 HTTP persistent connection

An HTTP persistent connection carries more than one HTTP request/HTTP response exchange over the same connection. HTTP/1.1 uses persistence by default unless its connection rules indicate closure. Reusing the connection avoids establishing a separate connection for every exchange.^[ref]

For a sequential example, a client retrieves /color and reads a response with Content-Length: 4 followed by blue. It then requests /shape on the same connection and reads Content-Length: 6 followed by circle. These are two distinct requests and two distinct responses, each with its own control information and HTTP message framing. Neither resource name nor method is inherited merely because the connection is reused.

This example relies on being able to finish the first message without closing the connection. To retain persistence, the message lengths must be self-defined rather than delimited by closure. A server must read the complete request body or close the connection after responding; otherwise leftover body bytes could be mistaken for a new request. A client likewise must consume the complete response body before reusing the connection.^[ref]

Connection: close signals that the connection will close after the current response. Persistence is not a promise of an unlimited lifetime: a connection can also close unexpectedly. Whether an outstanding operation can be retried automatically depends on its semantics, particularly HTTP idempotence, rather than on the fact that the connection had previously been reusable.^[ref]

Persistence also does not require sending requests before earlier responses arrive. That additional option is HTTP pipelining; in HTTP/1.1, the corresponding responses must retain request order. The sequential example uses persistence without pipelining.^[ref]