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What is the Sync phase (as opposed to PreSync/PostSync)? What are the hook phases and when does each run? How do sync waves and hooks interact? What are hook deletion policies? What is a Skip hook and when is it used? How are hook resources named and prevented from clashing across syncs? How long does ArgoCD wait between sync waves? What is a PreSync hook typically used for? What is a PostSync hook typically used for? Can hooks use sync waves to order among themselves?
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ArgoCD › Sync Waves & Hooks

How do you run a database migration safely before deploying a new version?

Intermediate

Answer

Use a PreSync hook Job that runs the migration; ArgoCD waits for it to complete successfully before applying the new app version (in a later phase/wave). If the migration fails, the sync fails and the new version isn't rolled out, protecting against deploying code against an unmigrated schema.
kind: Job
metadata:
  annotations:
    argocd.argoproj.io/hook: PreSync
    argocd.argoproj.io/hook-delete-policy: HookSucceeded
Real-world example The schema migration runs as a PreSync Job; only after it succeeds does the new app image get deployed.

Common follow-ups: What happens to the deploy if the migration fails? | Why PreSync rather than PostSync?

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All 29 Sync Waves & Hooks questions