apache/seatunnel

[Docs][Core] Add Javadoc to CheckpointCoordinator (checkpoint state machine)

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#10545 opened on Feb 28, 2026

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help wanted

Description

Sub-issue of #10533

Scope

CheckpointCoordinator implements Zeta's distributed checkpoint protocol: barrier injection, task ACK collection, state persistence, and restore. At 1 174 lines it has only ~21 Javadoc lines (~1.8 % coverage). The state machine and field semantics are entirely undocumented.

File

seatunnel-engine/seatunnel-engine-server/src/main/java/org/apache/seatunnel/engine/server/checkpoint/CheckpointCoordinator.java

Methods to document

Method Line What to explain
reportedTask(TaskReportStatusOperation) 256 Entry point for task status reports; maps WAITING_RESTORE → restoreTaskState, READY_START → allTaskReady
restoreTaskState(TaskLocation) 306 Looks up latest CompletedCheckpoint; distributes per-action states to the target task; parallelism remapping; COORDINATOR_INDEX special case for enumerator state
allTaskReady() 346 Called once all tasks report READY_START; triggers first checkpoint cycle or immediately completes if no checkpoint is needed
tryTriggerPendingCheckpoint(CheckpointType) 500 Enforces minimum checkpoint interval; guards against concurrent pending checkpoints; schedules barrier injection
acknowledgeTask(TaskAcknowledgeOperation) 905 Accumulates per-task ACKs inside the PendingCheckpoint; calls completePendingCheckpoint when all tasks have ACKed
completePendingCheckpoint(CompletedCheckpoint) 942 Persists checkpoint to storage; updates runningJobStateIMap; notifies all tasks; prunes oldest checkpoint from history
cleanPendingCheckpoint(CheckpointCloseReason) 853 Abort path: maps close reason to failure vs cancel; clears all pending checkpoints
readyToClose(TaskLocation) 424 Tracks source-complete signals; once all sources are ready-to-close, triggers final savepoint barrier
notifyTaskStart() 397 Broadcasts task-start event to all pipeline tasks; part of startup sequencing
scheduleSchemaChangeBeforeCheckpoint() 1111 Initiates the DDL-before-checkpoint phase of the schema-evolution protocol

Fields to document (add field-level Javadoc)

Field Line What to explain
readyToCloseStartingTask 119 Source tasks that have finished emitting data and are waiting for the final barrier
readyToCloseIdleTask 120 Source tasks currently in idle (no-data) state awaiting close
closedIdleTask 121 Idle tasks that have fully closed; used to gate coordinator completion
schemaChanging 136 Guards normal checkpoint triggers during an in-progress DDL schema-change event
isAllTaskReady 143 Set to true once; prevents duplicate allTaskReady calls on subsequent checkpoint cycles

How to contribute

  1. Fork the repo and create a branch
  2. Add Javadoc to the methods and fields above
  3. Run ./mvnw spotless:apply and ./mvnw -q -DskipTests verify
  4. Open a PR with title: [Docs][Core] Add Javadoc to CheckpointCoordinator

Contributor guide

Tech stack
java
Domain
backend
Issue type
documentation
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Java basicsUnderstanding of distributed checkpoint protocols
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
75
Research direction
Read the file 'CheckpointCoordinator.java' in the module seatunnel engine server. Understand the checkpoint state machine from the existing code and the table of methods and fields provided in the issue. Add Javadoc comments following the style of any existing Javadoc in the same file. After writing, run `./mvnw spotless:apply` to format and `./mvnw q DskipTests verify` to ensure no build errors. No external changes are needed; the issue is well scoped.