Kafka rack awareness

Learn about Kafka rack awareness and how it can be configured for Kafka brokers and clients.

Racks provide information about the physical location of a broker or a client. A Kafka deployment can be made rack aware by configuring rack awareness for the Kafka brokers and clients respectively. Enabling rack awareness can help in hardening your deployment, it provides durability guarantees for your Kafka service, and significantly decreases the chances of data loss.

Rack awareness for Kafka brokers

Learn about Kafka broker rack awareness and how rack aware Kafka brokers behave.

To avoid a single point of failure, instead of putting all brokers into the same rack, it is considered a best practice to spread your Kafka brokers among racks. In cloud environments Kafka brokers located in different availability zones or data centers are usually deployed in different racks. Kafka brokers have built in support for this type of cluster topology and can be configured to be aware of the racks they are in.

If you create, modify, or redistribute a topic in a rack-aware Kafka deployment, rack awareness ensures that replicas of the same partition are spread across as many racks as possible. This limits the risk of data loss if a complete rack fails. Replica assignment will try to assign an equal number of leaders for each broker, therefore, it is advised to configure an equal number of brokers for each rack to avoid uneven load of racks.

For example, assume you have a topic partition with 3 replicas and have the brokers configured in 3 different racks. If rack awareness is enabled, Kafka will try to distribute the replicas among the racks evenly in a round-robin fashion. In the case of this example, this means that Kafka will ensure to spread all replicas among the 3 different racks, significantly decreasing the chances of data loss in case of a rack failure.

Configuring rack awareness for Kafka brokers

Learn how to configure rack awareness for Kafka brokers

Rack awareness is enabled and configured by selecting the Enable Rack Awareness Kafka service property. Once selected, Enable Rack Awareness automatically configures racks for each Kafka broker based on the rack information available in Cloudera Manager.

  • In order for rack awareness to properly function, the brokers in your deployment must be spread across available racks. If all brokers are deployed on the same rack, enabling and configuring rack awareness will not provide you with any benefits.
  • If you previously configured and enabled rack awareness by manually configuring the broker.rack property with Kafka Broker Advanced Configuration Snippet (Safety Valve), ensure that you remove all broker.rack entries from the advanced configuration snippet. The advanced configuration snippet takes precedence over Enable Rack Awareness and overwrites the configuration set by Enable Rack Awareness.
  1. In Cloudera Manager, select the Kafka service.
  2. Go to Configuration.
  3. Find and select the Enable Rack Awareness property.
  4. Click Save Changes.
  5. Restart the Kafka service.
Rack awareness is enabled and configured for the Kafka brokers.
Configure rack awareness for Kafka clients.

Rack awareness for Kafka consumers

Learn about leader fetching, which can be used to make Kafka consumers rack aware

When a Kafka consumer tries to consume a topic partition, it fetches from the partition leader by default. If the partition leader and the consumer are not in the same rack, fetching generates significant cross-rack traffic, which has a number of disadvantages. For example, it can generate high costs and lead to lower consumer bandwidth and throughput.

For this reason, it is possible to provide the client with rack information so that the client fetches from the closest replica instead of the leader. If the configured closest replica does not exist (there is no replica for the needed partition in the configured closest rack), it uses the partition leader. This feature is called follower fetching and it can be used to mitigate the costs generated by cross-rack traffic or increase consumer throughput.

Configuring rack awareness for Kafka consumers

Learn how to make Kafka consumers rack aware by enabling and configuring follower fetching.

Kafka Consumers can be made rack aware enabling follower fetching for your Kafka deployment. Follower fetching can be enabled by configuring replica.selector.class property for the broker and configuring the client.rack property in the consumer’s configuration. The replica.selector.class property is not directly available for configuration in Cloudera Manager and you must use an advanced security snippet to configure it.

