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Published: March 1999.
Developing Distributed Applications on the WebNicholas Crofts, Direction des Systèmes d'Information (DSI), Switzerland
IntroductionMany museums today have assimilated the potential of the Web as a medium for distributing or publishing information. Presentations about the institution and its collections, sometimes virtual tours, are now commonplace. However, the web also has an enormous potential, at present almost completely unexploited by the museum community, as a front end for interactive applications which are used to input information. Rather than just pumping information out, the web can be used for information gathering, building distributed applications which are accessible to a far wider range of users than would be the case with traditional client- server application architectures.
The technology is still relatively new and different approaches exist. Based on the experience of Geneva's Musinfo project, the aim of the present paper is to outline the technical options, highlight some of the pitfalls and explore the potential impact of web enabled applications.
Musinfo exampleThe Musinfo application is an example of what can be achieved using readily available web technology and at relatively moderate cost. Based on the object oriented ICOM/CIDOC Conceptual Reference Model, the project was developed in-house for the city's museums by the Direction des Systèmes d'Information - the city's IT division. The application is currently used by three major institutions, which, together, represent close to 100 users. Other organisations will be joining in the near future. More than 350'000 records are already available on online, many with associated digital images. The application is a fully fledged collections management system and provides all the information management services for the for inventory, cataloguing and research: data entry and updating, queries, reports, thesaurus and authority lists, etc. Currently it is used by curators and administrators, and by some external researchers; public access is also planned for this year. The application covers all the disciplines represented by the participating institutions: natural science, ethnography, fine arts, and applied arts, archaeology, etc.
Fig. 1: Musinfo application screen
The entire Musinfo application can be accessed using a web browser such as Netscape or Internet Explorer . The web browser effectively functions as the client application. This allows for an unprecedented degree of flexibility and user mobility - the application and the entire database are potentially available on any work station connected to the Internet, anywhere in the world.
AdvantagesApart from the obvious major advantage of uncomplicated global access via the Internet, there are a number of other advantages to this approach which effectively leverage the technology and ease of use of the web browser:
How it worksA traditional client-server application uses a two tier architecture. Application software, installed on each client machine, communicates directly with the database engine via a protocol such as SQL*NET. The client software has to be installed on each machine which uses the database and a different version of the client application has to be compiled for each platform.
By contrast, the web enabled architecture is multi-tiered : application software executes at an intermediate level on an application server. No application specific software is installed on the client machine, only a web browser such as Netscape or IE. The client communicates with the application server using standard Internet protocols. The application server is takes over the responsibility of communicating with the database. It transforms client requests into native database queries, and processes database output into an appropriate format for the client.
Fig. 2: Three tier Web architecture
The Musinfo system uses an Oracle database running on a DEC alpha UNIX server. A powerful server is necessary to handle the number of users accessing the database. The application server can be installed either on the same machine as the database or on a separate platform. Geneva's system is based on a twin processor pentium machine running NT. In order to enhance security, the Musinfo system places a firewall between the application server and the database server. The application server software is Oracle's Web Application Server. Most client machines within the museums are connected via a fibre optic FDDI network, however, external access is also possible.
Fig. 3: Musinfo network architecture
DrawbacksNaturally, there are some drawbacks to the Web enabled approach which have to be taken into account.
Tips and tricks
Handling state informationState information is meta level data about the user's session. Such information is implicit in a standard database connection. After successfully logging in to an application, for example, it can be assumed that the user is authorised to access the data : the result of successful log in is implicit as long as the connection is maintained. Typically, when the connection is interrupted, for whatever reason, this information is lost and the user must reenter a password to reestablish the connection. The same logic applies to information about the user's context - the fact that the user has succeeded in navigating through a series of menu options and is now on screen X3 of module B14 in data entry mode, for example.
Web connections are normally discontinuous , so all this implicit meta data about the session state needs to be made explicit in order to maintain continuity. Each transaction between client and server effectively has to reestablish the user's authorisation and context, preferably without the user being aware of the fact.
Maintaining and managing state information is perhaps the most conceptually complex obstacle to designing Web enabled applications, although it s not in fact very difficult to overcome. Two different approaches exist, each with specific advantages:
Currently the Musinfo system uses the server side approach, primarily to allow for some extremely low-end client stations which are still in use. However, client side state information is being adopted for future developments as the relative balance of power between client work stations and the central server improves.
If dealing with state information sounds like a terrible headache which you would rather avoid, it is worth noting one very agreeable side effect which can make all the effort seem worthwhile: the highly robust nature of Web application sessions. Interrupted telecom links can be reestablished, even via a different provider, without any loss of session continuity. In most cases, sessions can even be reestablished after a server crash. So long as the state information is preserved, the application really doesn't notice any difference.
Record lockingOne of the minor inconveniences of discontinuous web connections is that record locking becomes slightly more complex. Record locking is the mechanism which prevents two users from modifying the same record simultaneously. In a standard environnement, any outstanding record locks are dropped as soon as a connection is lost. This is difficult to arrange in a web environment since the server receives no signal from the client to indicate that he has, for example, forgotten to log out before turning off the computer. A time out period could be used to remove outstanding locks, but this would mean that records might remained locked for relatively long periods at a time, blocking other transactions unnecessarily.
To avoid these problems, Musinfo uses a simple form of 'optimistic' locking, which assumes that update conflicts are relatively rare. Records are time stamped at the moment they are created or modified. When they are retrieved, this timestamp is included along with the data. When a user attempts to update a record, the timestamps are compared to ensure that the record has not been modified in the meantime. If it has, the user receives an error message. This approach is easy to implement and ensures database integrity, but has the potential drawback that users may be unable to save a record they have been working on. This policy is acceptable in an environment where conflicts are rare; a more restrictive locking scheme would be necessary in if conflicts were frequent.
Improving performanceMost standard client server development tools allow for a degree of load balancing between the client and server in order to distribute processing and thereby reduce network traffic. Simple data validation and screen updates, for example, can be performed by the client, which provides rapid response to the user and avoids a round trip to the server.
Load balancing is more difficult in a web environment because HTML provides few possibilities for dynamic screen control. The easy way out is to make a call to the server every time the display needs to be modified, but this can be extremely inefficient and time consuming since data, presentation information and processing results are all constantly sloshing back and forth across the network; only the data need really be transferred.
Potential impactSo far we have considered only the technical aspects developing web enabled application. I would like to turn now to consider briefly the potential impact of this technology in the museum context.
ConclusionThe use of web for the gathering of information has yet to be exploited by the museum community. The technical difficulties involved in creating web enabled distributed applications do not present major obstacles. Given a little imagination and careful design it is already possible to build attractive, fully functional applications which take advantage of the technology and familiarity of the web browser - today's most universally available software.
The commercial and scientific impact of having software applications available over the Web could be enormous. Museums will have the option of sharing or outsourcing the burden of running an information system, and thanks to the global availability of the Internet, a far broader and more flexible range of experts and sources becomes available for the creation and maintenance of cultural heritage resources.
Finally, by sharing application software and a common database, institutions will in effect be contributing to an information repository which breaks out of the confines of individual institutions, disciplines and departments: a step towards the creation of a global resource for cultural heritage.