Best practices for deploying Java webapps with minimal downtime?

Question

When deploying a large Java webapp (>100 MB .war) I\'m currently use the following deployment process:

• The application .war file is expanded locally on the develop

Answer 1

It has been noted that rsync does not work well when pushing changes to a WAR file. The reason for this is that WAR files are essentially ZIP files, and by default are created with compressed member files. Small changes to the member files (before compression) result in large scale differences in the ZIP file, rendering rsync's delta-transfer algorithm ineffective.

One possible solution is to use jar -0 ... to create the original WAR file. The -0 option tells the jar command to not compress the member files when creating the WAR file. Then, when rsync compares the old and new versions of the WAR file, the delta-transfer algorithm should be able to create small diffs. Then arrange that rsync sends the diffs (or original files) in compressed form; e.g. use rsync -z ... or a compressed data stream / transport underneath.

EDIT: Depending on how the WAR file is structured, it may also be necessary to use jar -0 ... to create component JAR files. This would apply to JAR files that are frequently subject to change (or that are simply rebuilt), rather than to stable 3rd party JAR files.

In theory, this procedure should give a significant improvement over sending regular WAR files. In practice I have not tried this, so I cannot promise that it will work.

The downside is that the deployed WAR file will be significantly bigger. This may result in longer webapp startup times, though I suspect that the effect would be marginal.

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A different approach entirely would be to look at your WAR file to see if you can identify library JARs that are likely to (almost) never change. Take these JARs out of the WAR file, and deploy them separately into the Tomcat server's common/lib directory; e.g. using rsync.