This document outlines the issues involved in dealing with concurrent database updates in our web apps - the situation where more than one user tries to update the same piece of data at the same time. See "Chosen Solution" below for... our chosen solution.
Example Situation
To set the scene, here's a typical situation in our web applications:
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We'll take these in reverse order:
Handling a Concurrent Update Situation
There are three approaches to handling a concurrent update situation:
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Unless there's a clear business requirement for the merge option, I think we need to take the second approach, "first commit wins". In order to pursue this approach, we do need to have a mechanism for detecting concurrent updates. Which leads to...
Detection
Hibernate does offer a facility for detecting concurrent updates by providing a version attribute. A database column (usually integer or timestamp) can be designated as a version field - for new tables we typically use the integer type.
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3. At the end of step 1, the Hibernate Session is stored in the user's HttpSession. The Hibernate Session remains active for the entire "conversation" - this technique is described as "session-per-conversation". Step 2 would retrieve the Hibernate Session from the user's HttpSession and all domain objects associated with the Hibernate Session in step 1 would be available to step 2. In this approach, it would be essential for the end of step 1 to disconnect the JDBC connection; and for the beginning of step 2 to establish a new JDBC connection.
Chosen Solution
For Education Systems web apps we have decided to use detection option 1 - passing the version number to the JSP in the GET request, and moving the version number into the retrieved Hibernate entity during the POST request. Hibernate itself will then take care of checking version number on update. If a concurrency problem is encountered, Spring should throw a org.springframework.orm.hibernate3.HibernateOptimisticLockingFailureException. This exception should be intercepted by the web app and an appropriate UI action should take place (for example, forwarding to an error page, or displaying a message on the current page).
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NOTE: if the web page displays multiple objects that can be updated, the version numbers of all the objects must be included in the JSP, and must all be applied to the relevant entity objects when the form data is posted.
Further Reading
Some useful information is available here:
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