Expand/Contract: The Pattern Behind Every Safe Migration
During a deploy, old and new versions of your app run side by side for a while.
If a schema change only the new code understands lands too early or too late, one version breaks. The expand/contract pattern keeps every step compatible with both, so nothing breaks in the overlap.
Pro members see the rest of this lesson
- A 4-step state diagram: Expand → Dual-write → Backfill → Contract
- The full mechanism: what PostgreSQL does internally, in source terms
- 1 SQL query you can run, each labelled by how it was verified
- The closing insight, the mistake it prevents, and what it changes in your work
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Anchored to postgres/postgres on REL_17_STABLE and cross-checked against the manual for PostgreSQL 15–18. Every query here was run and its output captured on a throwaway PostgreSQL 17.10 lab; corrections are noted inline. §5.6 Modifying Tables in the official docs →
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