Karl Matthias bio photo

Karl Matthias

DevOps, sysadmin, coder. Principal Systems Engineer at Nitro. Dublin, Ireland.




Discovering Pending Rails Migrations With Git

At MyDrive, Gavin and I came up with what I think is a pretty novel solution to discovering new Rails migrations at deployment time. We deploy often but not on every commit.  We usually know about new migrations from reading the commits—but no one is perfect and sometimes you miss a commit or three. The standard checks for pending migrations involve deploying the new code and verifying the schema version against the database before reloading the app. This works but it’s slow and you don’t find out about the migrations until the code has been pushed to at least one system. Automating this is still a good idea. But it’s often nice to find out about pending migrations right up front.  Sometimes you just don’t want to deploy right now if there are pending migrations.

But, do we need to talk to the DB to find this out? Doesn’t git tell us what has changed in our code? Why yes, it does.

We tag each deployed ref with a git tag representing the deployment environment (Capistrano stage) and the time and date. This is tells us what code was running in any environment at any time. We now also have a moving git tag that identifies the last deployment to any environment. After each deployment we move the tag to the current HEAD ref. Finding new migrations is now as simple as git diff on the db/migrate tree against the previous deployment tag for this environment. Capistrano does this as part of the deployment and lets us choose to abort if we don’t want to deploy with migrations. It’s not perfect: it will generate false positives on any change to any file in the migrations tree, but old migrations are rarely changed. Here’s what it looks like:

sleipnir:api karl$ cap production deploy
* 15:00:36 == Currently executing `production`
triggering start callbacks for `deploy`
* 15:00:36 == Currently executing `multistage:ensure`
* 15:00:36 == Currently executing `deploy`
triggering before callbacks for `deploy`
*** Deploying HEAD from branch 'master' to 'production'
*** Pending migrations!!!
create mode 100644 db/migrate/20120327113515_store_info_in_alerts.rb
create mode 100644 db/migrate/20120327190626_add_pending_data_table.rb
create mode 100644 db/migrate/20120329102140_populate_pending_data_table.rb
Do you want to continue deployment? (Y/N)
*** Aborting deployment!

Notably this needs to be done from a clean tree because you will be changing git tags on the local installation. So it should either be done from a deployment host or from a clean checkout of your code. It’s best to detect this in your deployment scripts as well so that you don’t get into trouble. If there is interest, we could probably release this as a Capistrano plugin. If you decide to implement it yourself, here are some notes on helpful git commands for doing so.

Update: I have released two gems to do this for you. The first is called capistrano-deploytags and handles all of the tagging for you at deployment time.  The second, capistrano-detect-migrations does what I described above.  It requires the functionality from capistrano-deploytags.

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