## Push Protocol (_PUSH_)

The ((_PUSH_ protocol))(((protocol, _PUSH_)))  is one half of a ((pipeline pattern)).
The other side is the xref:pull.adoc[_PULL_] protocol.

In the pipeline pattern, pushers distribute messages to pullers.
Each message sent by a pusher will be sent to one of its ready peer pullers, chosen in a round-robin fashion.
This property makes this pattern useful in ((load-balancing)) scenarios.

#### Socket Operations

The xref:nng_push_open.adoc[`nng_push0_open`] call creates a pusher socket.
This socket may be used to send messages, but is unable to receive them.
Attempts to receive messages will result in `NNG_ENOTSUP`.

Send operations will observe flow control (back-pressure), so that only peers capable of accepting a message will be considered.
If no peer is available to receive a message, then the send operation will wait until one is available, or the operation times out.

NOTE: Although the pipeline protocol honors flow control, and attempts to avoid dropping messages, no guarantee of delivery is made.
Furthermore, as there is no capability for message acknowledgment, applications that need reliable delivery are encouraged to consider the xref:nng_req.adoc[_REQ_] protocol instead.

#### Protocol Versions

Only version 0 of this protocol is supported.footnote:[At the time of writing, no other versions of this protocol have been defined.]

#### Protocol Options

xref:nng_options.adoc#NNG_OPT_SENDBUF[`NNG_OPT_SENDBUF`]::

    (`int`, 0 - 8192)
    Normally this is set to zero, indicating that send operations are unbuffered.
    In unbuffered operation, send operations will wait until a suitable peer is available to receive the message.
    If this is set to a positive value (up to 8192), then an intermediate buffer is provided for the socket with the specified depth (in messages).

NOTE: Transport layer buffering may occur in addition to any socket buffer determined by this option.

#### Protocol Headers

The _PUSH_ protocol has no protocol-specific headers.
