.

4.3.2. Message instance in the mailbox database : (Large database, small results)

In this case, the network did not seem to play any significant role as in the earlier case. This could be attributed to the small size of the query results (< 10 KB). The cold times are still significant as the database itself is large and therefore, a lot of pages need to be brought into memory at start-up.

4.3.3. University database : (Small database, small results)

In this case, the time taken is really small as the query results are very small. Even the cold time is quite small since the database itself is very small and hence, very few data pages would need to be paged in.

We also observed that closing the database does not actually result in a subsequent request taking a "cold time". This, we concluded was because Objectstore has a page server model which means that even when the database is closed, the data pages are still in memory and are not reclaimed unless the data of some other application is paged into the same place.

5. Conclusions

In this paper, we presented an architecture for browsing object-oriented databases over the World-Wide Web. This architecture consists mainly of a CGI dispatcher script, an intermediate data server, a simple intermediate data format for communicating between these two modules, and generic hypertext interfaces for browsing different database elements. We found remarkable performance improvements by using this Database Browser (DB) architecture. There is a need for further work on additional query options, since ObjectStore doesn't currently support OQL, the ODMG standard for OODB queries. We would also like to investigate further presentation issues for huge databases, as well as multimedia object handling.

We didn't consider in our study, security issues that would be crucial in extending our database operations. Currently, we only concentrated in browsing databases. There is a lot of research that needs to be done in authentication and authorization for databases operating in an open environment. [Bina et all. 94] is a good study of some of these security issues. They present a role wrapper-based framework, that assigns roles to clients and controls their database access according to these roles.

We believe that there is a need for more research and development in advancing the state-of-the-art in performance and security for connecting databases to the World-Wide Web. There are many applications that could benefit from this research including bibliographic databases, financial management systems, intelligent agents, data mining applications and countless others.

Acknowledgements

We would like to thank Dan Laliberte at NCSA for setting up our HyperNews project page; Mike Jerger for letting us use bunny, the database answering machine; Simon Kaplan for allowing us to use ObjectStore; International Systems Research, Tokyo and UMDS, Kobe for providing partial support to the first author; and last but not least, the WWW Organizing Committee for granting us a small extension to submit this paper.

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