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Status

Current state: Under Discussion

Discussion thread: here

JIRA: here

Released: <Kafka Version>

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

The most expensive part of a Kafka cluster is probably its storage system. At LinkedIn we use RAID-10 for storage and set Kafka’s replication factor = 2. This setup requires 4X space to store data and tolerates up to 1 broker failure. We are at risk of data loss with just 1 broker failure, which is not acceptable for e.g. financial data. On the other hand, it is prohibitively expensive to set replication factor = 3 with RAID-10 because it will increase our existing hardware cost and operational cost by 50%.

The solution is to use JBOD and set replication factor = 3 or higher. It is based on the idea that Kafka already has replication across brokers and it is unnecessary to use RAID-10 for replication. Let’s say we set the replication factor = 4 with JBOD. This setup requires 4X space to store data and tolerate up to 3 broker failures in order not to lose any data. In comparison to our existing setup, this allows us to obtain 3X broker failure tolerance without increasing storage hardware cost.

We have evaluated the possibility of using other RAID setup in LinkedIn. But none of them addresses our problem as JBOD does. RAID-0 stops working entirely with just one disk failure. RAID-5 or RAID-6 has sizable performance loss as compared to RAID-0 and probably JBOD as well, due to their use of block-level striping with distributed parity.

Unfortunately, JBOD is not recommended for Kafka because some important features are missing. For example, Kafka lacks good support for tools as well as load balancing across disks when multiple disks are used. Here is a list of problems that need to be addressed for JBOD to be useful:

1) Broker will shutdown if any disk fails. This means a single disk failure can bring down the entire broker. Instead, broker should still serve those replicas on the good disks as long as there is good disk available.

2) Kafka doesn’t provide the necessary tools for users to manage JBOD. For example, Kafka doesn’t provide script to re-assign replicas between disks of the same broker. These tools are needed before we can use JBOD with Kafka.

3) JBOD doesn’t by itself balance load across disks as RAID-10 does. This will be a new problem for us to solve in order for JBOD setup to work well. We should have a better solution than round-robin which we are using to select disk to place a new replica. And we should probably figure out how to re-assign replicas across disks of the same broker if we notice load imbalance across disks of a broker.

For ease of discussion, we have separated the design of JBOD support into two different KIPs. This KIP address the second problem. See KIP - Handle disk failure for JBOD to read our proposal of how to address the first problem.

Since Kafka configuration and implementation does not expose "disk", we will use log directory and disk interchangably in the rest of the KIP.

Goals

The goal of this KIP is to allow administrator to re-assign replicas to the specific log directories of brokers so that they can balance load across disks. This addresses the second problem raised in the motivation section. See KIP - Handle disk failure for JBOD to read our proposal of how to address the first problem.

Proposed change

1) How to move replica between log directories on the same broker

Problem statement:

Kafka doesn’t not allow user to move replica to another log directory on the same broker in runtime. This is not needed previously because we use RAID-10 and the load is already balanced across disks. But it will be needed to use JBOD with Kafka.

Currently a replica only has two possible states, follower or leader. And it is identified by the 3-tuple (topic, partition, broker). This works for the current implementation because there can be at most one such replica on a broker. However, now we can have two such replicas on a broker when we move replica from one log directory to another log directory on the same broker. Either a replica should be identified by its log directory as well, or the broker needs to persist information under the log directory to tell the destination replica from source replica that is being moved.

In addition, user needs to be able to query list of partitions and their size per log directory on any machine so that they can determine how to move replicas to balance the load. While these information may be retrieved via external tools if user can ssh to the machine that is running the Kafka broker, development of such tools may not be easy on all the operating systems that Kafka may run on. Further, a user may not even have the authorization to access the machine. Therefore Kafka needs to provide new request/response to provide this information to users.

Solution:

The idea is that user can send a ChangeReplicaDirRequest which tells broker to move topicPartition directory (which contains all log segments of the topicPartition replica) from the source log directory to a destination log directory. Broker can create a new directory with .move postfix on the destination log directory to hold all log segments of the replica. This allows broker to tell log segments of the replica on the destination log directory from log segments of the replica on the source log directory during broker startup. The broker can create new log segments for the replica on the destination log directory, push data from source log to the destination log, and replace source log with the destination log for this replica once the new log has caught up.

In the following we describe each step of the replica movement. 

1. Initiate replica movement using ChangeReplicaDirRequest

User uses kafka-reassignment-partitions.sh to send ChangeReplicaDirRequest to broker to initiate replica movement between its log directories. The flow graph below illustrates how broker handles ChangeReplicaDirRequest.

Notes:
- Broker will cancel existing movement of the replica if "any" is specified as destination log directory.
- If broker doesn't not have already replica created for the specified topicParition when it receives ChangeReplicaDirRequest, it will reply ReplicaNotAvailableException AND remember (replica, destination log directory) pair in memory to create the replica in the specified log directory when it receives LeaderAndIsrRequest later.

