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Kafka

The Challenge

Consider as an example: attempting to run Kafka on Kubernetes. When running a Kafka cluster, producer and consumer groups for topics are split into partitions which are replicated across a certain number of Kafka nodes. In Kubernetes, the concept of a "Kafka node" maps to a Pod. As such, we are faced with a dilemma as to when it is safe to terminate a Pod. On one hand, we need to ensure that producer and consumer groups are always available; if a partition's available replicas reduce to zero, then Kafka will suffer data loss in that partition, and data resilience will have been disrupted by the Kubernetes cluster operation. This is unacceptable in production clusters.

Proposed solutions that attempt to solve this, like using a PodDisruptionBudget or strimzi's KafkaRoller, implicitly assume the continued availability of the underlying Kubernetes nodes. Readiness checks are simply insufficient to express both that other pods in the StatefulSet should not be terminated (i.e. so that the PodDisruptionBudget will act) and that the Pod should continue to receive traffic (as it is still the leader of record for other partitions that are not currently under-replicated). As a result, a naive cluster operator draining production nodes can and will cause data loss.

The Solution

The Prometheus exporter for Kafka exposes partition health in two metrics: kafka_server_replicamanager_underreplicatedpartitions_value and kafka_server_replicamanager_offlinereplicacount_value. Writing a Prometheus alert to notify when a cluster is unhealthy is then as simple as writing the following two alerts:

- alert: KafkaUnderreplicatedPartitions
  expr: sum(kafka_server_replicamanager_underreplicatedpartitions_value) by (job) > 0
  labels:
    severity: warning
  annotations:
    summary: Kafka cluster {{$labels.job}} has underreplicated partitions
    description: The Kafka cluster {{$labels.job}} has {{$value}} underreplicated partitions
- alert: KafkaOfflinePartitions
  expr: sum(kafka_server_replicamanager_underreplicatedpartitions_value) by (job) > 0
  labels:
    severity: critical
  annotations:
    summary: Kafka cluster {{$labels.job}} has offline partitions
    description: The Kafka cluster {{$labels.job}} has {{$value}} offline partitions

When either one of these alerts are firing, the prometheus-alert-readiness pod will respond as NotReady and prevent the cluster tooling from draining any Kafka nodes, and therefore preventing the cluster tooling from evicting any additional Kafka pods. When the number of underreplicated partitions returns to zero, i.e. all partitions in the Kafka cluster are fully replicated again, then the prometheus-alert-readiness pod will respond as Ready and allow the cluster tooling to proceed.