97 lines
3.7 KiB
Plaintext
97 lines
3.7 KiB
Plaintext
2011/02/25 - Description of the different entities in haproxy - w@1wt.eu
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1) Definitions
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--------------
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Listener
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--------
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A listener is the entity which is part of a frontend and which accepts
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connections. There are as many listeners as there are ip:port couples.
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There is at least one listener instanciated for each "bind" entry, and
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port ranges will lead to as many listeners as there are ports in the
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range. A listener just has a listening file descriptor ready to accept
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incoming connections and to dispatch them to upper layers.
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Initiator
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---------
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An initiator is instanciated for each incoming connection on a listener. It may
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also be instanciated by a task pretending to be a client. An initiator calls
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the next stage's accept() callback to present it with the parameters of the
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incoming connection.
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Session
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-------
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A session is the only entity located between an initiator and a connector.
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This is the last stage which offers an accept() callback, and all of its
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processing will continue with the next stage's connect() callback. It holds
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the buffers needed to forward the protocol data between each side. This entity
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sees the native protocol, and is able to call analysers on these buffers. As it
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is used in both directions, it always has two buffers.
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When transformations are required, some of them may be done on the initiator
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side and other ones on the connector side. If additional buffers are needed for
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such transforms, those buffers cannot replace the session's buffers, but they
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may complete them.
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A session only needs to be instanciated when forwarding of data is required
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between two sides. Accepting and filtering on layer 4 information only does not
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require a session.
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For instance, let's consider the case of a proxy which receives and decodes
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HTTPS traffic, processes it as HTTP and recodes it as HTTPS before forwarding
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it. We'd have 3 layers of buffers, where the middle ones are used for
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forwarding of the protocol data (HTTP here) :
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<-- ssl dec --> <-forwarding-> <-- ssl enc -->
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,->[||||]--. ,->[||||]--. ,->[||||]--.
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client (|) (|) (|) (|) server
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^--[||||]<-' ^--[||||]<-' ^--[||||]<-'
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HTTPS HTTP HTTPS
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The session handling code is only responsible for monitoring the forwarding
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buffers here. It may declare the end of the session once those buffers are
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closed and no analyser wants to re-open them. The session is also the entity
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which applies the load balancing algorithm and decides the server to use.
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The other sides are responsible for propagating the state up to the session
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which takes decisions.
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Connector
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---------
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A connector is the entity which permits to instantiate a connection to a known
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destination. It presents a connect() callback, and as such appears on the right
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side of diagrams.
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Connection
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----------
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A connection is the entity instanciated by a connector. It may be composed of
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multiple stages linked together. Generally it is the part of the stream
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interface holding a file descriptor, but it can also be a processing block or a
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transformation block terminated by a connection. A connection presents a
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server-side interface.
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2) Sequencing
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-------------
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Upon startup, listeners are instanciated by the configuration. When an incoming
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connection reaches a listening file descriptor, its read() callback calls the
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corresponding listener's accept() function which instanciates an initiator and
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in turn recursively calls upper layers' accept() callbacks until
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accept_session() is called. accept_session() instanciates a new session which
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starts protocol analysis via process_session(). When all protocol analysis is
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done, process_session() calls the connect() callback of the connector in order
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to get a connection.
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