2. Complete replica data movement

Here we describe how a broker moves a Log from source to destination log directory and swaps the Log.  This corresponds to the "Initiate replica data movement" box in the flow graph above. Note that the broker responds to ChangeReplicaDirRequest with MoveInProgress after step 1) described below.

1) The Replica instance is updated to track two instances of Log, one referencing the directory topicPartition on the source log directory and the other referencing the directory topicPartition.move on the destination log directory.
2) If there is thread available in ReplicaMoveThreadPool, one thread gets allocated to move this replica. Otherwise, the movement of this replica gets delayed until there is available thread.
3) The ReplicaMoveThread keeps reading data from the Log in the source log directory to the Log (i.e. toipcPartition.move) in the destination log directory using zero-copy. ReplicaMoveThread may need to sleep to ensure that total throughput in byte-rate used by all ReplicaMoveThread instances does not exceed the configured value of intra.broker.throttled.rate.
4) If the Log in the destination log directory has caught up with the Log in the source log directory, the ReplicaMoveThread grabs lock on the Replica instance.
5) The ReplicaMoveThread continues to move data as specified in step 3) until the Log in the destination log directory has caught up with the Log in the source log directory. We need to do this again because RequestHandlerThread or ReplicaFetcherThread may be appending data to the log in the source log directory concurrently after the check in step 4).
6) The ReplicaMoveThread renames directory topicPartition to topicPartition.delete on the source log directory. topicParition.delete will be subject to asynchronous delete.
7) The ReplicaMoveThread renames directory topicParition.move to topicParition on the destination log directory 
8) The ReplicaMoveThread updates the corresponding Replica instance to track only the Log in the destination log directory.
9) The ReplicaMoveThread releases lock on the Replica instance.
 

3. Handle failure that happens when broker is moving data or swapping replica

Broker does the following to recover from failure when it starts up.

- If both the directory topicPartition and the directory topicPartition.move exist on good log directories, broker will start ReplicaMoveThread to copy data from topicPartition to topicPartition.move. The effect is the same as if broker has received ChangeReplicaDirRequest to move replica from topicPartition to topicPartition.move.
- If topicPartition.move exists but topicPartition doesn't exist on any good log directory, and if there is no bad log directory, then broker renames topicPartition.move to topicPartition.
- If topicPartition.move exists but topicPartition doesn't exist on any good log directory, and if there is bad log directory, then broker considers topicPartition as offline and would not touch topicPartition.move.
- If topicPartition.delete exists, the broker schedules topicParition.delete for asynchronous delete.

2) How to reassign replica between log directories across brokers

Problem statement:

kafka-reassign-partitions.sh should provide an option for user to specify destination log directory of the replica in the reassignment json file. And user should be able to verify that the replica has been moved to the specified log directory after the reassignment is completed. This is needed by user to balance load across log directories of brokers in the cluster.

Solution:

The idea is that user should be able to specify log directory when using kafka-reassign-partitions.sh to reassign partition. If user has specified log directory on the destination broker, the script should send ChangeReplicaDirRequest directly to the broker so that broker can start ReplicaMoveThread to move the replica. Finally, the script should send DescribeDirsRequest to broker to verify that the replica has been created/moved in the specified log directory when user requests to verify the assignment.

Here are the steps to execute partition reassignment:

- User specifies a list of log directories, one log directory per replica, for each topic partition in the reassignment json file that is provided to kafka-reassignment-partitions.sh. The log directory specified by user must be either "any", or absolute path which begins with '/'. See Scripts section for the format of this json file.
- The script sends ChangeReplicaDirRequest to those brokers which need to move replicas to user-specified log directory. This step can be skipped if user has specified "any" as log directory for all replicas. The script exits with error if the broker to receive ChangeReplicaDirRequest is offline or if the ChangeReplicaDirResponse contains any error that is not ReplicaNotAvailableException.
- The script creates reassignment znode in zookeeper.
- The script retries ChangeReplicaDirRequest to those brokers which have responded with ReplicaNotAvailableException in the ChangeReplicaDirResponse previously. The script keeps retrying up to user-specified timeout. The timeout is 10 seconds by default. The script exits with error if the broker to receive ChangeReplicaDirRequest is offline or if the ChangeReplicaDirResponse contains any error that is not ReplicaNotAvailableException.
- The script returns result to user.

Here are the steps to verify partition assignment:

- kafka-reassignment-partitions.sh will verify partition assignment across brokers as it does now. 
- For those replicas with destination log directory != "any", kafka-reassignment-partitions.sh groups those replicas according to their brokers and and sends DescribeDirsRequest to those brokers. The DescribeDirsRequest should provide the log directories and partitions specified in the expected assignment.
- Broker replies with DescribeDirsResponse which shows the current log directory for each partition specified in the DescribeDirsRequest.
- kafka-reassignment-partitions.sh determines whether the replica has been moved to the specified log directory based on the DescribeDirsResponse.

