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 development machine.
• The expanded application is rsync:ed from the development machine to the live environment.
• The app server in the live environment is restarted after the rsync. This step is not strictly needed, but I've found that restarting the application server on deployment avoids "java.lang.OutOfMemoryError: PermGen space" due to frequent class loading.

Good things about this approach:

• The rsync minimizes the amount of data sent from the development machine to the live environment. Uploading the entire .war file takes over ten minutes, whereas an rsync takes a couple of seconds.

Bad things about this approach:

• While the rsync is running the application context is restarted since the files are updated. Ideally the restart should happen after the rsync is complete, not when it is still running.
• The app server restart causes roughly two minutes of downtime.

I'd like to find a deployment process with the following properties:

• Minimal downtime during deployment process.
• Minimal time spent uploading the data.
• If the deployment process is app server specific, then the app server must be open-source.

Question:

• Given the stated requirements, what is the optimal deployment process?

Solution 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.

Solution 2

Update:

Since this answer was first written, a better way to deploy war files to tomcat with zero downtime has emerged. In recent versions of tomcat you can include version numbers in your war filenames. So for example, you can deploy the files ROOT##001.war and ROOT##002.war to the same context simultaneously. Everything after the ## is interpreted as a version number by tomcat and not part of the context path. Tomcat will keep all versions of your app running and serve new requests and sessions to the newest version that is fully up while gracefully completing old requests and sessions on the version they started with. Specifying version numbers can also be done via the tomcat manager and even the catalina ant tasks. More info here.

Original Answer:

Rsync tends to be ineffective on compressed files since it's delta-transfer algorithm looks for changes in files and a small change an uncompressed file, can drastically alter the resultant compressed version. For this reason, it might make good sense to rsync an uncompressed war file rather than a compressed version, if network bandwith proves to be a bottleneck.

What's wrong with using the Tomcat manager application to do your deployments? If you don't want to upload the entire war file directly to the Tomcat manager app from a remote location, you could rsync it (uncompressed for reasons mentioned above) to a placeholder location on the production box, repackage it to a war, and then hand it to the manager locally. There exists a nice ant task that ships with Tomcat allowing you to script deployments using the Tomcat manager app.

There is an additional flaw in your approach that you haven't mentioned: While your application is partially deployed (during an rsync operation), your application could be in an inconsistent state where changed interfaces may be out of sync, new/updated dependencies may be unavailable, etc. Also, depending on how long your rsync job takes, your application may actually restart multiple times. Are you aware that you can and should turn off the listening-for-changed-files-and-restarting behavior in Tomcat? It is actually not recommended for production systems. You can always do a manual or ant scripted restart of your application using the Tomcat manager app.

Your application will be unavailable to users during a restart, of course. But if you're so concerned about availability, you surely have redundant web servers behind a load balancer. When deploying an updated war file, you could temporarily have the load balancer send all requests to other web servers until the deployment is over. Rinse and repeat for your other web servers.

Solution 3

In any environment where downtime is a consideration, you are surely running some sort of cluster of servers to increase reliability via redundancy. I'd take a host out of the cluster, update it, and then throw it back into the cluster. If you have an update that cannot run in a mixed environment (incompatible schema change required on the db, for example), you are going to have to take the whole site down, at least for a moment. The trick is to bring up replacement processes before dropping the originals.

Using tomcat as an example - you can use CATALINA_BASE to define a directory where all of tomcat's working directories will be found, separate from the executable code. Every time I deploy software, I deploy to a new base directory so that I can have new code resident on disk next to old code. I can then start up another instance of tomcat which points to the new base directory, get everything started up and running, then swap the old process (port number) with the new one in the load balancer.

If I am concerned about preserving session data across the switch, I can set up my system such that every host has a partner to which it replicates session data. I can drop one of those hosts, update it, bring it back up so that it picks the session data back up, and then switch the two hosts. If I've got multiple pairs in the cluster, I can drop half of all pairs, then do a mass switch, or I can do them a pair at a time, depending upon the requirements of the release, requirements of the enterprise, etc. Personally, however, I prefer to just allow end-users to suffer the very occasional loss of an active session rather than deal with trying to upgrade with sessions intact.

It's all a tradeoff between IT infrastructure, release process complexity, and developer effort. If your cluster is big enough and your desire strong enough, it is easy enough to design a system that can be swapped out with no downtime at all for most updates. Large schema changes often force actual downtime, since updated software usually cannot accommodate the old schema, and you probably cannot get away with copying the data to a new db instance, doing the schema update, and then switching the servers to the new db, since you will have missed any data written to the old after the new db was cloned from it. Of course, if you have resources, you can task developers with modifying the new app to use new table names for all tables that are updated, and you can put triggers in place on the live db which will correctly update the new tables with data as it is written to the old tables by the prior version (or maybe use views to emulate one schema from the other). Bring up your new app servers and swap them into the cluster. There are a ton of games you can play in order to minimize downtime if you have the development resources to build them.

Perhaps the most useful mechanism for reducing downtime during software upgrades is to make sure that your app can function in a read-only mode. That will deliver some necessary functionality to your users but leave you with the ability to make system-wide changes that require database modifications and such. Place your app into read-only mode, then clone the data, update schema, bring up new app servers against new db, then switch the load balancer to use the new app servers. Your only downtime is the time required to switch into read-only mode and the time required to modify the config of your load balancer (most of which can handle it without any downtime whatsoever).

Solution 4

My advice is to use rsync with exploded versions but deploy a war file.

1. Create temporary folder in the live environment where you'll have exploded version of webapp.
2. Rsync exploded versions.
3. After successfull rsync create a war file in temporary folder in the live environment machine.
4. Replace old war in the server deploy directory with new one from temporary folder.

Replacing old war with new one is recommended in JBoss container (which is based on Tomcat) beacause it'a atomic and fast operation and it's sure that when deployer will start entire application will be in deployed state.

Solution 5

Can't you make a local copy of the current web application on the web server, rsync to that directory and then perhaps even using symbolic links, in one "go", point Tomcat to a new deployment without much downtime?

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