Sometimes a new consumer group does not work

问题

I've seen this in production once (I don't remember how we solved it) and now I can repeat it in the integration tests, which always start with a brand new Kafka installation. Here's how it goes:

Step 1: A consumer of a group that doesn't exist yet subscribes to a topic that does not exist yet and starts polling.

self.kafka_consumer = confluent_kafka.Consumer({
    'group.id': 'mygroup',
    'bootstrap.servers': 'kafka:9092',
    'enable.auto.commit': False,
    'auto.offset.reset': 'earliest',
})
self.kafka_consumer.subscribe('mytopic')

Step 2: A producer writes a message to the topic.

Result:

• About half the times it works fine; the consumer reads the message alright.
• The other half times the consumer seems stuck. I've tried waiting times up to 10 minutes to see if it would get unstuck, but no.
• Even if the two steps are reversed, i.e. the consumer tries to subscribe to an already existing topic that already has a message, the behavior is the same (however the group is always new).

More details

The consumer is polling with a timeout of 2 seconds, and if there's no result it loops over.

While the topic doesn't exist, poll() returns None. After the topic exists, poll() returns an msg whose error().code() is _PARTITION_EOF.

While the consumer seems stuck, I ask kafka what's going on with mygroup, and here's what it tells me:

root@e7b124b4039c:/# /usr/local/kafka/bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --group mygroup --describe
Note: This will not show information about old Zookeeper-based consumers.


TOPIC                          PARTITION  CURRENT-OFFSET  LOG-END-OFFSET  LAG        CONSUMER-ID                                       HOST                           CLIENT-ID
root@e7b124b4039c:/#

I try to make it unstuck by trying to read another nonexistent topic as mygroup:

root@e7b124b4039c:/# /usr/local/kafka/bin/kafka-console-consumer.sh --bootstrap-server localhost:9092 --group mygroup --topic nonexistent --from-beginning
[2018-03-15 16:36:59,369] WARN [Consumer clientId=consumer-1, groupId=pixelprocessor] Error while fetching metadata with correlation id 2 : {nonexistent=LEADER_NOT_AVAILABLE} (org.apache.kafka.clients.NetworkClient)
^CProcessed a total of 0 messages
root@e7b124b4039c:/#

After I do that, here's what Kafka has to say about mygroup:

root@e7b124b4039c:/# /usr/local/kafka/bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --group mygroup --describe
Note: This will not show information about old Zookeeper-based consumers.


TOPIC                          PARTITION  CURRENT-OFFSET  LOG-END-OFFSET  LAG        CONSUMER-ID                                       HOST                           CLIENT-ID
mytopic                        0          -               1               -          rdkafka-a172d013-08e6-4ee2-92f3-fdb07d163d57      /172.20.0.6                    rdkafka
(another topic)                0          -               0               -          rdkafka-a172d013-08e6-4ee2-92f3-fdb07d163d57      /172.20.0.6                    rdkafka
(a third topic)                0          -               0               -          rdkafka-a172d013-08e6-4ee2-92f3-fdb07d163d57      /172.20.0.6                    rdkafka
nonexistent                    0          0               0               0          -                                                 -                              -

This is Kafka 1.0.1, librdkafka 0.11.3, confluent_kafka 0.11.0, on Ubuntu 16.04 dockers (with the OS's packaged zookeeper 3.4.8) which are running on a Debian stretch (9.4) with Linux 4.9.0-6-amd64.

回答1:

The problem seems to have been in the Consumer() arguments. This doesn't work properly:

self.kafka_consumer = confluent_kafka.Consumer({
    'group.id': 'mygroup',
    'bootstrap.servers': 'kafka:9092',
    'auto.offset.reset': 'earliest',
})

But this does:

self.kafka_consumer = confluent_kafka.Consumer({
    'group.id': 'mygroup',
    'bootstrap.servers': 'kafka:9092',
    'default.topic.config': {
        'auto.offset.reset': 'earliest',
    },
})

来源：https://stackoverflow.com/questions/49319885/sometimes-a-new-consumer-group-does-not-work