3) How to retrieve information to determine the new replica assignment across log directories

Problem statement:

In order to optimize replica assignment across log directories, user would need to figure out the list partitions per log directory, the size of each partition. As of now Kafka doesn't expose this information via any RPC and user would need to either query the JMX metrics of the broker, or use external tools to log onto each machine to get this information. While it is possible to retrieve these information via JMX, users would have to manage JMX port and related credentials. It is better if Kafka can expose this information via RPC.

Solution:

We introduce DescribeDirsRequest and DescribeDirsResponse. When a broker receives DescribeDirsRequest with empty list of log directories, it will respond with a DescribeDirsResponse which shows the size of each partition and lists of partitions per log directory for all log directories. If user has specified a list of log directories in the DescribeDirsRequest, the broker will provide the above information for only log directories specified by the user. If user has specified an empty list of topics in the DescribeDirsRequest, all topics will be queried and included in the response. Otherwise, only those topics specified in the DescribeDirsRequest will be queried. Non-zero error code will be specified in the DescribeDirsResponse for each log directory that is either offline or not found by the broker.

Public interface

Protocol

Create ChangeReplicaDirRequest

ChangeReplicaDirRequest => topics
  topics => [ChangeReplicaDirRequestTopic]
 
ChangeReplicaDirRequestTopic => topic partitions
  topic => str
  partitions => [ChangeReplicaDirRequestPartition]

ChangeReplicaDirRequestPartition => partition dir
  partition => int32
  dir => str

ChangeReplicaDirResponse => topics
  topics => [ChangeReplicaDirResponseTopic]
 
ChangeReplicaDirResponseTopic => topic partitions
  topic => str
  partitions => [ChangeReplicaDirResponsePartition]
 
ChangeReplicaDirResponsePartition => partition error_code
  partition => int32
  error_code => int16

// log_dirs and topics are used to filter the results to include only the specified log_dir/topic. The result is the intersection of both filters.
DescribeDirsRequest => log_dirs topics
  log_dirs => [str]  // If this is null, then all log directories will be queried
  topics => [str] // If this is null, all topics will be queried

DescribeDirsResponse => log_dirs
  log_dirs => [DescribeDirsResponseDirMetadata]

DescribeDirsResponseDirMetadata => error_code path topics
  error_code => int16
  path => str
  topics => [DescribeDirsResponseTopic]
 
DescribeDirsResponseTopic => topic partitions
  topic => str
  partitions => [DescribeDirsResponsePartition]
  
DescribeDirsResponsePartition => partition size log_end_offset is_temporary
  partition => int32
  size => int64
  log_end_offset => int64
  is_temporary => boolean  // True if replica is *.move

Scripts

1) Add kafka-log-dirs.sh which allows user to get list of replicas per log directory on a broker.

{
  "version" : 1,
  "log_dirs" : [
    {
      "is_live" : boolean,
      "path" : str,
      "partitions": [
        {
          "topic" : str, 
          "partition" : int32, 
          "size" : int64,
          "is_temporary" : boolean
        },
        ...
      ]
    },

    ...
  ]
}

{
  "version" : int,
  "partitions" : [
    {
      "topic" : str,
      "partition" : int,
      "replicas" : [int],
      "log_dirs" : [str]    <-- NEW. A log directory can be either "any", or a valid absolute path that begins with '/'
    },
    ...
  ]
}

Note: the quota specified by the argument `–throttle` will be applied to only inter-broker replica reassignment. It does not affect the quota for replica movement between log directories.

3) Add optional argument --timeout to kafka-reassignment-partitions.sh. This is because kafka-reassignment-partitions.sh may need to re-send ChangeReplicaDirRequest to broker if replica hasn't already been created there. The timeout is set to 10 seconds by default.

Changes in Operational Procedures

In this section we describe the expected changes in operational procedures in order to run Kafka with JBOD. Administrators of Kafka cluster need to be aware of these changes before switching from RAID-10 to JBOD.

- Need to load balance across log directories

When running Kafka with RAID-10, we only need to take care of load imbalance across brokers. Administrator can balance load across brokers using the script kafka-reassign-partitions.sh. After switching from RAID-10 to JBOD, we will start to see load imbalance across log directories. In order to address this problem, administrator needs to get the partition assignment and their size per log directory using kafka-log-dirs.sh, determine the reassignment of replicas per log directory per broker, and provide partition to (broker, log_directory) mapping as input to kafka-reassign-partitions.sh to execute the new assignment.

Administrator also needs to be prepared that the need to rebalance across log directories will probably be much more frequent than the need to rebalance across brokers since the capacity of individual disk is likely much smaller than the capacity of existing RAID-10 setup.

Compatibility, Deprecation, and Migration Plan

This KIP is a pure addition. So there is no backward compatibility concern.

The KIP changes the inter-broker protocol. Therefore the migration requires two rolling bounce. In the first rolling bounce we will deploy the new code but broker will still communicate using the existing protocol. In the second rolling bounce we will change the config so that broker will start to communicate with each other using the new protocol.

Test Plan

The new features will be tested through unit, integration, and system tests. In the following we explain the system tests only. 

Rejected Alternatives