Implementation:Apache Kafka CoordinatorLoaderImpl Load

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

Overview

Concrete tool for loading coordinator state from partition logs provided by CoordinatorLoaderImpl.

Description

The CoordinatorLoaderImpl.load method reads all records from a coordinator topic-partition and replays them through a CoordinatorPlayback instance. It schedules the loading on an internal single-threaded scheduler, reads records in batches using BufferSupplier, deserializes them, and periodically commits progress. Returns a CompletableFuture<LoadSummary> with loading statistics.

Usage

Called by the CoordinatorRuntime after a partition transitions to LOADING state. Not invoked directly by user code.

Code Reference

Source Location

Repository: Apache Kafka
File: coordinator-common/src/main/java/org/apache/kafka/coordinator/common/runtime/CoordinatorLoaderImpl.java
Lines: L104-351

Signature

public CompletableFuture<LoadSummary> load(
    TopicPartition tp,
    CoordinatorPlayback<T> coordinator
)

Import

import org.apache.kafka.coordinator.common.runtime.CoordinatorLoaderImpl;
import org.apache.kafka.coordinator.common.runtime.CoordinatorLoader.LoadSummary;

I/O Contract

Inputs

Name	Type	Required	Description
tp	TopicPartition	Yes	Partition to load records from
coordinator	CoordinatorPlayback<T>	Yes	Replay target for deserialized records

Outputs

Name	Type	Description
CompletableFuture<LoadSummary>	future	Completes with startOffset, endOffset, numRecords, numBatches

Usage Examples

// Internal usage within CoordinatorRuntime
CoordinatorLoaderImpl<Record> loader = new CoordinatorLoaderImpl<>(
    time, partitionLogSupplier, partitionLogEndOffsetSupplier,
    deserializer, loadBufferSize, DEFAULT_COMMIT_INTERVAL_OFFSETS
);

CompletableFuture<LoadSummary> future = loader.load(
    new TopicPartition("__consumer_offsets", 0),
    coordinatorPlayback
);

future.thenAccept(summary ->
    log.info("Loaded {} records in {} batches from offset {} to {}",
        summary.numRecords(), summary.numBatches(),
        summary.startOffset(), summary.endOffset()));

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