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Restarting Kafka Connect connectors and tasks

This proposal adds support for restarting Kafka Connect connectors and connector tasks via annotations added to the KafkaConnector and KafkaMirrorMaker2 custom resources and via overridable default behaviour of the KafkaConnector and KafkaMirrorMaker2 operators.

Current situation

When Kafka Connect connectors or tasks throw uncaught exceptions, the Kafka Connect framework stops the connector/task and puts it into a FAILED state. The Kafka Connect REST API includes two endpoints to restart a connector and a task:

  • POST /connectors/{connector-name}/restart
  • POST /connectors/{connector-name}/tasks/{task-id}/restart Sending a request to one of these endpoints will cause the Connect framework to restart the connector/task.

Currently, the Strimzi operators do not have any special handling for connectors or tasks that are in the FAILED state. Users must make HTTP POST requests to the Connect restart endpoints if they want to restart a connector/task.

A connector/task does not need to be in the FAILED state to be restarted via the Connect REST API; a connector or task can be restarted at any time. Restarting a connector or task causes the Connect framework to re-initialise the connector/task and reactivate message processing. Note that restarting a connector does not restart its tasks.

The default network policies created by the Strimzi operators restrict incoming Connect REST API calls to only allow access from the operator pod, so users must add supplementary network policies to be able to make calls to the restart REST APIs.

Motivation

Enabling users to restart connectors and tasks means there are almost (1) no further Connect REST APIs that users must call to interact with connectors/tasks when using CRs that create connectors. This means the CRs and their operators can exclusively be used to manage connectors and tasks - Strimzi users do not have to call the Connect REST API directly. As a corollary, this means the Connect REST listener does not need to be available to users; it could be secured so that only the operator can interact with the REST API.

(1) - Kafka 2.5 introduced the GET /connectors/{connector-name}/topics Connect endpoint to list the topics that a connector has used since the connector was created or the list was reset. Kafka 2.5 also introduced the PUT /connectors/{connector-name}/topics/reset Connect endpoint to clear the list of topics that a connector has used.

Proposal

This proposal adds two new separate but related features:

  1. New annotations that cause the operator to restart connectors or tasks. They can be applied to the KafkaConnector, and the KafkaMirrorMaker2 custom resources. The annotation acts as a trigger for a single restart call by the operator, and is removed from the CR when the restart REST API call is successfully called.

  2. New default behaviour in the KafkaConnector and KafkaMirrorMaker2 operators that automatically attempts to restart connectors and tasks if they are in the FAILED state. The KafkaConnector and KafkaMirrorMaker2 CRs can use a new field in the spec to disable the automatic restarting behaviour if required.

These two features could be delivered separately as they do not rely on each other.

strimzi.io/restart annotation

An annotation named strimzi.io/restart can be applied to the KafkaConnector custom resource. The CR it is applied to determines the connector that is restarted. Once the operator successfully calls the restart REST API endpoint, the CR is patched to remove the annotation. If the restart REST API endpoint does not return a successful (HTTP 200 OK) response, the operator will continue to attempt to call the restart REST API endpoint once per reconcile cycle.

When applied to a KafkaConnector CR, the connector defined by the CR is restarted. Any value can be assigned to the annotation. For example:

apiVersion: strimzi.io/v1alpha1
kind: KafkaConnector
metadata:
  name: my-source-connector
  annotations:
    strimzi.io/restart: "true"
  ...

strimzi.io/restart-connector annotation

An annotation named strimzi.io/restart-connector can be applied to the KafkaMirrorMaker2 custom resource. The value of the annotation and the CR it is applied to determines the connector that is restarted. Once the operator successfully calls the restart REST API endpoint, the CR is patched to remove the annotation. If the restart REST API endpoint does not return a successful (HTTP 200 OK) response, the operator will continue to attempt to call the restart REST API endpoint once per reconcile cycle.

When applied to a KafkaMirrorMaker2 CR, the annotation value contains the name of the connector that is restarted. For example:

apiVersion: strimzi.io/v1alpha1
kind: KafkaMirrorMaker2
metadata:
  name: my-mm2-cluster
  annotations:
    strimzi.io/restart: "source-cluster->target-cluster.MirrorSourceConnector"
  ...

strimzi.io/restart-task annotation

An annotation named strimzi.io/restart-task can be applied to the KafkaConnector custom resources. The value of the annotation and the CR it is applied to determines the connector task that is restarted. Once the operator successfully calls the restart REST API endpoint, the CR is patched to remove the annotation. If the restart REST API endpoint does not return a successful (HTTP 200 OK) response, the operator will continue to attempt to call the restart REST API endpoint once per reconcile cycle.

