Executable tutorial submission: Safe database migrations in CD (tmldjb-cesarah)

[tomasmbrito]

Assignment Proposal

Title

Safe Database Migrations in Continuous Delivery: Migration Linting and the Expand/Contract Pattern

Names and KTH ID

• Tomás Brito (tmldjb@kth.se)
• Cesar Aceves Hernández (cesarah@kth.se)

Deadline

• Task 3

Category

• Executable tutorial

Description

We will create an executable tutorial on Killercoda about changing a database schema without breaking a running application. Code changes are easy to roll back, schema changes are not, and during a rolling deploy the old and the new version of an app run against the same database at the same time.

The tutorial starts from a small web service backed by PostgreSQL, with traffic running against it. First, the learner applies a naive migration (renaming a column) with dbmate and watches the old version of the service start failing. Then they add squawk, a linter for PostgreSQL migrations, as a check in a small CI script, and see it block the same migration before it reaches the database. Finally, they redo the change with the expand/contract pattern: add the new column with a trigger that keeps both columns in sync, backfill it, roll out the new version, and only drop the old one once nothing reads it anymore. A last step shows the difference between a normal index build, which blocks writes, and CREATE INDEX CONCURRENTLY.

Everything runs in the browser with pinned versions and no accounts. Each step has a check script, and the tutorial includes an architecture diagram and a reflection on when this approach is worth the extra steps and when it is not.

Relevance

Continuous delivery assumes that any commit can be deployed safely, but database migrations are often the part that still needs a maintenance window. Zero-downtime deploys require the schema to work with two versions of the application at once, which is the same problem canary and blue-green releases deal with. Checking migrations automatically in CI is a shift-left practice that turns knowledge about locks and breaking changes into a repeatable gate.

Link to tutorial

• Tutorial on Killercoda: https://killercoda.com/tomasmbrito/scenario/safe-db-migrations
• Source code: https://github.com/tomasmbrito/safe-db-migrations-tutorial

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[frankreyesgarcia]

Good, make sure the small CI script is good for the tutorial.