Sometimes when working with a team of people, or with multiple teams of people, it’s possible that more than one change to the database can be made at a time. Past solutions to this problem have been to centralize the management or responsibility for change to one person or team. But this creates bureaucracy and slows down the development process. It also hurts automation and continuous integration. Therefore we need a better approach.
MyBatis Migrations simply allows this situation to occur, but makes it very obvious that it has happened. Then the teams can review the situation, downgrade their schemas and re-run the migrations in order, and re-assess the situation. It allows teams to work autonomously, while encouraging communication, team work and good source control practices. When someone creates migration in another workspace before you, but commits to the source control system later than you, you’ll end up with an orphaned pending migration. They’re easy to spot with the status command:
/home/cbegin/testdb$ migrate status ID Applied At Description ================================================================== 20090802210445 2009-08-04 22:51:17 create changelog 20090804225207 2009-08-04 22:51:17 create author table 20090804225328 ...pending... create blog table 20090804225333 2009-08-04 22:51:17 create post table
MyBatis Migrations will not run this orphaned migration simply by running the up command. Instead, you’d have to downgrade to the point just before the orphaned migration, then run up to run all of the migrations in order. This is the safest and recommended approach.
However, if you and the other team review the change, and decide it’s completely isolated and not a conflicting change, then there is a way to run the pending migration(s) out of order. The pending command does just that. It runs all pending migrations regardless of their order or position in the status log. So if we were to run the pending command given the situation above, the results would be as we expect:
/home/cbegin/testdb$ migrate pending /home/cbegin/testdb$ migrate status ID Applied At Description ================================================================== 20090802210445 2009-08-04 22:51:17 create changelog 20090804225207 2009-08-04 22:51:17 create author table 20090804225328 2009-08-05 24:55:23 create blog table 20090804225333 2009-08-04 22:51:17 create post table
Even after the fact, you’ll be able to identify any migrations run in this way, as the applied date will give them away. An out-of-order applied date is clear indication that a migration was run out of order. No surprises!
Some commands like pending and down are highly unlikely to ever be needed in production. By the time you promote migrations to production, you’ve hopefully decided on your final schema and tested and approved the schema for release. While they won’t be used in production, they are highly valuable during the development process. Once you get used to the idea, you won’t be able to work without it again.