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30 lines
1.3 KiB
Markdown
30 lines
1.3 KiB
Markdown
---
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title: "ADR: Breaking DB changes"
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---
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## Background
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During the evolution of a feature different clients may use different version of code e.g. behind a feature flag.
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If the code relies on breaking DB changes (column delete, table rename, deleting DB entries etc.) it may lead to errors.
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The very same problem occurs when you apply a breaking migration just before the new version of the application starts e.g. during a zero-downtime deployment (whatever strategy you use).
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The code is still running against the old schema as the migration takes a few seconds to apply.
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## Decision
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First please make sure to avoid breaking DB changes in the first place if possible.
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If breaking change is inevitable please use the "expand/contract" pattern.
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In the "expand phase":
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* maintain old and new DB schema in parallel
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* maintain code that works with old and new DB schema
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* keep it for 2 minor releases to give all clients a chance to upgrade the code
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* with a fallback of 2 version we can also downgrade in this range without running down migrations
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In the "contract phase":
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* remove the old schema when you know that no client is using the old version
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Action for a code reviewer:
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* when you spot a migration with `ALTER table DROP COLUMN` or `ALTER table RENAME TO` please raise a flag if the "expand phase" was missed
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