Syncing applications with Argo CD: CAPA practice questions

A sync applies the target state from Git to the cluster. It can be started by hand from the UI or with argocd app sync, or automatically by a sync policy. A manual sync can be narrowed: --resource limits it to chosen resources, which is called a selective sync, --revision syncs a specific commit or tag, --prune allows the deletion of resources that are no longer in Git, and --dry-run previews the apply without changing the cluster. Hooks do not run during a selective sync.

Automated sync is turned on under spec.syncPolicy.automated. It only acts when the application is OutOfSync and it is cautious by default. prune is off, so a resource deleted from Git stays in the cluster until pruning is allowed. selfHeal is off, so a change made directly in the cluster does not trigger a new sync. allowEmpty must be set before automated pruning may remove every resource of an application. A retry block adds a limit and an exponential backoff for failed sync attempts. An application that syncs automatically cannot be rolled back with argocd app rollback; automated sync has to be disabled first.

Sync options change how resources are applied, either for the whole application under syncPolicy.syncOptions or for one resource with the argocd.argoproj.io/sync-options annotation. CreateNamespace=true creates the destination namespace, PruneLast=true prunes at the end of the sync, ApplyOutOfSyncOnly=true skips resources that already match, ServerSideApply=true uses server-side apply, Replace=true uses replace instead of apply and can be destructive, and Prune=false protects one resource from pruning.

Ordering is controlled by phases and waves. The argocd.argoproj.io/hook annotation places a resource in the PreSync, Sync, PostSync, SyncFail or PostDelete phase, and a PreSync hook that fails stops the whole sync. Within a phase, the argocd.argoproj.io/sync-wave annotation orders resources from the lowest number up; the default wave is 0, so a negative wave runs earlier. Pruning runs in reverse wave order. A hook-delete-policy of HookSucceeded, HookFailed or BeforeHookCreation, the assumed default, decides when hook resources are removed. Sync windows on an AppProject can allow or deny syncs on a cron schedule, and a deny window wins over an allow window.