What are the downsides of pipelining PUT and/or POST requests?

Question

I've heard PUT and POST requests should not be pipelined. Why ?

Answer 1

What this comes down to is Idempotence

Non-idempotent requests should not be pipelined, since the effects of N > 1 requests may produce a different result than a single request would do. This means the POST requests should not be pipelined, but any non-idempotent method (just about any request other than POST method) can safely be.

See:

• RFC2616 section 9.1.2
• Idempotence (Wikipedia)
• HTTP pipelining (Wikipedia)

Answer 2

I don't think that pipelining PUT requests poses much of an issue, but you should not pipeline POST requests. POST requests can alter the state of objects on the server. If a POST request is sent before the response to a previous POST request is received, the results may be indeterminate. This is especially true if the connection is terminated during the session.