Getting the last message sent to a kafka topic

问题

I'm new to Kafka and working on a prototype to connect a proprietary streaming service into Kafka.

I'm looking to get the key of the last message sent on a topic as our in-house stream consumer needs to logon with the ID of the last message it received when connecting.

Is it possible, using either the KafkaProducer or a KafkaConsumer to do this?

I've attempted to do the following using a Consumer, but when also running the console consumer I see messages replayed.

    // Poll so we know we're connected
    consumer.poll(100);
    // Get the assigned partitions
    Set<TopicPartition> assignedPartitions = consumer.assignment();
    // Seek to the end of those partitions
    consumer.seekToEnd(assignedPartitions);

    for(TopicPartition partition : assignedPartitions) {
        final long offset = consumer.committed(partition).offset();
        // Seek to the previous message
        consumer.seek(partition,offset - 1);
    }

    // Now get the last message
    ConsumerRecords<String, String> records = consumer.poll(100);
    for (ConsumerRecord<String, String> record : records) {
        lastKey = record.key();
    }
    consumer.close();

Is this expected behaviour or am I on the wrong path?

回答1:

The problem is on line final long offset = consumer.committed(partition).offset(), as link api refers committed method is to get the last committed offset for the given partition, i.e: the last offset your consumer tell kafka server that it had already read. So, definitely you will got messages replayed, because you always read from specific offset. As I think I only have to remove the first for block.

回答2:

Check the record count and get the last message:

    // Poll so we know we're connected
    consumer.poll(100);
    // Get the assigned partitions
    Set<TopicPartition> assignedPartitions = consumer.assignment();
    // Seek to the end of those partitions
    consumer.seekToEnd(assignedPartitions);

    for (TopicPartition partition : assignedPartitions) {
        final long offset = consumer.committed(partition).offset();
        // Seek to the previous message
        consumer.seek(partition, offset - 1);
    }

    // Now get the last message
    ConsumerRecords<String, String> records = consumer.poll(100);
    int size = records.count();
    int index = 0;
    for (ConsumerRecord<String, String> record : records) {
        index = index + 1;
        if (index == size) {
            String value = record.value();
            System.out.println("Last Message = " + value);
        }
    }
    consumer.close();

来源：https://stackoverflow.com/questions/47164878/getting-the-last-message-sent-to-a-kafka-topic