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Episode 3 - Decoupling Database Migrations at Application Startup

13 mins
  • Data models change and evolve with your application.
  • There’s plenty of tools that keep track of database schemas and automatically generate scripts to upgrade or downgrade them.
  • It’s common for developers to run a migration at the start of their app before running app code.
  • Our author explains two common problems with this approach.
    1. Modern day production deployments and horizontal scaling can get you into a race condition.
    2. You start assuming that new code will only ever run with the new schema.
  • You can decouple migrations from code changes by disabling parallelism during this time.
  • Make it a separate command or lock the database during the upgrade.
  • We can easily implement locking ourselves in any language.
    • Use Redis locks if you’re ok with something external to the DB.
    • Use the DB itself by writing to an extra table to say that you’re upgrading it.
  • Plan your deployment appropriately so you can run old code with new by making migrations additive in the short term.
  • Using a script at startup that optionally performs the migration based on an environment variable integrates wel with Docker and cloud services.
  • Upgrades of both code and data should be part of your testing BEFORE releasing to production.

References

Background music by Vendredi from Audio Library+
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