Database Versioning - How does branch switching work?

Question

This is a question for those of you developing on a team of devs where all of you have separate databases. You're versioning your database using source control and other tools which will automatically bring dev databases up to date to the latest version of the database (schema, data, SP's, functions, etc.).

OK Great! But wait! What if you are developing on version 4.0 of your software, but now you need to switch branches to the 3.2 branch to fix a bug? The schema could be (almost assuredly is) very different by now...

I suppose if you went through the extra effort to write rollback scripts along with your change scripts, this could work. But that seems like a lot of work - is it really worth it?

Answer 1

Much easier would be to create a new 3.2-branch database and work with that while working on the 3.2-branch code. It doesn't seem reasonable to me to require that each developer has exactly one database to work with.

Answer 2

I'm going on a limb and assume that you are versioning the database as a binary? If all your database assets were in the form of constructive code (eg SQL scripts and/or text data dumps), the solution would be simple, as suggested by Mark: store these assets as part of the development branch. To work on version 3.2, switch the branch, re-run the create scripts and presto, 3.2 database. Merging would be just as easy as with regular code (or just as painful, depending on your version control system of choice).

Here are some suggestions to work in this mode:

• If creating the database instances from text is too slow, make a cache on a shared disk volume, keyed by the contents of all the schema / data files (or the MD5 sum thereof).
• Write a pre-commit hook to ensure that the schema and data dumps in the developer's instance are the same as the ones under version control. This prevents people from making changes to their dev database with an interactive tool, and then forgetting to commit them.
• You mention change scripts; treat them as a liability. While they may be required by your deployment scenario (eg for customers who want to upgrade in-place), they duplicate information from the version history of the database, and per Murphy's law duplication means desynchronization sooner or later. Try to auto-generate the change scripts from the versioned database assets using "diff"; or if this cannot be achieved, dedicate some serious unit tests to database upgrades.