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
This protocol uses a backtrace in the header. This form uses a "stack" of 32-bit big-endian identifiers. There must be at least one identifier, the request ID, which will be the last element in the array, and must have the most significant bit set.
There may be additional peer IDs preceeding the request ID. These will be distinguishable from the request ID by having their most significant bit clear.
When a request message is received by a forwarding node (see nng_device(3)), the forwarding node prepends a 32-bit peer ID (which must have the most significant bit clear), which is the forwarder’s way of identifying the directly connected peer from which it received the message. (This peer ID, except for the most significant bit, has meaning only to the forwarding node itself.)
It may help to think of prepending a peer ID as "pushing" a peer ID onto the front of the stack of headers for the message. (It will use the peer ID it popped from the front to determine the next intermediate destination for the reply.)
When a reply message is created, it is created using the same headers that the request contained.
A forwarding node can "pop" the peer ID it originally pushed on the message, stripping it from the front of the message as it does so.
When the reply finally arrives back at the initiating requestor, it should have only a single element in the message, which will be the request ID it originally used for the request.