consumer.c

/*
 * librdkafka - Apache Kafka C library
 *
 * Copyright (c) 2019-2022, Magnus Edenhill
 *               2023, Confluent Inc.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

/**
 * Simple high-level balanced Apache Kafka consumer
 * using the Kafka driver from librdkafka
 * (https://github.com/confluentinc/librdkafka)
 */

#include <stdio.h>
#include <signal.h>
#include <string.h>
#include <ctype.h>


/* Typical include path would be <librdkafka/rdkafka.h>, but this program
 * is builtin from within the librdkafka source tree and thus differs. */
//#include <librdkafka/rdkafka.h>
#include "rdkafka.h"


static volatile sig_atomic_t run = 1;

/**
 * @brief Signal termination of program
 */
static void stop(int sig) {
        run = 0;
}



/**
 * @returns 1 if all bytes are printable, else 0.
 */
static int is_printable(const char *buf, size_t size) {
        size_t i;

        for (i = 0; i < size; i++)
                if (!isprint((int)buf[i]))
                        return 0;

        return 1;
}


int main(int argc, char **argv) {
        rd_kafka_t *rk;          /* Consumer instance handle */
        rd_kafka_conf_t *conf;   /* Temporary configuration object */
        rd_kafka_resp_err_t err; /* librdkafka API error code */
        char errstr[512];        /* librdkafka API error reporting buffer */
        const char *brokers;     /* Argument: broker list */
        const char *groupid;     /* Argument: Consumer group id */
        char **topics;           /* Argument: list of topics to subscribe to */
        int topic_cnt;           /* Number of topics to subscribe to */
        rd_kafka_topic_partition_list_t *subscription; /* Subscribed topics */
        int i;

        /*
         * Argument validation
         */
        if (argc < 4) {
                fprintf(stderr,
                        "%% Usage: "
                        "%s <broker> <group.id> <topic1> <topic2>..\n",
                        argv[0]);
                return 1;
        }

        brokers   = argv[1];
        groupid   = argv[2];
        topics    = &argv[3];
        topic_cnt = argc - 3;


        /*
         * Create Kafka client configuration place-holder
         */
        conf = rd_kafka_conf_new();

        /* Set bootstrap broker(s) as a comma-separated list of
         * host or host:port (default port 9092).
         * librdkafka will use the bootstrap brokers to acquire the full
         * set of brokers from the cluster. */
        if (rd_kafka_conf_set(conf, "bootstrap.servers", brokers, errstr,
                              sizeof(errstr)) != RD_KAFKA_CONF_OK) {
                fprintf(stderr, "%s\n", errstr);
                rd_kafka_conf_destroy(conf);
                return 1;
        }

        /* Set the consumer group id.
         * All consumers sharing the same group id will join the same
         * group, and the subscribed topic' partitions will be assigned
         * according to the partition.assignment.strategy
         * (consumer config property) to the consumers in the group. */
        if (rd_kafka_conf_set(conf, "group.id", groupid, errstr,
                              sizeof(errstr)) != RD_KAFKA_CONF_OK) {
                fprintf(stderr, "%s\n", errstr);
                rd_kafka_conf_destroy(conf);
                return 1;
        }

        /* If there is no previously committed offset for a partition
         * the auto.offset.reset strategy will be used to decide where
         * in the partition to start fetching messages.
         * By setting this to earliest the consumer will read all messages
         * in the partition if there was no previously committed offset. */
        if (rd_kafka_conf_set(conf, "auto.offset.reset", "earliest", errstr,
                              sizeof(errstr)) != RD_KAFKA_CONF_OK) {
                fprintf(stderr, "%s\n", errstr);
                rd_kafka_conf_destroy(conf);
                return 1;
        }

