Just a Theory

By David E. Wheeler

Data Deployment with Sqitch

I’ve been thinking about data migrations. I love how well Sqitch works for schema changes, but so far have avoided data changes. Some data ought to be managed by the deployment process, rather than by end-user applications. Lists of countries, for example. Yet none of our Sqitch-managed databases include INSERTs, UPDATEs, or DELETEs in deploy scripts. Why not? Two reasons:

  1. These are mainly Postgres ports of existing Oracle databases. As such, I’ve written independent migration scripts that use oracle_fdw to copy data from Oracle. It made no sense to commit hard-coded changes to the deploy script, even for static data, as it was all to be copied from the old production Oracle databases — often months after I wrote the migrations.

  2. These projects include extensive pgTAP unit tests that expect to run many times against an empty database with no side effects. Having different data in testing than in production increases the likelihood of unforeseen behavioral variations. Better to expect no data in tests, freeing them to focus on units of behavior without regard to preexisting data.

Now that we have multiple Sqitch-deployed databases in production, the time has come to address these issues.

Deploy Hooks for External Sources

I propose to resolve the one-time migration requirement with deploy hooks. The idea is similar to Git hooks: Before or after any sqitch deploy, one or more hook scripts can run. The impetus was to ensure some higher level of consistency after every deploy. For example, a post-deploy hook might grant privileges on all tables in a database. Another might run VACCUM; ANALZYE;.

But we could also use it for one-time data migrations. An option to deploy will disable them, which would be useful for development and test databases. Once the migration has been run in production, we just delete the migration hook scripts from the project. Sqitch won’t record hook executions, so adding or removing them will be no problem.

I like this approach, as Sqitch will automatically run migration scripts, but hooks will not change the interface of Sqitch itself. And it’s more generally useful. Hell, I might want deploy hook script that sends an email notification announcing a deployment (though that might require adding support for non-SQL scripts). There are all kinds of things for which hooks will prove useful.

Changes for Static Data Maintenance

For data that must be tied to the deployment process, there are two complications that have prevented me from simply managing them in normal Sqitch changes:

  1. There might be side-effects to the deployment. For example, a foreign key constraint to the users table, to identify the users who added rows to the database. But in a new database, perhaps there are no users — and Sqitch doesn’t create them, the app does. Chicken, meet egg.

  2. The initial data set might derived from some external source, such as another database. Consequently, none of that data was defined in Sqitch deploy scripts. This situation complicates future updates of the data. We can add data via Sqitch in the future, but then we don’t have the canonical list of all rows that should exist in all such databases.

However, I can think of no alternative that does not over-complicate Sqitch itself. I considered adding another change-related script type, named “update”, to complement the existing deploy, verify, and revert scripts. But oftentimes a data change would not be tied to a schema change, so the corresponding deploy script would be a no-op. Seems silly.

I also considered adding a completely separate command specifically for deploying data changes. Yet these data migrations are exactly like schema changes: Sqitch must execute them in the proper order relative to other changes, record successful or failed deployment, and be able to revert them when required. The only difference is what’s defined in them: data modification rather than definition.

Through several drafts of this post, I have come around to the idea that I should change nothing in Sqitch with regard to data deployments. A better solution than the above, I believe, is organizational.

Data Deploy Best Practice

Let the best practice for data deploys be this: they should be contained in Sqitch changes, but such changes should contain only data modifications. No change script should both define a table and insert its initial rows. Keep the table and its data in separate changes — keep DML separate from DDL.

For our list of countries, we might have a change named “countries”, which creates the countries table, and another, named “country_data”, which inserts the appropriate data into that table. Where necessary and appropriate, these changes may use conditional paths to bring the data up-to-date and in sync across deployments.

Conditions must deal with side-effects, such as foreign key constraints. Where possible, such side effects ought be removed from deployment-managed data. For tracking the user or users who added data to a database, for example, one can use the tools of the source code repository (git log, git blame) to assign blame. Other side-effects may be more necessary, but to the extent possible, deployed data should be independent.

Naturally, unit tests must expect static data to be present, and be updated when that data changes. We are, after all, talking about infrequently-updated data. Frequently-updated data should have separate interfaces provided by applications to change the data. Otherwise, how static is it, really?

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