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

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).