Ensure that brokers have rack awareness enabled. For more information, see Configuring rack awareness for Kafka brokers.
  1. In Cloudera Manager, select the Kafka service.
  2. Go to Configuration.
  3. Find the Kafka Broker Advanced Configuration Snippet (Safety Valve) for kafka.properties property.
  4. Add the following configuration entry to the advanced configuration snippet.
    replica.selector.class=org.apache.kafka.common.replica.RackAwareReplicaSelector
  5. Click Save Changes.
  6. Restart the Kafka service.
  7. Add the following to your consumer configuration.
    client.rack=[***RACK ID***]
    
    Replace [***RACK ID***] with the ID of the rack that the consumer is running in. The rack ID should match one of the rack ID’s you configured for the brokers. Ensure that you configure each consumer and add its corresponding rack ID. If the consumer is deployed in a rack with no brokers, specify the rack ID of a broker that is closest to the rack that the consumer is running in.
Follower fetching is enabled for the Kafka deployment. Kafka consumers are now rack aware and attempt to consume from the replica that is in the closet rack instead of consuming from the replica leader.

Rack awareness for Kafka producers

Learn about rack awareness for Kafka producers.

Compared to brokers or consumers, there are no producer specific rack-awareness features or toggles that you can enable. However, in a deployment where rack awareness is an important factor, you can make configuration changes so that producers make use of rack awareness and have messages replicated to multiple racks.

Specifically, Cloudera recommends a configuration that ensures that the produced messages are replicated to at least two different racks before the messages are considered to be successful. This involves configuring acks to all in the producer configuration and setting up min.insync.replicas for the topics in a way that ensures a minimum of two racks get the message before the produce request is considered successful.

The configuration of the acks property is fixed. If you want to make your producers rack aware, the property must be set to all no matter the cluster topology or deployment.

The exact value you set for min.insync.replicas on the other hand depends on your cluster deployment. Specifically, the min.insync.replicas value you must set will depend on the number of racks, brokers, and the replication factor of your topics. Cloudera recommends that you exercise caution and review the following examples to better understand configuration.

For example, consider a Cloudera recommended deployment that has three racks with topic replication set to 3. In a case like this, a min.insync.replicas setting of 2 ensures that you always have data written to at least two different racks even if one replica is lagging.

Understand however, that setting min.insync.replicas to 2 does not universally work for all deployments and may not guarantee that you always have your produced message in at least two racks. Configuration depends on the number of replicas, as well as the number of racks and brokers.

If you have more replicas and brokers than racks, you will have at least two replicas in the same rack. In a case like this, setting min.insync.replicas to 2 is not sufficient, a partition might become unavailable under certain circumstances.

For example, assume you have three racks with topic replication factor set to 4, meaning that there are a total of four replicas. Additionally, assume that only two of the replicas are in the in-sync replica set (ISR), the leader and one of the followers, and both are located in the same rack. The other two replicas are lagging. Unclean leader election is disabled to avoid data loss.

When the leader and the in-sync follower (located in the same rack) successfully append a produced message to the log, message production is considered successful. The leader does not wait for acknowledgement from the lagging replicas. This is because acks=all only guarantees that the leader waits for the replicas that are in the ISR (including itself). This means that while the latest messages are available on two brokers, both are located on the same rack. If the rack goes down at the same time or shortly after production is successful, the partition will become unavailable as only the two lagging replicas remain, which cannot become leaders.

In cases like this, a correct value for min.insync.replicas would be 3 instead of 2 as three ISRs would guarantee that messages are produced to at least two different racks.

Configuring rack awareness for Kafka producers

Learn how to enable and configure rack awareness for Kafka producers.

Enabling rack awareness for Kafka producers involves configuring your Kafka deployment in a way that ensures that producers commit messages to at least two separate brokers that are deployed on different racks. This can be done by configuring your producers to provide the highest available guarantee on message delivery and configuring min.insync.replicas for your topics.

Ensure that brokers have rack awareness enabled. For more information, see Configuring rack awareness for Kafka brokers.
  1. Add the following to your producer configuration.
    acks=all

    This is the default configuration for producer version 3.0.0 or later. As a result, configuring this property might not be required.

  2. Configure min.insync.replicas for the produced topics to a value that ensures the desired number of racks (minimum of 2) get the message before the produce request is considered successful.
Rack awareness for Kafka producers is configured. Producers will now ensure that messages are produced to at least 2 (or more) of the available racks.