13.11 Write-Through Mandatory

All methods that might be expected to cause modifications to the origin server's resources MUST be written through to the origin server. This currently includes all methods except for GET and HEAD. A cache MUST NOT reply to such a request from a client before having transmitted the request to the inbound server, and having received a corresponding response from the inbound server. This does not prevent a proxy cache from sending a 100 (Continue) response before the inbound server has sent its final reply.

The alternative (known as "write-back" or "copy-back" caching) is not allowed in HTTP/1.1, due to the difficulty of providing consistent updates and the problems arising from server, cache, or network failure prior to write-back.