        /*
         * Create consumer instance.
         *
         * NOTE: rd_kafka_new() takes ownership of the conf object
         *       and the application must not reference it again after
         *       this call.
         */
        rk = rd_kafka_new(RD_KAFKA_CONSUMER, conf, errstr, sizeof(errstr));
        if (!rk) {
                fprintf(stderr, "%% Failed to create new consumer: %s\n",
                        errstr);
                return 1;
        }

        conf = NULL; /* Configuration object is now owned, and freed,
                      * by the rd_kafka_t instance. */


        /* Redirect all messages from per-partition queues to
         * the main queue so that messages can be consumed with one
         * call from all assigned partitions.
         *
         * The alternative is to poll the main queue (for events)
         * and each partition queue separately, which requires setting
         * up a rebalance callback and keeping track of the assignment:
         * but that is more complex and typically not recommended. */
        rd_kafka_poll_set_consumer(rk);


        /* Convert the list of topics to a format suitable for librdkafka */
        subscription = rd_kafka_topic_partition_list_new(topic_cnt);
        for (i = 0; i < topic_cnt; i++)
                rd_kafka_topic_partition_list_add(subscription, topics[i],
                                                  /* the partition is ignored
                                                   * by subscribe() */
                                                  RD_KAFKA_PARTITION_UA);

        /* Subscribe to the list of topics */
        err = rd_kafka_subscribe(rk, subscription);
        if (err) {
                fprintf(stderr, "%% Failed to subscribe to %d topics: %s\n",
                        subscription->cnt, rd_kafka_err2str(err));
                rd_kafka_topic_partition_list_destroy(subscription);
                rd_kafka_destroy(rk);
                return 1;
        }

        fprintf(stderr,
                "%% Subscribed to %d topic(s), "
                "waiting for rebalance and messages...\n",
                subscription->cnt);

        rd_kafka_topic_partition_list_destroy(subscription);


        /* Signal handler for clean shutdown */
        signal(SIGINT, stop);

        /* Subscribing to topics will trigger a group rebalance
         * which may take some time to finish, but there is no need
         * for the application to handle this idle period in a special way
         * since a rebalance may happen at any time.
         * Start polling for messages. */

        while (run) {
                rd_kafka_message_t *rkm;

                rkm = rd_kafka_consumer_poll(rk, 100);
                if (!rkm)
                        continue; /* Timeout: no message within 100ms,
                                   *  try again. This short timeout allows
                                   *  checking for `run` at frequent intervals.
                                   */

                /* consumer_poll() will return either a proper message
                 * or a consumer error (rkm->err is set). */
                if (rkm->err) {
                        /* Consumer errors are generally to be considered
                         * informational as the consumer will automatically
                         * try to recover from all types of errors. */
                        fprintf(stderr, "%% Consumer error: %s\n",
                                rd_kafka_message_errstr(rkm));
                        rd_kafka_message_destroy(rkm);
                        continue;
                }

                /* Proper message. */
                printf("Message on %s [%" PRId32 "] at offset %" PRId64
                       " (leader epoch %" PRId32 "):\n",
                       rd_kafka_topic_name(rkm->rkt), rkm->partition,
                       rkm->offset, rd_kafka_message_leader_epoch(rkm));

                /* Print the message key. */
                if (rkm->key && is_printable(rkm->key, rkm->key_len))
                        printf(" Key: %.*s\n", (int)rkm->key_len,
                               (const char *)rkm->key);
                else if (rkm->key)
                        printf(" Key: (%d bytes)\n", (int)rkm->key_len);

                /* Print the message value/payload. */
                if (rkm->payload && is_printable(rkm->payload, rkm->len))
                        printf(" Value: %.*s\n", (int)rkm->len,
                               (const char *)rkm->payload);
                else if (rkm->payload)
                        printf(" Value: (%d bytes)\n", (int)rkm->len);

                rd_kafka_message_destroy(rkm);
        }


        /* Close the consumer: commit final offsets and leave the group. */
        fprintf(stderr, "%% Closing consumer\n");
        rd_kafka_consumer_close(rk);


        /* Destroy the consumer */
        rd_kafka_destroy(rk);

        return 0;
}