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Kuberentes Kafka

This project contains tools to facilitate the deployment of Apache Kafka on Kubernetes using StatefulSets. It requires Kubernetes 1.7 or greater.netes.

Limitations

  1. Persistent Volumes must be used. emptyDirs will likely result in a loss of data.
  2. Storage media I/O isolation is not generally possible at this time. Consider using Pod Anti-Affinity rules to place noisy neighbors on separate Nodes.

Kafka Docker Image

The docker directory contains the Makefile for a Docker image that runs a Kafka broker.

Manifests

The manifests directory contains server Kubernetes manifests that can be used for demonstration purposes or production deployments. If you primarily deploy manifests directly you can modify any of these to fit your use case.

ZooKeeper

Kafka requires an installation of Apache Zookeeper for broker configuration storage and coordination. Manifests and helm charts for deploying an ensemble can be found here. It is recommended to deploy a separate small, ZooKeeper ensemble for each Kafka cluster instead of using a large multi-tennant ensemble.

Configuration and Administration

This section contains common configuration and administration items for running Kafka on Kubernetes.

OS Image tuning

For production use, it is important to configure the base OS image to allow for a sufficient number of file descriptors for your workload.

  • For each broker, (partition) * (partition_size/segment_size) determines the number of files the Broker will have open at any give time. You must ensure that this will not result in the Broker process dying because it has exhausted its allowable number of file descriptors.

Cluster Size

The size of the Kafka cluster, the number of brokers, is controlled by the .spec.replicas field of the StatefulSet. You should ensure that the size of the cluster supports your planned throughput and latency requirements for all topics. If the size of the cluster gets too large, you should consider segregating your topics into multiple smaller clusters.

CPU

Typical production Kafka broker deployments run on dual processor Xeon's with multiple hardware threads per core. However, CPU is unlikely to be your bottleneck. An 8 CPU deployment should be more than sufficient for good performance. You should start by simulating your workload with 2-4 CPUs and titrating up from there. The amount of CPU is controlled by the StatefulSet's spec.template.containers[0].resources.cpus.

Memory

Kafka utilizes the OS page cache heavily to buffer data. To fully understand the interaction of Kafka and Linux containers you should read the Kafka file system design and memory cgroups documentation. In particular, its is important to understand the accounting and isolation offered for the page cache for a mem cgroup. If your primary concern is isolation and performance you should do the following.

  1. Determine the number of seconds of data you want to buffer t (time).
  2. Determine the total write throughput of the deployment tp (storage/time).
  3. tp * t gives the memory storage requirement that you should reserve. This should be set as the memory request for the container using the StatefulSet's .spec.template.containers[0].resources.mem. You must also account for the JVM heap and process memory. Adding an extra 2-4 Gib is generally adequate.

The JVM heap size of the Kafka brokers is controlled by KAFKA_HEAP_OPTS environment variable. "-Xms=2G -Xmx=2G" is sufficient for most deployments.

Disk

Disk throughput is the most common bottleneck that users encounter with Kafka. Given that Persistent Volumes are backed by network attached storage, the throughput is, in most cases, capped on a per Node basis without respect to the number of Persistent Volumes that are attached to the Node. For instance, if you are deploying Kafka onto a GKE or GCP based Kubernetes cluster, and if you use the standard PD type, your maximum sustained per instance throughput is 120 MB/s (Write) and 180 MB/s (Read). If you have multiple applications, each with a Persistent Volume mounted, these numbers represent the total achievable throughput. If you find that you have contention you should consider using Pod Anti-Affinity rules to ensure that noisy neighbors are not collocated on the same Node. You can control the amount of disk allocated by your provisioner using the .spec.volume.volumeClaimTemplates[0].resources.requests.storage field.

Networking

The kafka-hs Headless Service must specify a server port that corresponds to the server port in the spec.template.containers[0].ports field of the kafka StatefulSet. The --override listeners port must also correspond

Spreading

The Kafka Pod in the StatefulSet's PodTemplateSpec contains a Pod Anti-Affinity and a Pod Anti-Affinity rule.

    affinity:
        podAntiAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
            - labelSelector:
                matchExpressions:
                  - key: "app"
                    operator: In
                    values: 
                    - kafka
              topologyKey: "kubernetes.io/hostname"
        podAffinity:
          preferredDuringSchedulingIgnoredDuringExecution:
             - weight: 1
               podAffinityTerm:
                 labelSelector:
                    matchExpressions:
                      - key: "app"
                        operator: In
                        values: 
                        - zk
                 topologyKey: "kubernetes.io/hostname"

The Pod Anti-Affinity rule ensures that two Kafka Broker's will never be launched on the same Node. This isn't strictly necessary, but it helps provide stronger availability guarantees in the presence of Node failure, and it helps alleviate disk throughput bottlenecks. The Pod Affinity rule attempts to collocate Kafka and ZooKeeper on the same Nodes. You will likely have more Kafka brokers than ZooKeeper servers, but the Kubernetes scheduler will attempt to, where possible, collocate Kafka brokers and ZooKeeper servers while respecting the hard spreading enforced by the Pod Anti-Affinity rule. This optimization attempts to minimize the amount of network I/O between the ZooKeeper ensemble and the Kafka cluster. However, if disk contention becomes an issue, it is equally valid to express a Pod Anti-Affinity rule to ensure that ZooKeeper servers and Kafka brokers are not scheduled onto the same node.

Log Retention

Kafka will periodically truncate or compact logs is a partition to reclaim disk space. You can configure the log retention using --override log.retention.bytes and --override log.retention.hours parameters passed to the StatefulSet's Pods' container's command. The default will retain logs for 168 hours and never purge messages due to log size. You should adjust this based on the desired retention and expected load for your cluster.

Kafka Application Logging

Kafka's application logs are written to standard out so they can be captured by the default Kuberentes logging infrastructure (as is considered to be the best practice for containerized applications). The logging level can be controlled by the KAFKA_OPTS environment variable. Setting its value to "-Dlogging.level=", where level is one of INFO, DEBUG, WARNING, ERROR, or FATAL controls the logging threshold

Readiness and Liveness

The Liveness of a broker is decided by whether or not the JVM process running the broker is still alive. Readiness is decided by a readiness check to determine if the application can accept requests.

readinessProbe:
  exec:
   command:
    - sh
    - -c
    - "/opt/kafka/bin/kafka-broker-api-versions.sh --bootstrap-server=localhost:9093"