NNG pattern guide

pipeline communication topology

Pipeline (A One-Way Pipe)

This pattern is useful for solving producer/consumer problems, including load-balancing. Messages flow from the push side to the pull side. If multiple peers are connected, the pattern attempts to distribute fairly.

pipeline.c
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.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"

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

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

        if ((rv = nng_pull0_open(&sock)) != 0) {
                fatal("nng_pull0_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);
                }
                printf("NODE0: RECEIVED \"%s\"\n", buf); (1)
                nng_free(buf, sz);
        }
}

int
node1(const char *url, char *msg)
{
        int sz_msg = strlen(msg) + 1; // '\0' too
        nng_socket sock;
        int rv;
        int bytes;

        if ((rv = nng_push0_open(&sock)) != 0) {
                fatal("nng_push0_open", rv);
        }
        if ((rv = nng_dial(sock, url, NULL, 0)) != 0) {
                fatal("nng_dial", rv);
        }
        printf("NODE1: SENDING \"%s\"\n", msg);
        if ((rv = nng_send(sock, msg, strlen(msg)+1, 0)) != 0) {
                fatal("nng_send", rv);
        }
        sleep(1); // wait for messages to flush before shutting down
        nng_socket_close(sock);
	return (0);
}

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

        if ((argc > 2) && (strcmp(NODE1, argv[1]) == 0))
                return (node1(argv[2], argv[3]));

        fprintf(stderr, "Usage: pipeline %s|%s <URL> <ARG> ...'\n",
                NODE0, NODE1);
        return (1);
}
1 Blithely assumes message is ASCIIZ string. Real code should check it.
Compilation
gcc pipeline.c -lnng -o pipeline
Execution
./pipeline node0 ipc:///tmp/pipeline.ipc & node0=$! && sleep 1
./pipeline node1 ipc:///tmp/pipeline.ipc "Hello, World!"
./pipeline node1 ipc:///tmp/pipeline.ipc "Goodbye."
kill $node0
Output
NODE1: SENDING "Hello, World!"
NODE0: RECEIVED "Hello, World!"
NODE1: SENDING "Goodbye."
NODE0: RECEIVED "Goodbye."

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.