The service is currently unavailable. This is most likely a transient condition, which can be corrected by retrying with a backoff. Note that it is not always safe to retry non-idempotent operations. See the guidelines above for deciding between `FAILED_PRECONDITION`, `ABORTED`, and `UNAVAILABLE`. HTTP Mapping: 503 Service Unavailable
변경 내역: Extracted enum names, numeric aliases, and official source comments; comment syntax removed; original Korean contexts added. Successful OK is labeled status, not error.