Liquibase in a load balanced world.

General Discussion
1

I am working on an application that will be deployed to a cloud architecture like AWS and this system will have multiple application servers deployed behind a load balancer. All of the app servers connect to the same replicated database on the back-end. I was wondering what the best practice is for running Liquibase in an environment such as this? I want to minimize downtime when I make updates to the application (or eliminate downtime if possible) but I can’t seem to wrap my head around how database updates would work. If I update the database, the running app servers will error and stop working since the model has changed. Which Liquibase method would I use (ant/maven, Spring, etc)? Does anyone have any tips or advice for this kind of situation. Any suggestions would be greatly appreciated. Thanks.

2

From a liquibase standpoint, there isn’t anything you need to do because liqubase uses a databasechangeloglock table for exactly this situation so the database is only updated by one instance at a time.


Having your application work with a database schema that may not match what it was coded against is a different issue that really falls outside the scope of what liquibase tries to manage.


Depending on your technologies (hibernate, sql, etc) used, using a policy of “only backwards-compatible changes allowed” may be a good start. So, for example, you can’t do a normal column rename because it will break code using the old name. Instead, copy the data over to a new column and in a future release (when all servers are running against the new column) you can drop the old. Using views may help with some of the conversion as well.


Nathan

3

Thanks for the reply. I will definitely look into backward-compatible only changes for my schema. I appreciate the feedback.