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https://issues.apache.org/jira/browse/FLINK-3257?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15941700#comment-15941700
]
ASF GitHub Bot commented on FLINK-3257:
---------------------------------------
Github user StephanEwen commented on a diff in the pull request:
https://github.com/apache/flink/pull/1668#discussion_r108034229
--- Diff:
flink-streaming-java/src/main/java/org/apache/flink/streaming/runtime/tasks/StreamIterationTail.java
---
@@ -64,6 +70,22 @@ public void init() throws Exception {
super.init();
}
+ @Override
+ protected boolean performCheckpoint(CheckpointMetaData
checkpointMetaData, CheckpointOptions checkpointOptions, CheckpointMetrics
checkpointMetrics) throws Exception {
+ LOG.debug("Starting checkpoint {} on task {}",
checkpointMetaData.getCheckpointId(), getName());
+
+ synchronized (getCheckpointLock()) {
+ if (isRunning()) {
+ dataChannel.put(new Either.Right(new
CheckpointBarrier(checkpointMetaData.getCheckpointId(),
checkpointMetaData.getTimestamp(), checkpointOptions)));
+
getEnvironment().acknowledgeCheckpoint(checkpointMetaData.getCheckpointId(),
checkpointMetrics);
--- End diff --
Can the `IterationTailTask` contain operators as well, or is it always a
task without operators? If it has operators, we cannot immediately acknowledge
here, but need to delegate to superclass checkpoint method instead.
> Add Exactly-Once Processing Guarantees in Iterative DataStream Jobs
> -------------------------------------------------------------------
>
> Key: FLINK-3257
> URL: https://issues.apache.org/jira/browse/FLINK-3257
> Project: Flink
> Issue Type: Improvement
> Components: DataStream API
> Reporter: Paris Carbone
> Assignee: Paris Carbone
>
> The current snapshotting algorithm cannot support cycles in the execution
> graph. An alternative scheme can potentially include records in-transit
> through the back-edges of a cyclic execution graph (ABS [1]) to achieve the
> same guarantees.
> One straightforward implementation of ABS for cyclic graphs can work as
> follows along the lines:
> 1) Upon triggering a barrier in an IterationHead from the TaskManager start
> block output and start upstream backup of all records forwarded from the
> respective IterationSink.
> 2) The IterationSink should eventually forward the current snapshotting epoch
> barrier to the IterationSource.
> 3) Upon receiving a barrier from the IterationSink, the IterationSource
> should finalize the snapshot, unblock its output and emit all records
> in-transit in FIFO order and continue the usual execution.
> --
> Upon restart the IterationSource should emit all records from the injected
> snapshot first and then continue its usual execution.
> Several optimisations and slight variations can be potentially achieved but
> this can be the initial implementation take.
> [1] http://arxiv.org/abs/1506.08603
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