This post explains the inherent problem of running online schema changes in MySQL, on tables participating in a foreign key relationship. We’ll lay some ground rules and facts, sketch a simplified schema, and dive into an online schema change operation. Our discussion applies to gh-ost, pt-online-schema-change, and VReplication based migrations, or any other online schema change tool that works with a shadow/ghost table like the Facebook tools. Why Online DDL? # Online schema change tools come as workarounds to an old problem: schema migrations in MySQL were blocking, uninterruptible, aggressive in resources, replication unfriendly.

Vitess introduces a new way to run schema migrations: non-blocking, asynchronous, scheduled online DDL. With online DDL Vitess simplifies the schema migration process by taking ownership of the operational overhead, and providing the user a simple, familiar interface: the standard ALTER TABLE statement. Let’s first give some background and explain why schema migrations are such an issue in the databases world, and then dive into implementation details The relational model and the operational overhead # The relational model is one of the longest surviving models in the software world, introduced decades ago and widely used until today.