An Argo CD application spec provides several different ways of track kubernetes resource manifests in git. This document describes the different techniques and the means of deploying those manifests to the target environment.
If a branch name, or a symbolic reference (like HEAD) is specified, Argo CD will continually compare live state against the resource manifests defined at the tip of the specified branch or the dereferenced commit of the symbolic reference.
To redeploy an application, a user makes changes to the manifests, and commit/pushes those the changes to the tracked branch/symbolic reference, which will then be detected by Argo CD controller.
If a tag is specified, the manifests at the specified git tag will be used to perform the sync comparison. This provides some advantages over branch tracking in that a tag is generally considered more stable, and less frequently updated, with some manual judgement of what constitutes a tag.
To redeploy an application, the user uses git to change the meaning of a tag by retagging it to a different commit SHA. Argo CD will detect the new meaning of the tag when performing the comparison/sync.
If a git commit SHA is specified, the application is effectively pinned to the manifests defined at the specified commit. This is the most restrictive of the techniques and is typically used to control production environments.
Since commit SHAs cannot change meaning, the only way to change the live state of an application which is pinned to a commit, is by updating the tracking revision in the application to a different commit containing the new manifests. Note that parameter overrides can still be set on an application which is pinned to a revision.
In all tracking strategies, the application has the option to sync automatically. If auto-sync is configured, the new resources manifests will be applied automatically -- as soon as a difference is detected between the target state (git) and live state. If auto-sync is disabled, a manual sync will be needed using the Argo UI, CLI, or API.
Note that in all tracking strategies, any parameter overrides set in the application instance take precedence over the git state.