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:
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.
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',
},
})