key为null时Kafka会将消息发送给哪个分区?

当你编写kafka Producer时, 会生成KeyedMessage对象。

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KeyedMessage<K, V> keyedMessage = new KeyedMessage<>(topicName, key, message)

这里的key值可以为空,在这种情况下, kafka会将这个消息发送到哪个分区上呢?依据Kafka官方的文档, 默认的分区类会随机挑选一个分区:

The third property "partitioner.class" defines what class to use to determine which Partition in the Topic the message is to be sent to. This is optional, but for any non-trivial implementation you are going to want to implement a partitioning scheme. More about the implementation of this class later. If you include a value for the key but haven't defined a partitioner.class Kafka will use the default partitioner. If the key is null, then the Producer will assign the message to a random Partition.

但是这句话相当的误导人。

从字面上来讲,这句话没有问题, 但是这里的随机是指在参数"topic.metadata.refresh.ms"刷新后随机选择一个, 这个时间段内总是使用唯一的分区。 默认情况下每十分钟才可能重新选择一个新的分区。 但是相信大部分的程序员和我一样, 都理解成每个消息都会随机选择一个分区。
可以查看相关的代码:

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private def getPartition(topic: String, key: Any, topicPartitionList: Seq[PartitionAndLeader]): Int = {
val numPartitions = topicPartitionList.size
if(numPartitions <= 0)
throw new UnknownTopicOrPartitionException("Topic " + topic + " doesn't exist")
val partition =
if(key == null) {
// If the key is null, we don't really need a partitioner
// So we look up in the send partition cache for the topic to decide the target partition
val id = sendPartitionPerTopicCache.get(topic)
id match {
case Some(partitionId) =>
// directly return the partitionId without checking availability of the leader,
// since we want to postpone the failure until the send operation anyways
partitionId
case None =>
val availablePartitions = topicPartitionList.filter(_.leaderBrokerIdOpt.isDefined)
if (availablePartitions.isEmpty)
throw new LeaderNotAvailableException("No leader for any partition in topic " + topic)
val index = Utils.abs(Random.nextInt) % availablePartitions.size
val partitionId = availablePartitions(index).partitionId
sendPartitionPerTopicCache.put(topic, partitionId)
partitionId
}
} else
partitioner.partition(key, numPartitions)
if(partition < 0 || partition >= numPartitions)
throw new UnknownTopicOrPartitionException("Invalid partition id: " + partition + " for topic " + topic +
"; Valid values are in the inclusive range of [0, " + (numPartitions-1) + "]")
trace("Assigning message of topic %s and key %s to a selected partition %d".format(topic, if (key == null) "[none]" else key.toString, partition))
partition
}

如果key为null, 它会从sendPartitionPerTopicCache查选缓存的分区, 如果没有,随机选择一个分区,否则就用缓存的分区。

LinkedIn工程师Guozhang Wang在邮件列表中解释了这一问题,
最初kafka是按照大部分用户理解的那样每次都随机选择一个分区, 后来改成了定期选择一个分区, 这是为了减少服务器段socket的数量。不过这的确很误导用户,据称0.8.2版本后又改回了每次随机选取。但是我查看0.8.2的代码还没看到改动。

所以,如果有可能,还是为KeyedMessage设置一个key值吧。

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