## Reply Protocol (_REP_)

The ((_REP_ protocol))(((protocol, _REP_))) is one half of a ((request/reply pattern)).
In this pattern, a requester sends a message to one replier, whois expected to reply.
The request is resent if no reply arrives, until a reply is received or the request times out.

This protocol is useful in setting up RPC-like services.
It is also "reliable", in that a requester will keep retrying until a reply is received.

The _REP_ protocol is the replier side, and the xref:req.adoc[_REQ_] protocol is the requester side.

### Socket Operations

The xref:../socket/nng_rep_open.adoc[`nng_rep_open`] functions create a replier socket.
This socket may be used to receive messages (requests), and then to send replies.

A reply can only be sent after receiving a request.
Send operations will result in `NNG_ESTATE` if no corresponding request was previously received.

Likewise, only one receive operation may be pending at a time.footnote:[However contexts can be used to allow concurrent request processing.]
Any additional concurrent receive operations will result in `NNG_ESTATE`.

xref:../sock/nng.adoc#raw_mode[Raw mode] sockets ignore all these restrictions.

### Context Operations

This protocol supports the creation of xref:../ctx/index.adoc[contexts] for concurrent processing using xref:../ctx/nng_ctx_open.adoc[`nng_ctx_open`].

Each context may have at most one outstanding request, and operates independently of the others.
The restrictions for order of operations with sockets apply equally well for contexts, except that each context will be treated as if it were a separate socket.

### 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

The _rep_ protocol has no protocol-specific options.

### Protocol Headers

(((backtrace)))
The _REP_ protocol uses a _backtrace_ in the header.
This is documented in the xref:req.adoc[_REQ_] protocol.