[External] : Re: mapConcurrent() with InterruptedException

Wed Feb 5 16:18:23 UTC 2025

Hi Jige,

I opened an issue to track the concern, and I have proposed a change which seems to align well with how parallel streams behave under caller thread interruption.

I've opened the following PR for review:
https://github.com/openjdk/jdk/pull/23467

If you are able to make a local OpenJDK build with that solution you could check if it addresses your use-cases (or not).
[https://opengraph.githubassets.com/00e04f8a63bde12217df087df7ef8edee563adf7e925d07c75bdeae092180094/openjdk/jdk/pull/23467]<https://github.com/openjdk/jdk/pull/23467>
8349462: Gatherers.mapConcurrent could support async interrupts by viktorklang-ora · Pull Request #23467 · openjdk/jdk<https://github.com/openjdk/jdk/pull/23467>
This change is likely going to need some extra verbiage in the spec for mapConcurrent, and thus a CSR. This behavior aligns mapConcurrent with how parallel streams work in conjunction with interrup...
github.com

Cheers,
√

Viktor Klang
Software Architect, Java Platform Group
Oracle
________________________________
From: Jige Yu <yujige at gmail.com>
Sent: Wednesday, 5 February 2025 16:24
To: Viktor Klang <viktor.klang at oracle.com>
Cc: core-libs-dev at openjdk.org <core-libs-dev at openjdk.org>
Subject: Re: [External] : Re: mapConcurrent() with InterruptedException

Thanks Viktor!

I understand the problem.

The main reason I asked is because I want to understand how the core Java team thinks of throwing an unchecked exception.

As explained above, I consider losing cancellability a big deal, a deal breaker even. And I thought throwing unchecked is more acceptable. Because the most common reason the mapConcurrent() VT can be interrupted is due to cancellation from a parent mapConcurrent(), or a parent Structured Concurrency scope. The cancellation could be either from an organic exception, or from the downstream not needing more elements, like maybe due to findFirst() already getting an element.

In both cases, since the concurrent operation is already cancelled (result ignored), what exception pops up to the top level isn't that big of a deal (perhaps only a log record will be seen?)

But if the core Java team considers it a bad idea, I would love to learn and adjust.

On Tue, Feb 4, 2025 at 4:41 AM Viktor Klang <viktor.klang at oracle.com<mailto:viktor.klang at oracle.com>> wrote:
Hi,

The problem is that mapConcurrent cannot throw InterruptedException because that is a checked exception, so we cannot clear the interrupted flag and throw that exception.

So the updated semantics is to not cut the stream short but instead run to completion, restoring the interruption flag.

There exists a couple of alternatives to this approach which I am contemplating, but they need to be further explored before I consider moving forward with any of them.

Cheers,
√

Viktor Klang
Software Architect, Java Platform Group
Oracle
________________________________
From: Jige Yu <yujige at gmail.com<mailto:yujige at gmail.com>>
Sent: Monday, 27 January 2025 17:00
To: Viktor Klang <viktor.klang at oracle.com<mailto:viktor.klang at oracle.com>>
Cc: core-libs-dev at openjdk.org<mailto:core-libs-dev at openjdk.org> <core-libs-dev at openjdk.org<mailto:core-libs-dev at openjdk.org>>
Subject: Re: [External] : Re: mapConcurrent() with InterruptedException

Thanks Viktor!

It looks like the current fix ignores interruption.

I want to make sure my concern of it defeating cancellation is heard and understood.

The scenarios I worry about is for a mapConcurrent() that fans out to another method call, which internally calls mapConcurrent() as implementation detail.

An example:

List<RefundResponse> refundHelper(transaction) {
  transaction.creditCardAccounts.stream()
    .gather(mapConcurrent(acct -> service.refund(acct))
    .toList();
}

transactions.stream()
    .gather(mapConcurrent(transaction -> refundHelper(transaction));

It seems undesirable that in such a case all the service.refund() calls become non cancellable, because the only way the outer mapConcurrent() cancels the refundHelper() calls is through Thread.interrupt() the virtual threads that call refundHelper(), which would then be disabled by the inner mapConcurrent().

Does this example make sense to you? I can further explain if anything isn't clear.

But I want to make sure the decision to disable interruption is deliberate judgement call that such nested mapConcurrent() is unlikely,or not important.

Cheers,

On Mon, Jan 27, 2025 at 6:11 AM Viktor Klang <viktor.klang at oracle.com<mailto:viktor.klang at oracle.com>> wrote:
Hi!

Please see: https://github.com/openjdk/jdk/pull/23100<https://urldefense.com/v3/__https://github.com/openjdk/jdk/pull/23100__;!!ACWV5N9M2RV99hQ!IzQs0G26M7ZGPwJ3YJpCcS0gxi6BjqoBux2T5u0cHud_zb_mHLfiIrASSZiP0ynNgnaAuwuOh__WinK8$>

Cheers,
√

Viktor Klang
Software Architect, Java Platform Group
Oracle
________________________________
From: Jige Yu <yujige at gmail.com<mailto:yujige at gmail.com>>
Sent: Sunday, 26 January 2025 23:03
To: Viktor Klang <viktor.klang at oracle.com<mailto:viktor.klang at oracle.com>>
Cc: core-libs-dev at openjdk.org<mailto:core-libs-dev at openjdk.org> <core-libs-dev at openjdk.org<mailto:core-libs-dev at openjdk.org>>
Subject: [External] : Re: mapConcurrent() with InterruptedException

Checking in on what you've found out, Viktor.

