REQ / REP — I Ask, You Answer · NNG getting started

NNG pattern guide

reqrep communication topology

Request/Reply (I ask, you answer)

Request/Reply is used for synchronous communications where each question is responded with a single answer, for example remote procedure calls (RPCs). Like Pipeline, it also can perform load-balancing. This is the only reliable messaging pattern in the suite, as it automatically will retry if a request is not matched with a response.

reqprep.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <time.h>

#include <nng/nng.h>

static int
nng_recv_alloc_(nng_socket sock, void **buf, size_t *size)
{
        nng_msg *msg = NULL;
        int rv;
        if ((rv = nng_recvmsg(sock, &msg, 0)) != 0) return rv;
        *size = nng_msg_len(msg);
        if ((*buf = nng_alloc(*size)) == NULL) { nng_msg_free(msg); return NNG_ENOMEM; }
        memcpy(*buf, nng_msg_body(msg), *size);
        nng_msg_free(msg);
        return 0;
}
#define nng_recv_alloc(sock, buf, size) nng_recv_alloc_((sock), (void **)(buf), (size))

#define NODE0 "node0"
#define NODE1 "node1"
#define DATE "DATE"

void
fatal(const char *func, int rv)
{
        fprintf(stderr, "%s: %s\n", func, nng_strerror(rv));
        exit(1);
}

char *
date(void)
{
        time_t now = time(&now);
        struct tm *info = localtime(&now);
        char *text = asctime(info);
        text[strlen(text)-1] = '\0'; // remove '\n'
        return (text);
}

int
node0(const char *url)
{
        nng_socket sock;
        int rv;

        if ((rv = nng_rep0_open(&sock)) != 0) {
                fatal("nng_rep0_open", rv);
        }
          if ((rv = nng_listen(sock, url, NULL, 0)) != 0) {
                fatal("nng_listen", rv);
        }
        for (;;) {
                char *buf = NULL;
		size_t sz;
                if ((rv = nng_recv_alloc(sock, &buf, &sz)) != 0) {
                        fatal("nng_recv", rv);
                }
                if ((sz == (strlen(DATE) + 1)) && (strcmp(DATE, buf) == 0)) {
                        printf("NODE0: RECEIVED DATE REQUEST\n");
                        char *d = date();
                        printf("NODE0: SENDING DATE %s\n", d);
                        if ((rv = nng_send(sock, d, strlen(d) + 1, 0)) != 0) {
                                fatal("nng_send", rv);
                        }
                }
		nng_free(buf, sz);
        }
}

int
node1(const char *url)
{
        nng_socket sock;
        int rv;
	size_t sz;
        char *buf = NULL;

        if ((rv = nng_req0_open(&sock)) != 0) {
                fatal("nng_socket", rv);
        }
        if ((rv = nng_dial(sock, url, NULL, 0)) != 0) {
                fatal("nng_dial", rv);
        }
        printf("NODE1: SENDING DATE REQUEST %s\n", DATE);
        if ((rv = nng_send(sock, DATE, strlen(DATE)+1, 0)) != 0) {
                fatal("nng_send", rv);
        }
        if ((rv = nng_recv_alloc(sock, &buf, &sz)) != 0) {
                fatal("nng_recv", rv);
        }
        printf("NODE1: RECEIVED DATE %s\n", buf);  (1)
        nng_free(buf, sz);
        nng_socket_close(sock);
	return (0);
}

int
main(const int argc, const char **argv)
{
        if ((argc > 1) && (strcmp(NODE0, argv[1]) == 0))
                return (node0(argv[2]));

        if ((argc > 1) && (strcmp(NODE1, argv[1]) == 0))
                return (node1(argv[2]));

      fprintf(stderr, "Usage: reqrep %s|%s <URL> ...\n", NODE0, NODE1);
      return (1);
}

1	Blithely assumes message is ASCIIZ string. Real code should check it.

Compilation

gcc reqrep.c -lnng -o reqrep

Execution

./reqrep node0 ipc:///tmp/reqrep.ipc & node0=$! && sleep 1
./reqrep node1 ipc:///tmp/reqrep.ipc
kill $node0

Output

NODE1: SENDING DATE REQUEST DATE
NODE0: RECEIVED DATE REQUEST
NODE0: SENDING DATE Tue Jan  9 09:17:02 2018
NODE1: RECEIVED DATE Tue Jan  9 09:17:02 2018

This migrated example has been checked against NNG main. It uses the current socket API and receives messages through nng_recvmsg; the original NNG_FLAG_ALLOC form is no longer available.