Hazelcast Jet 4.3 is Released!

Marko Topolnik | Oct 23, 2020

Today we’re releasing Hazelcast Jet 4.3, our fourth release of 2020!

We took part in Google Summer of Code that ended just a few weeks ago, and this release already brings a production-ready piece of work by our student, Mohamed Mandouh: distributed in-memory sorting. Mohamed’s primary focus was research into the feasibility of integrating RocksDB or a similar DB library as a disk-based state backend for Jet’s hash join, aggregation and sorting, and we plan to continue with this work for some more time.

Here are the main improvements in this release:

Benchmarking and Tuning for Low Latency

Continuing the story from the previous release, we benchmarked and fine-tuned Jet with a focus on low-latency processing. Jet can now give you a 99.99th percentile latency of fewer than 10 milliseconds at a pipeline throughput of 60M items/second! Based on this work we significantly expanded the Operations Guide section on Garbage Collection with many new latency-squashing tricks.


As mentioned, this is the work coming out of this year’s GSoC. You can now sort the data coming out of a batch pipeline stage. For example, this starts from an ascending sequence 0..9,999, sorts it in descending order, and prints the result:

var pipeline = Pipeline.create();
var integerSequence = TestSources.items(IntStream.range(0, 10_000)
        .sort(ComparatorEx.comparing(i -> -i))
try {
} finally {

Jet’s current execution model allows reordering, e.g., when maximizing parallel throughput in stateless transform stages, which means you may easily lose the sort order. In the next release, we’ll add the ability to set limits on these optimizations so that the ordering survives.


Our community contributor Guenter Hesse took the ad-hoc work we did for our low latency GC benchmarks and productized it to be included in our library. If you want to benchmark Hazelcast Jet in a way that doesn’t depend on the specifics of an actual data source, you can use this distributed event generator that produces a timestamped sequence of Long numbers. You can then transform the sequence numbers to whichever mock events you are using for the benchmark:

StreamStage<String> trades = pipeline
        .readFrom(TestSources.longStream(1_000_000, 25))
        .map(i -> String.format("Trade %09d", i));

This stage will generate a steady stream of a million events per second, keeping the latency of emitting any given event at a minimum. If you run it in a cluster, every cluster node will generate its share of the events.

Preserve Job State on Exception

Jet’s default behavior (and so far the only choice) is to cancel and dispose of a job that throws an exception from any part of the pipeline. This is usually user code, but it could also be IO errors while contacting outside services. We are now introducing an option that applies to jobs with enabled fault tolerance: Jet can now keep the job in a suspended state, with the latest snapshot attached to it. Once you remove the cause of the exception, you can resume the job and it will continue executing without data loss.

As a part of these improvements, we added a whole new section on error handling in the Programming Guide.

Make Continuous Progress in Pipelines Based on Ingestion Time

Hazelcast Jet is primarily built to respect the original event timestamps instead of just noting the time it received them. Time advances in the pipeline when events with fresh timestamps arrive. This system has an Achilles heel for the case where the event stream is very sparse: without events, time doesn’t pass. When you have a partitioned data source, each partition has its own event time, and Jet must consolidate them into a single global event time. For this to work out without losses, time advances according to the “slowest” partition, with the lowest local event time. So all it takes is a single partition out of potentially hundreds or thousands, to experience very low traffic and your entire pipeline experiences stalls.

In this release we bring a partial solution to this general problem: if you happen to work with a pipeline based on ingestion time instead of event time, Jet can be certain that the time advances in any partition with or without events. We use this to improve our watermark emission logic and make progress regardless of actual events coming in. We expect to invest more effort into heuristic approaches that will improve the progress of event time-based pipelines as well.

Full Release Notes

Members of the open source community that appear in these release notes:

  • @caioguedes
  • @guenter-hesse
  • @MohamedMandouh

Thank you for your valuable contributions!

New Features

  • [pipeline-api] [014] @MohamedMandouh implemented distributed sorting: BatchStage.sort() (#2469, #2544)
  • [core] [012] Added JobConfig.suspendOnFailure: suspend a job on exception instead of cancelling it (#2411)
  • [cdc] Improved the consistency of reconnect behaviour across CDC sources, new uniform API to configure the reconnect strategy (#2419)


  • [core] @guenter-hesse contributed a test source to benchmark Jet’s throughput and latency (#2382)
  • [core] [013] Improved watermark semantics that prevents low event rate from stalling an ingestion time-based pipeline (#2485, #2514)
  • [cdc] Exposed the sequence number in the CDC ChangeRecord that orders the events (#2390)
  • [core] Two new DAG edge types: distributeToOne (sending all data to one member) and ordered (maintaining the sort order) (#2394, #2469, #2544)
  • [core] Disabled access to external XML entities when parsing XML config, this was a potential XXE attack vector (#2528)


  • [core] Fixed error handling during job startup that could result in an inconsistent job state (#2383)
  • [core] Fixed an internal exception that leaked out of Observable (#2313, #2389)
  • [core] Prevented Observable from processing in-flight items after cancellation (#2415, #2418)
  • [cli] @caioguedes fixed an issue with --targets option in CLI where it would overwrite other settings (#2373, #2421)
  • [metrics] Fixed a problem where an internally added DAG vertex would show up as a source instead of the actual source vertex (#2475, #2476)
  • [core] Fixed a race that could cause getJobStatus() to throw an exception if called right after newJob() (#2481, #2484)
  • [core] Fix a race between snapshotting and restarting (#2487, #2503)
  • [core] Fixed a race where getJobStatus() would report RUNNING even though it was actually COMPLETING. (#2507)
  • [core] Fixed an issue where a DONE_ITEM could get lost due to connection failure, preventing the job from completing (#2158, #2532)
  • [core] Fixed a job failure related to the coordinator member failing (#2461, #2546)
  • [core] Fixed a job failure related to a member reconnecting (#2542, #2547)
  • [core] Improved robustness related to Jet’s internal IMap operations (#2533, #2550)
  • [cdc] Upgraded Jackson jr dep, solving a null handling issue (#2459)
  • [cdc] Upgraded the Debezium dep, solving a Postgress issue resulting in data loss when snapshotting (#2406)
  • [core] Fixed a bug where a non-keyed aggregating stage would produce no output when no input (#2560, #2567)

Breaking Changes

  • [pipeline-api] Breaking signature change to Sources.streamFromProcessorWithWatermarks()
  • [pipeline-api] Deprecated Pipeline.toDag(), made Pipeline and all its components Serializable.
  • [core-api] Breaking signature change to StreamEventJournalP, methods streamRemoteMapSupplier() and streamRemoteCacheSupplier

Relevant Resources

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About the Author

Marko Topolnik

Senior Software Engineer, Hazelcast

Marko Topolnik, PhD, has been a Java professional since 2001. His current position is in the core team of Hazelcast Jet, where he co-wrote the core execution engine based on coroutine-like suspendable code that runs many concurrent tasks on a fixed thread pool. Marko is also an active contributor on Stack Overflow on the kotlin-coroutines tag.