HTTP: The Definitive Guide

Pipelined Connections

HTTP/1.1 permits optional request pipelining over persistent connections. This is a further performance optimization over keep-alive connections. Multiple requests can be enqueued before the responses arrive. While the first request is streaming across the network to a server on the other side of the globe, the second and third requests can get underway. This can improve performance in high-latency network conditions, by reducing network round trips.

Figure 4-18a-c shows how persistent connections can eliminate TCP connection delays and how pipelined requests (Figure 4-18c) can eliminate transfer latencies.

Figure 4-18. Four transactions (pipelined connections)

There are several restrictions for pipelining:

• HTTP clients should not pipeline until they are sure the connection is persistent.

• HTTP responses must be returned in the same order as the requests. HTTP messages are not tagged with sequence numbers, so there is no way to match responses with requests if the responses are received out of order.

• HTTP clients must be prepared for the connection to close at any time and be prepared to redo any pipelined requests that did not finish. If the client opens a persistent connection and immediately issues 10 requests, the server is free to close the connection after processing only, say, 5 requests. The remaining 5 requests will fail, and the client must be willing ...