Redis Streams one message per consumer with Java

Question

I'am trying to implement a java application with redis streams where every consomer consumes exactly one message. Like a pipeline/queue where every consumer takes exactly one message, processes it and after finishing the consumer takes the next message which was not processed so far in the stream. What works is that every message is consumed by exactly one consumer (with xreadgroup).

I started with this tutorial from redislabs

The code:

    RedisClient redisClient = RedisClient.create("redis://pw@host:port");
    StatefulRedisConnection<String, String> connection = redisClient.connect();
    RedisCommands<String, String> syncCommands = connection.sync();

    try {
        syncCommands.xgroupCreate(XReadArgs.StreamOffset.from(STREAM_KEY, "0-0"), ID_READ_GROUP);
    } catch (RedisBusyException redisBusyException) {
        System.out.println(String.format("\t Group '%s' already exists", ID_READ_GROUP));
    }

    System.out.println("Waiting for new messages ");

    while (true) {
        List<StreamMessage<String, String>> messages = syncCommands.xreadgroup(
                Consumer.from(ID_READ_GROUP, ID_WORKER), ReadArgs.StreamOffset.lastConsumed(STREAM_KEY));

        if (!messages.isEmpty()) {
            System.out.println(messages.size()); // 
            for (StreamMessage<String, String> message : messages) {
                System.out.println(message.getId());
                Thread.sleep(5000);
                syncCommands.xack(STREAM_KEY, ID_READ_GROUP, message.getId());
            }
        }

    }

My current problem is that a consumer takes more that one message from the queue and in some situations the other consumers are waiting and one consumer is processing 10 messages at once.

Thanks in advance!

Answer 1

Notice that XREADGROUP can get COUNT argument.

See the JavaDoc how to do it in Lettuce xreadgroup , by passing XReadArgs.