Replication tools
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1. Controlled Shutdown

What is controlled shutdown?

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(in the in-sync set) on the remaining brokers.

Basically, it reduces the unavailable window. If we simply bring down a broker without controlled shutdown, partitions with leader on the broker are not available until the new leaders are elected. Since we currently elect leaders sequentially one partition at a time and each leader election involves reads/writes to ZK, it may take some time for all leaders to be elected, especially when there are many partitions. Doing the controlled shutdown allows us to move the leader proactively one at a time and thus reduce the unavailable window.

There are two ways to do controlled shutdown:

The first approach is to set "controlled.shutdown.enable" to true in the broker. Then, the broker will try to move all leaders on it to other brokers one at time before shutting itself down. One can tweak controlled.shutdown.max.retries and controlled.shutdown.retry.backoff.ms to control the max amount time the broker spends during controlled shutdown.

The second approach is to run a command line tool.

A summary of the steps that the tool does is shown below -

1. Finds the jmx port of the broker where the controller resides.
2. Issues a shutdown command to the controller using the port specifying the broker to be shutdown.
3. The controller finds the list of partitions the broker is currently leading.
4. For each of the partitions, the controller finds another broker that is in the "in-sync" replica set and makes it the leader for the partition.
5. For a given partition, if the controller cannot find any other replica that is in the 'in-sync" set, it fails the shutdown.
6. The shutdown tool waits for the response of the controller. If it finds that the shutdown was not successful, it retries (this is configurable) and eventually succeeds or fails based on the controller response.

Note that the shutdown tool only moves the leadership of the partitions to the other available brokers. It does not terminate the broker. This needs to be done manually after running the tool.

How to use the tool?

The tool takes in a list of zookeeper hosts (including the namespace if any) and the broker id that needs to be shutdown. Both these arguments are mandatory. The tool also allows to specify an optional number of retries and the interval between those retries. If the shutdown attempt by the controller fails, the tool retries after the specified interval.

FAQ

How does the tool work when ack = 0?

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.

What happens when there are no other replicas in the "in-sync" set for a partition?

When a partition does not have any other replicas that are in the "in-sync" set, the tool fails to shutdown the broker. This is to ensure that we do not have an unclean leader election (choosing a replica that is outside the "in-sync" set) and thereby causing data loss.

2. Preferred Replica Leader Election Tool

What does the tool do?

With replication, each partition can have multiple replicas. The list of replicas for a partition is called the "assigned replicas". The first replica in this list is the "preferred replica". When topic/partitions are created, Kafka ensures that the "preferred replica" for the partitions across topics are equally distributed amongst the brokers in a cluster. In an ideal scenario, the leader for a given partition should be the "preferred replica". This guarantees that the leadership load across the brokers in a cluster are evenly balanced. However, over time the leadership load could get imbalanced due to broker shutdowns (caused by controlled shutdown, crashes, machine failures etc). This tool helps to restore the leadership balance between the brokers in the cluster. A summary of the steps that the tool does is shown below -

1. The tool updates the zookeeper path "/admin/preferred_replica_election" with the list of topic partitions whose leader needs to be moved to the preferred replica.
2. The controller listens to the path above. When a data change update is triggered, the controller reads the list of topic partitions from zookeeper.
3. For each topic partition, the controller gets the preferred replica (the first replica in the assigned replicas list). If the preferred replica is not already the leader and it is present in the isr, the controller issues a request to the broker that owns the preferred replica to become the leader for the partition.

Note that the tool only updates the zookeeper path and exits. The controller moves the leader for a partition to the preferred replica asynchronously.

How to use the tool?

The tool takes a mandatory list of zookeeper hosts and an optional list of topic partitions provided as a json file. If the list is not provided, the tool queries zookeeper and gets all the topic partitions for the cluster. The tool exits after updating the zookeeper path "/admin/preferred_replica_election" with the topic partition list.

Example json file (This is optional. This can be specified to move the leader to the preferred replica for specific topic partitions)

FAQ

What happens if the preferred replica is not in the ISR?

The controller will fail to move the leadership to the preferred replica if it is not in the ISR. This is to ensure that there is no dataloss. When the replica becomes "in-sync" with the leader, the tool can be run again to move the leader.

How to find if all the partitions have been moved to the "preferred replica" after running the tool?

ListTopicCommand is an excellent tool that provides an overview of all the topic partitions in the cluster. For each topic partition, it displays the leader, assigned replicas and current "in-sync" replica set. If the leader and the first replica in the assigned replica set are the same then the Preferred replica leader election" tool succeeded. If not, the tool failed and may have to be run again.

3. List Topic Tool

What does the tool do?

