THIS IS A TEST INSTANCE. ALL YOUR CHANGES WILL BE LOST!!!!
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In the current world, a broker which can contact ZooKeeper but which is partitioned from the active controller will continue serving user requests, but will not receive any metadata updates. This can lead to some confusing and difficult situations. For example, a producer using acks=1 might continue to produce to a leader that actually was not the leader any more, but which failed to receive the controller's LeaderAndIsrRequest moving the leadership.
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When a broker is stopping, it is still running, but we are trying to migrate the partition leaders off of the broker.
Eventually, the active controller will ask the broker to finally go offline, by returning a special result code in the MetadataFetchResponse. Alternately, the broker will shut down if the leaders can't be moved in a predetermined amount of time.
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New versions of the clients should send these operations directly to the active controller. This is a backwards compatible change: it will work with both old and new clusters. In order to preserve compatibility with old clients that sent these operations to a random broker, the brokers will forward these requests to the active controller.
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In some cases, we will need to create a new API to replace an operation that was formerly done via ZooKeeper. One example of this is that when the leader of a partition wants to modify the in-sync replica set, it currently modifies ZooKeeper directly In the post-ZK world, the leader will make an RPC to the active controller instead.
Removing Direct ZooKeeper Access from Tools
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We will preserve compatibility with the existing Kafka clients. In some cases, the existing clients will take a less efficient code path. For example, the brokers may need to forward their requests to the active controller.
Bridge Release
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We do not need to worry about the ZooKeeper state getting concurrently modified during this loading process. In the bridge release, neither the tools nor the non-controller brokers will modify ZooKeeper.
The new active controller will monitor ZooKeeper for legacy broker node registrations. It will know how to send the legacy "push" metadata requests to those nodes, during the transition period.
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