When applied to a KafkaConnector CR, the annotation value contains the ID of the task that is restarted. For example:

apiVersion: strimzi.io/v1alpha1
kind: KafkaConnector
metadata:
  name: my-source-connector
  annotations:
    strimzi.io/restart-task: "0"
  ...

strimzi.io/restart-connector-task annotation

An annotation named strimzi.io/restart-connector-task can be applied to the KafkaMirrorMaker2 custom resource. The value of the annotation and the CR it is applied to determines the connector task that is restarted. Once the operator successfully calls the restart REST API endpoint, the CR is patched to remove the annotation. If the restart REST API endpoint does not return a successful (HTTP 200 OK) response, the operator will continue to attempt to call the restart REST API endpoint once per reconcile cycle.

When applied to a KafkaMirrorMaker2 CR, the annotation value contains the name of the connector and the ID of the task that is restarted, delimited by a : character. For example:

apiVersion: strimzi.io/v1alpha1
kind: KafkaMirrorMaker2
metadata:
  name: my-mm2-cluster
  annotations:
    strimzi.io/restart-task: "source-cluster->target-cluster.MirrorSourceConnector:2"
  ...

Automatically restarting FAILED connectors and tasks

The KafkaConnector and KafkaMirrorMaker2 operators will have new default behaviour that automatically restarts connectors or tasks that are in the FAILED state. For each connector or task that is in the FAILED state, the operator will call the Connect REST API restart endpoint once per reconcile cycle to attempt to restart the connector/task.

To disable the new default behaviour a new field in the KafkaConnector spec named autoRestart with an enabled field value of false can be applied. This will be useful when users know that external factors will cause connectors/tasks to be in the FAILED state and they do not want the operator attempting automatic restarts. It also means users can keep the current behaviour of the KafkaConnector and KafkaMirrorMaker2 operators if they wish.

For example, the following KafkaConnector CR disables the automatic restarting of the connector and it's tasks when they are in the FAILED state:

apiVersion: strimzi.io/v1alpha1
kind: KafkaConnector
metadata:
  name: my-source-connector
spec:
  autoRestart:
    enabled: false
  ...

Similarly, a new field in the KafkaMirrorMaker2 spec named autoRestartConnectorsAndTasks with an enabled field value of false can be applied, to disable auto-restarting connectors and tasks.

For example, the following KafkaMirrorMaker2 CR disables the automatic restarting of the connector and it's tasks when they are in the FAILED state:

apiVersion: strimzi.io/v1alpha1
kind: KafkaMirrorMaker2
metadata:
  name: my-mm2-cluster
spec:
  autoRestartConnectorsAndTasks:
    enabled: false
  ...

Repeated automatic restarts

Connectors and tasks that are in the FAILED state and are automatically restarted might cause other connectors and tasks to be rebalanced across the Kafka Connect cluster. If the connectors and tasks repeatedly transition to the FAILED state, repeated auto-restarts could be detrimental to the health of the Kafka Connect cluster. A back-off algorithm is applied to automatic restarts of each connector and it's tasks, incrementing the time between automatic restarts by 2 minutes, up to a maximum of 30 minutes, at which point no further automatic restarts are attempted, and the connector or tasks are left in the FAILED state.

For example, if a connector repeatedly transitions to the FAILED state, it is automatically restarted at minute 0, 2, 6, 12, 20 and 30.

Status of automatic restarts

A new field in the resource status records the number of times that connectors and tasks have been auto-restarted. The restart count is incremented every time the restart REST API is invoked and record the time when it was last auto-restarted:

In the KafkaConnector resource, the field is named autoRestart. For example:

apiVersion: strimzi.io/v1alpha1
kind: KafkaConnector
...
status:
  autoRestart:
    count: 2
    lastRestartTimestamp: "2020-07-13T21:59:09.832Z"

In the KafkaMirrorMaker2 resource, the field is named autoRestartConnectorsAndTasks and records the number of times that each connector has been auto-restarted. For example:

apiVersion: strimzi.io/v1alpha1
kind: KafkaConnector
...
status:
  autoRestartConnectorsAndTasks:
  - count: 2
    connectorName: "source-cluster->target-cluster.MirrorSourceConnector"
    lastRestartTimestamp: "2020-07-13T21:59:09.832Z"
  - count: 1
    connectorName: "source-cluster->target-cluster.MirrorCheckpointConnector"
    lastRestartTimestamp: "2020-07-14T20:55:34.678Z"

Compatibility

Automatically restarting connectors and tasks in the FAILED state means that the default behaviour of the KafkaConnector and KafkaMirrorMaker2 operators will be slightly different. This can be overridden using the spec.autoRestart and spec.autoRestartConnectorsAndTasks fields if desired.

Rejected Alternatives

The default behaviour of the KafkaConnector and KafkaMirrorMaker2 operators could be left as-is, and the new spec.autoRestart and spec.autoRestartConnectorsAndTasks fields could be applied to explicitly set to true to enable the automatic restarting behaviour. It was felt that the most useful 'Kubernetes-native' behaviour would be to attempt automatic restarts, in a similar way to how failing pods are recreated by their parent deployments.