This tool lists the information for a given list of topics. If no topics are provided in the command line, the tool queries zookeeper to get all the topics and lists the information for them. The fields that the tool displays are - topic name, partition, leader, replicas, isr. Two optional arguments can be provided to the tool. If "under-replicated-partitions" is specified, the tool only provides information for those topic / partitions which have replicas that are under replicated. If "unavailable-partitions" is specified, the tool only provides information for those topic/partitions whose leader is not available.

How to use the tool?

4. Create Topic Tool

What does the tool do?

By default, Kafka auto creates topic if "auto.create.topics.enable" is set to true on the server. This creates a topic with a default number of partitions, replication factor and uses Kafka's default scheme to do replica assignment. Sometimes, it may be required that we would like to customize a topic while creating it. This tool helps to create a topic and also specify the number of partitions, replication factor and replica assignment list for the topic.

How to use the tool?

5. Add Partition Tool

What does the tool do?

In Kafka partitions act as the unit of parallelism: messages of a single topic are distributed to multiple partitions that can be stored and served on different servers. Upon creation of a topic, the number of partitions for this topic has to be specified. Later on more partitions may be needed for this topic when the volume of this topic increases. This tool helps to add more partitions for a specific topic and also allow manual replica assignment of the added partitions. This tool is only available in the 0.8 branch for now.

How to use the tool?

6. Reassign Partitions Tool

What does the tool do?

The goal of this tool is similar to the Referred Replica Leader Election Tool as to achieve load balance across brokers. But instead of only electing a new leader from the assigned replicas of a partition, this tool allows to change the assigned replicas of partitions – remember that followers also need to fetch from leaders in order to keep in sync, hence sometime only balance the leadership load is not enough.

A summary of the steps that the tool does is shown below -

1. The tool updates the zookeeper path "/admin/reassign_partitions" with the list of topic partitions and (if specified in the Json file) the list of their new assigned replicas.
2. The controller listens to the path above. When a data change update is triggered, the controller reads the list of topic partitions and their assigned replicas from zookeeper.
3. For each topic partition, the controller does the following:
3.1. Start new replicas in RAR - AR (RAR = Reassigned Replicas, AR = original list of Assigned Replicas)
3.2. Wait until new replicas are in sync with the leader
3.3. If the leader is not in RAR, elect a new leader from RAR
3.4 4. Stop old replicas AR - RAR
3.5. Write new AR
3.6. Remove partition from the /admin/reassign_partitions path

Note that the tool only updates the zookeeper path and exits. The controller reassign the replicas for the partitions asynchronously.

This tool is only available in the 0.8 branch for now.

How to use the tool?

WARNING: The tool was released in beta in 0.8 and has some bugs that can render the topic unusable. The tool is known to be stable in 0.8.1.

Please note that by default the script runs in a dry-run mode and does not initiate the partition movement. Only when the --execute option is specified, the tool proceeds to start the partition movement

Cluster Expansion

The partition reassignment tool can be used to expand an existing 0.8 cluster. Cluster expansion involves including brokers with new broker ids in a Kafka 08 cluster. Typically, when you add new brokers to a cluster, they will not receive any data from existing topics until this tool is run to assign existing topics/partitions to the new brokers. The tool allows 2 options to make it easier to move some topics in bulk to the new brokers. These 2 options are a) topics to move b) list of newly added brokers. Using these 2 options, the tool automatically figures out the placements of partitions for the topics on the new brokers.

The following example moves 2 topics (foo1, foo2) to newly added brokers in a cluster (5,6,7)

Selectively moving some partitions to a broker

The partition movement tool can also be moved to selectively move some replicas for certain partitions over to a particular broker. Typically, if you end up with an unbalanced cluster, you can use the tool in this mode to selectively move partitions around. In this mode, the tool takes a single file which has a list of partitions to move and the replicas that each of those partitions should be assigned to.

The following example moves 1 partition (foo-1) from replicas 1,2,3 to 1,2,4

7. StateChangeLogMerger Tool

What does the tool do ?

The goal of this tool is to collect data from the brokers in a cluster and format it in a central log to help troubleshoot issues with state changes. Every broker in a Kafka cluster emits a state-change.log that logs the lifecycle of every state change received by the broker. Often times, there is some problem with leader election for a subset of topics/partitions and the question is what caused the problem. In order to answer this question, we need a global view of state changes in the kafka cluster, possibly filtered on a time range and/or specific topics/partitions. This is exactly what the StateChangeLogMerger tool does. It takes in a list of state-change.log files, merges them in time order, filters on a certain time range if specified by the user, filters on topics/partitions if specified by the user, and outputs a merged and formatted state-change.log that is easy to query and understand the root cause.

How to use the tool ?

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