An additional problem, beyond that of impedance mismatch, can occur because the cache used by an application may not be tightly integrated with an RDBMS. This problem is referred to as cache synchronization. It occurs after the relational data has been transformed into an object format placed in an application cache.
When only one application is using the data, cache synchronization is not a problem. As soon as a second application accesses the database, however, the problem can occur.
An example of the impact of a second application is shown the figure below. In the example, a second application is accessing the same DBMS server that is being used by an object application that uses a cache with object-relational mapping. Application B can change the data being used by Application A. Cache A would need to be synchronized in some manner to obtain the changed data. How the cache is implemented and what types of locks are used will have a big impact on the problem. Also see concurrency control and locking.
The data in the cache must be synchronized with the data in the database managed by the DBMS server when a cache is used by an application that is separate from the underlying DBMS server.
Also see caching for object database management systems.
More on the general topic: Mapping Layer
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The Savvy Manager's Guide
Douglas K Barry is also the author of a book that explains Web Services, service-oriented architecture, and Cloud Computing in an easy-to-understand, non-technical manner.
Web Services, Service-Oriented Architectures, and Cloud Computing: The Savvy Manager's Guide (Second Edition)
by Douglas K Barry with David Dick
This is a guide for the savvy manager who wants to capitalize on the wave of change that is occurring with Web Services, service-oriented architecture, and—more recently—Cloud Computing. The changes wrought by these technologies will require both a basic grasp of the technologies and an effective way to deal with how these changes will affect the people who build and use the systems in our organizations. This book covers both issues. Managers at all levels of all organizations must be aware of both the changes that we are now seeing and ways to deal with issues created by those changes.