Chapter 25. Rack schema reference Used in: KafkaBridgeSpec , KafkaClusterSpec , KafkaConnectSpec , KafkaMirrorMaker2Spec Full list of Rack schema properties The rack option configures rack awareness. A rack can represent an availability zone, data center, or an actual rack in your data center. The rack is configured through a topologyKey . topologyKey identifies a label on OpenShift nodes that contains the name of the topology in its value. An example of such a label is topology.kubernetes.io/zone (or failure-domain.beta.kubernetes.io/zone on older OpenShift versions), which contains the name of the availability zone in which the OpenShift node runs. You can configure your Kafka cluster to be aware of the rack in which it runs, and enable additional features such as spreading partition replicas across different racks or consuming messages from the closest replicas. For more information about OpenShift node labels, see Well-Known Labels, Annotations and Taints . Consult your OpenShift administrator regarding the node label that represents the zone or rack into which the node is deployed. 25.1. Spreading partition replicas across racks When rack awareness is configured, Streams for Apache Kafka will set broker.rack configuration for each Kafka broker. The broker.rack configuration assigns a rack ID to each broker. When broker.rack is configured, Kafka brokers will spread partition replicas across as many different racks as possible. When replicas are spread across multiple racks, the probability that multiple replicas will fail at the same time is lower than if they would be in the same rack. Spreading replicas improves resiliency, and is important for availability and reliability. To enable rack awareness in Kafka, add the rack option to the .spec.kafka section of the Kafka custom resource as shown in the example below. Example rack configuration for Kafka apiVersion: kafka.strimzi.io/v1beta2 kind: Kafka metadata: name: my-cluster spec: kafka: # ... rack: topologyKey: topology.kubernetes.io/zone # ... Note The rack in which brokers are running can change in some cases when the pods are deleted or restarted. As a result, the replicas running in different racks might then share the same rack. Use Cruise Control and the KafkaRebalance resource with the RackAwareGoal to make sure that replicas remain distributed across different racks. When rack awareness is enabled in the Kafka custom resource, Streams for Apache Kafka will automatically add the OpenShift preferredDuringSchedulingIgnoredDuringExecution affinity rule to distribute the Kafka brokers across the different racks. However, the preferred rule does not guarantee that the brokers will be spread. Depending on your exact OpenShift and Kafka configurations, you should add additional affinity rules or configure topologySpreadConstraints for both ZooKeeper and Kafka to make sure the nodes are properly distributed accross as many racks as possible. For more information see Configuring pod scheduling . 25.2. Consuming messages from the closest replicas Rack awareness can also be used in consumers to fetch data from the closest replica. This is useful for reducing the load on your network when a Kafka cluster spans multiple datacenters and can also reduce costs when running Kafka in public clouds. However, it can lead to increased latency. In order to be able to consume from the closest replica, rack awareness has to be configured in the Kafka cluster, and the RackAwareReplicaSelector has to be enabled. The replica selector plugin provides the logic that enables clients to consume from the nearest replica. The default implementation uses LeaderSelector to always select the leader replica for the client. Specify RackAwareReplicaSelector for the replica.selector.class to switch from the default implementation. Example rack configuration with enabled replica-aware selector apiVersion: kafka.strimzi.io/v1beta2 kind: Kafka metadata: name: my-cluster spec: kafka: # ... rack: topologyKey: topology.kubernetes.io/zone config: # ... replica.selector.class: org.apache.kafka.common.replica.RackAwareReplicaSelector # ... In addition to the Kafka broker configuration, you also need to specify the client.rack option in your consumers. The client.rack option should specify the rack ID in which the consumer is running. RackAwareReplicaSelector associates matching broker.rack and client.rack IDs, to find the nearest replica and consume from it. If there are multiple replicas in the same rack, RackAwareReplicaSelector always selects the most up-to-date replica. If the rack ID is not specified, or if it cannot find a replica with the same rack ID, it will fall back to the leader replica. Figure 25.1. Example showing client consuming from replicas in the same availability zone You can also configure Kafka Connect, MirrorMaker 2 and Kafka Bridge so that connectors consume messages from the closest replicas. You enable rack awareness in the KafkaConnect , KafkaMirrorMaker2 , and KafkaBridge custom resources. The configuration does does not set affinity rules, but you can also configure affinity or topologySpreadConstraints . For more information see Configuring pod scheduling . When deploying Kafka Connect using Streams for Apache Kafka, you can use the rack section in the KafkaConnect custom resource to automatically configure the client.rack option. Example rack configuration for Kafka Connect apiVersion: kafka.strimzi.io/v1beta2 kind: KafkaConnect # ... spec: # ... rack: topologyKey: topology.kubernetes.io/zone # ... When deploying MirrorMaker 2 using Streams for Apache Kafka, you can use the rack section in the KafkaMirrorMaker2 custom resource to automatically configure the client.rack option. Example rack configuration for MirrorMaker 2 apiVersion: kafka.strimzi.io/v1beta2 kind: KafkaMirrorMaker2 # ... spec: # ... rack: topologyKey: topology.kubernetes.io/zone # ... When deploying Kafka Bridge using Streams for Apache Kafka, you can use the rack section in the KafkaBridge custom resource to automatically configure the client.rack option. Example rack configuration for Kafka Bridge apiVersion: kafka.strimzi.io/v1beta2 kind: KafkaBridge # ... spec: # ... rack: topologyKey: topology.kubernetes.io/zone # ... 25.3. Rack schema properties Property Property type Description topologyKey string A key that matches labels assigned to the OpenShift cluster nodes. The value of the label is used to set a broker's broker.rack config, and the client.rack config for Kafka Connect or MirrorMaker 2.

Commands:
apiVersion: kafka.strimzi.io/v1beta2 kind: Kafka metadata: name: my-cluster spec: kafka: # rack: topologyKey: topology.kubernetes.io/zone #
apiVersion: kafka.strimzi.io/v1beta2 kind: Kafka metadata: name: my-cluster spec: kafka: # rack: topologyKey: topology.kubernetes.io/zone config: # replica.selector.class: org.apache.kafka.common.replica.RackAwareReplicaSelector #
apiVersion: kafka.strimzi.io/v1beta2 kind: KafkaConnect spec: # rack: topologyKey: topology.kubernetes.io/zone #
apiVersion: kafka.strimzi.io/v1beta2 kind: KafkaMirrorMaker2 spec: # rack: topologyKey: topology.kubernetes.io/zone #
apiVersion: kafka.strimzi.io/v1beta2 kind: KafkaBridge spec: # rack: topologyKey: topology.kubernetes.io/zone #