Implementation:Apache Kafka CoordinatorTimerImpl Schedule

Knowledge Sources	Apache Kafka
Domains	Distributed_Systems, Scheduling
Last Updated	2026-02-09 12:00 GMT

Overview

Concrete tool for scheduling and cancelling timed operations provided by CoordinatorTimerImpl.

Description

The CoordinatorTimerImpl manages a map of TimerTask objects keyed by unique string identifiers. The schedule method creates a TimerTask that, when it expires, schedules a write operation through the runtime's scheduler. The cancel method removes and cancels a previously scheduled timer. Overriding a timer by scheduling with the same key automatically cancels the previous one.

Usage

Used internally by coordinator shards to schedule session timeouts, heartbeat deadlines, and delayed operations.

Code Reference

Source Location

Repository: Apache Kafka
File: coordinator-common/src/main/java/org/apache/kafka/coordinator/common/runtime/CoordinatorTimerImpl.java
Lines: L58-89 (schedule), L152-155 (cancel)

Signature

public void schedule(
    String key,
    long delay,
    TimeUnit unit,
    boolean retry,
    TimeoutOperation<U> operation
)

public void cancel(String key)

Import

import org.apache.kafka.coordinator.common.runtime.CoordinatorTimerImpl;
import java.util.concurrent.TimeUnit;

I/O Contract

Inputs

Name	Type	Required	Description
key	String	Yes	Unique timer identifier
delay	long	Yes	Delay before execution
unit	TimeUnit	Yes	Time unit for the delay
retry	boolean	Yes	Retry on failure
operation	TimeoutOperation	Yes	Operation to execute on timeout

Outputs

Name	Type	Description
schedule	void	Timer registered; fires write operation on expiration
cancel	void	Timer cancelled if it exists

Usage Examples

// Schedule a session timeout
timer.schedule(
    "session-timeout-" + memberId,
    30000, TimeUnit.MILLISECONDS,
    true,  // retry on failure
    () -> coordinator.expireSession(memberId)
);

// Cancel when member heartbeats
timer.cancel("session-timeout-" + memberId);

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