Google Calendar API ETags support conditional modifications that help prevent overwriting another person’s newer edit. Use the version from the resource you actually read. A timestamp in your local database is not automatically equivalent to the server’s documented concurrency control.
Retain the retrieved version
When retrieving the intended resource, store its etag alongside the data used to prepare the change. Confirm the calendar and resource identifier. Avoid preparing a broad replacement from an old or partial local record when only a narrow field should change.
Send a conditional modification
Google’s versioning guide describes including an If-Match header with the previous ETag for an update or deletion. If the version is unchanged, the operation can succeed and return the new version. A changed resource produces 412 Precondition failed.
Do not remove the condition and retry blindly after that conflict. Retrieve the current resource, inspect the competing change, and reapply only the authorized intended difference where appropriate.
Verify conflict handling
Test a controlled concurrent-edit scenario and confirm that the stale write is rejected rather than silently replacing newer information. Review the merged result before another conditional attempt. Google’s guide says conditional modifications do not apply to insert operations; identifier-based creation rules are a separate mechanism. It also distinguishes If-None-Match conditional retrieval and 304 Not Modified from write-conflict handling. The completed design should preserve legitimate concurrent edits and clearly report unresolved conflicts, without claiming that any ETag automatically provides locking across every resource or that a transport-level retry is a safe resolution to a failed precondition.
References
Reviewed against the linked official product guidance on November 4, 2026. Features, limits, screen labels, and access can vary by account, platform, region, and organization policy.