Coordinating database migrations

By default at startup PuppetDB will attempt to perform any database updates that might be needed. If there is a possibiity that multiple PuppetDB instances could run at the same time, it's important to coordinate the update process so that only one server attempts to perform updates. The configuration documentation explains how to arrange that, and the broader context, but not the detailed process, which is described here.

Assuming all of the PuppetDB instances have been configured appropriately, then only one will have [database] migrate set to true, so that it will be the only one that should attempt a migration. That will guard against concurrent migration attempts, but doesn't help if the servers are misconfigured, and it doesn't do anything to protect against another concern, the possibility that PuppetDB instances might try to operate on a database that's at the incorrect migration level during normal operations.

This might happen because, for example, a newer version of PuppetDB has migrated the database to a version they don't understand, or because they're not configured to perform a migration and the database is too old. And note that these concerns apply all of the time, not just at startup, since the database might accidentally be migrated while older PuppetDB instances are still running.

In an attempt to guard against all of these possibilities, PuppetDB does the following:

When acting as a migrator (migrate = true)

  • Connects to the database as the migrator-username using a completely independent connection pool.

  • Begins a transaction.

  • Grabs an exclusive migration lock.

  • Checks to see if any migrations are required, and if not, ends the transaction, releasing the lock, and then proceeds with normal operations.

  • If migrations are required, proceeds as follows.

  • Revokes connection privileges from the normal username. This prevents any new connections from being established.

  • Terminates any existing database connections from the normal username. The revocation above may not affect them.

  • Changes its role to the normal username so that all new database objects will be owned by the normal user. If nothing else, this ensures the normal user will be able to drop those partitions during routine garbage collection (e.g. for reports).

  • Performs all the required migrations.

  • Commits the transaction, releasing the lock.

  • Restores the normal user's connection privileges.

  • Resumes normal operations.

All PuppetDB instances, including a migrator after migration

  • Always connects to the database as the normal username using the routine read and write pools.

  • Sets a HikariCP connectionInitSql guard that prevents any new connections to a database that isn't at the correct migration level by examining the levels in the schema_migrations table.

  • Establishes a periodic check that will shut PuppetDB down if it detects a database that's either newer or older than it's prepared to handle. This also makes sure that PuppetDB won't continue using any existing (pool) connections to the database (since the connectionInitSql check above only prevents new connections).

    When a version mismatch is detected, exits with a with a status of 77 (ASCII M) if the migration level is too new, and 109 (ASCII m) if it's too old. (Note: in versions before 6.10 the exit statuses are only required to be non-zero, not necessarily m or M).