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This chapter is from the book

Shared Database


by Martin Fowler

An enterprise has multiple applications that are being built independently, with different languages and platforms. The enterprise needs information to be shared rapidly and consistently.

How can I integrate multiple applications so that they work together and can exchange information?

How can I integrate multiple applications so that they work together and can exchange information?

File Transfer enables applications to share data, but it can lack timeliness—yet timeliness of integration is often critical. If changes do not quickly work their way through a family of applications, you are likely to make mistakes due to the staleness of the data. For modern businesses, it is imperative that everyone have the latest data. This not only reduces errors, but also increases people's trust in the data itself.

Rapid updates also allow inconsistencies to be handled better. The more frequently you synchronize, the less likely you are to get inconsistencies and the less effort they are to deal with. But however rapid the changes, there are still going to be problems. If an address is updated inconsistently in rapid succession, how do you decide which one is the true address? You could take each piece of data and say that one application is the master source for that data, but then you'd have to remember which application is the master for which data.

File Transfer also may not enforce data format sufficiently. Many of the problems in integration come from incompatible ways of looking at the data. Often these represent subtle business issues that can have a huge effect. A geological database may define an oil well as a single drilled hole that may or may not produce oil. A production database may define a well as multiple holes covered by a single piece of equipment. These cases of semantic dissonance are much harder to deal with than inconsistent data formats. (For a much deeper discussion of these issues, it's really worth reading Data and Reality [Kent].) What is needed is a central, agreed-upon datastore that all of the applications share so each has access to any of the shared data whenever it needs it.

Integrate applications by having them store their data in a single Shared Database, and define the schema of the database to handle all the needs of the different applications.


Integrate applications by having them store their data in a single Shared Database, and define the schema of the database to handle all the needs of the different applications.


If a family of integrated applications all rely on the same database, then you can be pretty sure that they are always consistent all of the time. If you do get simultaneous updates to a single piece of data from different sources, then you have transaction management systems that handle that about as gracefully as it ever can be managed. Since the time between updates is so small, any errors are much easier to find and fix.

Shared Database is made much easier by the widespread use of SQL-based relational databases. Pretty much all application development platforms can work with SQL, often with quite sophisticated tools. So you don't have to worry about multiple file formats. Since any application pretty much has to use SQL anyway, this avoids adding yet another technology for everyone to master.

Since every application is using the same database, this forces out problems in semantic dissonance. Rather than leaving these problems to fester until they are difficult to solve with transforms, you are forced to confront them and deal with them before the software goes live and you collect large amounts of incompatible data.

One of the biggest difficulties with Shared Database is coming up with a suitable design for the shared database. Coming up with a unified schema that can meet the needs of multiple applications is a very difficult exercise, often resulting in a schema that application programmers find difficult to work with. And if the technical difficulties of designing a unified schema aren't enough, there are also severe political difficulties. If a critical application is likely to suffer delays in order to work with a unified schema, then often there is irresistible pressure to separate. Human conflicts between departments often exacerbate this problem.

Another, harder limit to Shared Database is external packages. Most packaged applications won't work with a schema other than their own. Even if there is some room for adaptation, it's likely to be much more limited than integrators would like. Adding to the problem, software vendors usually reserve the right to change the schema with every new release of the software.

This problem also extends to integration after development. Even if you can organize all your applications, you still have an integration problem should a merger of companies occur.

Multiple applications using a Shared Database to frequently read and modify the same data can turn the database into a performance bottleneck and can cause deadlocks as each application locks others out of the data. When applications are distributed across multiple locations, accessing a single, shared database across a wide-area network is typically too slow to be practical. Distributing the database as well allows each application to access the database via a local network connection, but confuses the issue of which computer the data should be stored on. A distributed database with locking conflicts can easily become a performance nightmare.

To integrate applications' functionality rather than their data, use Remote Procedure Invocation. To enable frequent exchanges of small amounts of data using a format per datatype rather than one universal schema, use Messaging.

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