1.1. The HTTP transaction model

The HTTP protocol is transaction-driven. This means that each request will lead to one and only one response. Traditionally, a TCP connection is established from the client to the server, a request is sent by the client on the connection, the server responds and the connection is closed. A new request will involve a new connection :

[CON1] [REQ1] … [RESP1] [CLO1] [CON2] [REQ2] … [RESP2] [CLO2] …

In this mode, called the “HTTP close” mode, there are as many connection establishments as there are HTTP transactions. Since the connection is closed by the server after the response, the client does not need to know the content length.

Due to the transactional nature of the protocol, it was possible to improve it to avoid closing a connection between two subsequent transactions. In this mode however, it is mandatory that the server indicates the content length for each response so that the client does not wait indefinitely. For this, a special header is used: “Content-length”. This mode is called the “keep-alive” mode :

[CON] [REQ1] … [RESP1] [REQ2] … [RESP2] [CLO] …

Its advantages are a reduced latency between transactions, and less processing power required on the server side. It is generally better than the close mode, but not always because the clients often limit their concurrent connections to a smaller value.

A last improvement in the communications is the pipelining mode. It still uses keep-alive, but the client does not wait for the first response to send the second request. This is useful for fetching large number of images composing a page :

[CON] [REQ1] [REQ2] … [RESP1] [RESP2] [CLO] …

This can obviously have a tremendous benefit on performance because the network latency is eliminated between subsequent requests. Many HTTP agents do not correctly support pipelining since there is no way to associate a response with the corresponding request in HTTP. For this reason, it is mandatory for the server to reply in the exact same order as the requests were received.

By default HAProxy operates in keep-alive mode with regards to persistent connections: for each connection it processes each request and response, and leaves the connection idle on both sides between the end of a response and the start of a new request.

HAProxy supports 5 connection modes :

  • keep alive : all requests and responses are processed (default)
  • tunnel : only the first request and response are processed, everything else is forwarded with no analysis.
  • passive close : tunnel with “Connection: close” added in both directions.
  • server close : the server-facing connection is closed after the response.
  • forced close : the connection is actively closed after end of response.
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