SYNOPSIS
#include <nng/protocol/reqrep0/req.h>
int nng_req0_open(nng_socket *s);DESCRIPTION
The nng_req protocol is one half of a request/reply pattern. In this pattern, a requester sends a message to one replier, who is 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 the requester will keep retrying until a reply is received. |
| Because requests are resent, it is important that they be idempotent to ensure predictable and repeatable behavior even in the face of duplicated requests, which can occur (for example if a reply message is lost for some reason.) |
The requester generally only has one outstanding request at a time unless
in "raw" mode (via NNG_OPT_RAW), and it will generally attempt to spread
work requests to different peer repliers.
| This property, when combined with a device can help provide a degree of load-balancing. |
The nng_req protocol is the requester side, and the nng_rep(7) protocol is the replier side.
Socket Operations
The nng_req0_open() call creates a requester socket. This socket
may be used to send messages (requests), and then to receive replies. Generally
a reply can only be received after sending a request. (Attempts to receive
a message will result in NNG_ESTATE if there is no outstanding request.)
Requests may be canceled by sending a different request. This will cause the requester to discard any reply from the earlier request, but it will not stop a replier from processing a request it has already received or terminate a request that has already been placed on the wire.
Attempts to receive on a socket with no outstanding requests will result
in NNG_ESTATE.
Raw mode sockets (set with NNG_OPT_RAW) ignore all these restrictions.
Protocol Versions
Only version 0 of this protocol is supported. (At the time of writing, no other versions of this protocol have been defined.)
Protocol Options
The following protocol-specific options are available.
NNG_OPT_REQ_RESENDTIME-
This read/write option is a duration (32-bit unsigned integer) representing a relative number of milliseconds. When a new request is started, a timer of this duration is also started. If no reply is received before this timer expires, then the request will be resent. (Requests are also automatically resent if the peer to whom the original request was sent disconnects, or if a peer becomes available while the requester is waiting for an available peer.)
NNG_OPT_MAXTTL-
Maximum time-to-live. This option is an integer value between 0 and 255, inclusive, and is the maximum number of "hops" that a message may pass through until it is discarded. The default value is 8. A value of 0 may be used to disable the loop protection, allowing an infinite number of hops.
Protocol Headers
The nng_req protocol uses a backtrace in the header. This form uses an array of 32-bit big-endian identifiers, where the first element in the array identifies the local peer identifier to which the message will next be sent. This is a hop-by-hop header where each element in a path adds routing information to the end when sending a request, and when replying removes elements to obtain the next hop information. The request ID is at the end of this header and is inserted into the header as its first element by the originating surveyor. (Request IDs are distinguished from hops by having their high order bit set to one. They are generated automatically and randomly when a request is first issued.)
SEE ALSO
COPYRIGHT
Copyright 2017 Garrett D’Amore
Copyright 2017 Capitar IT Group BV
This document is supplied under the terms of the MIT License.