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1. Controlled Shutdown Tool

What does the tool do?

In 0.8, each partition can have multiple replicas. These replicas are distributed across different brokers for better availability. At any given instance of time, only one of these replicas will serve reads and writes. In other words, one replica acts as the leader. When a broker needs to be shutdown (A broker can be shutdown for doing a new release, changing config etc), it would be useful to still serve the requests for the partitions on this broker using the other in-sync replicas. This is what the Controlled shutdown tool helps you to achieve. It transfers the leadership of the partitions from the broker (to be shutdown) to the other available replicas on the remaining brokers. A summary of the steps that the tool does is shown below -

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When the leadership for a partition is changed, the clients (producer and consumer) gets an error when they try to produce or consume from the old leader when they wait for a response. The client then refreshes the partition metadata from zookeeper and gets the new leader for the partition and retries. This does not work for the producer client when ack = 0. This is because the producer does not wait for a response and hence does not know about the leadership change. The client would end up loosing messages till the shutdown broker is brought back up.

2. Preferred Replica Leader Election Tool

What does the tool do?

With replication, each partition can have multiple replicas.

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