From where we left off, I understand that you were looking at alternatives instead of silent truncation?

Have you reached any conclusion?

We touched on disallowing interruption during mapConcurrent(). I still have concerns with disabling cancellation, because it basically undoes this API note from the javadoc<https://cr.openjdk.org/~alanb/sc-20240503/java.base/java/util/stream/Gatherers.html#mapConcurrent(int,java.util.function.Function)>:

API Note:

In progress tasks will be attempted to be cancelled, on a best-effort basis, in situations where the downstream no longer wants to receive any more elements.
In reality, people will use mapConcurrent() to fan out rpcs. Sometimes these rpcs are just a single blocking call; yet sometimes they may themselves be a Structured Concurrency scope, with 2 or 3 rpcs that constitute a single logical operation. Under two conditions, cancellation is imho important semantic:

  1.  The downstream code uses filter().findFirst(), and when it sees an element, it will return and no longer needs the other pending rpcs to complete. If cancellation is disabled, these unnecessary rpcs will waste system resources.
  2.  One of the rpc throws and the Stream pipeline needs to propagate the exception. Again, if the other rpcs cannot be cancelled, we'll have many zombie rpcs.

Zombie rpcs may or may not be a deal breaker, depending on the specific use case. But for a JDK library, losing cancellation would have a negative impact on usability.

My 2c,

On Fri, Jan 3, 2025 at 9:18 AM Viktor Klang <viktor.klang at oracle.com<mailto:viktor.klang at oracle.com>> wrote:
Hi Ben,

Thanks for raising these questions—getting feedback is crucial in the Preview stage of features.

I wrote a reply to the Reddit thread so I'll just summarize here:

It is important to note that mapConcurrent() is not a part of the Structured Concurrency JEPs, so it is not designed to join SC scopes.

I'm currently experimenting with ignoring-but-restoring interrupts on the "calling thread" for mapConcurrent(), as well as capping work-in-progress to maxConcurrency (not only capping the concurrency but also the amount of completed-but-yet-to-be-pushed work). Both of these adjustments should increase predictability of behavior in the face of blocking operations with variable delays.

Another adjustment I'm looking at right now is to harden/improve the cleanup to wait for concurrent tasks to acknowledge cancellation, so that once the finisher is done executing the VTs are known to have terminated.

As for not preserving the encounter order, that would be a completely different thing, and I'd encourage you to experiment with that if that functionality would be interesting for your use-case(s).

Again, thanks for your feedback!

Cheers,
√

Viktor Klang
Software Architect, Java Platform Group
Oracle
________________________________
From: core-libs-dev <core-libs-dev-retn at openjdk.org<mailto:core-libs-dev-retn at openjdk.org>> on behalf of Jige Yu <yujige at gmail.com<mailto:yujige at gmail.com>>
Sent: Friday, 3 January 2025 17:53
To: core-libs-dev at openjdk.org<mailto:core-libs-dev at openjdk.org> <core-libs-dev at openjdk.org<mailto:core-libs-dev at openjdk.org>>
Subject: mapConcurrent() with InterruptedException

Hi Java Experts,

I sent this email incorrectly to loom-dev@ and was told on Reddit that core-libs-dev is the right list.

The question is about the behavior of mapConcurrent() when the thread is interrupted.

Currently mapConcurrent()'s finisher phase will re-interrupt the thread, then stop at whatever element that has already been processed and return.

This strikes me as a surprising behavior, because for example if I'm running:

   Stream.of(1, 2, 3)
        .gather(mapConcurrent(i -> i * 2))
        .toList()

and the thread is being interrupted, the result could be any of [2], [2, 4] or [2, 4, 6].

Since thread interruption is cooperative, there is no guarantee that the thread being interrupted will just abort. It's quite possible that it'll keep going and then will use for example [2] as the result of doubling the list of [1, 2, 3], which is imho incorrect.

In the Reddit<https://urldefense.com/v3/__https://www.reddit.com/r/java/comments/1hr8xyu/observations_of_gatherersmapconcurrent/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button__;!!ACWV5N9M2RV99hQ!L1LHRE2pnYPg43nM0J0dCoV4agscV_rybIV9jY97xn9XJi9d7VoPma4jhx4J4GBeoeQmVud8M4PjPb7L$> thread, someone argued that interruption rarely happens so it's more of a theoretical issue. But interruption can easily happen in Structured Concurrency or in mapConcurrent() itself if any subtask has failed in order to cancel/interrupt the other ongoing tasks.

There had been discussion about alternative strategies:

  1.  Don't respond to interruption and just keep running to completion.
  2.  Re-interrupt thread and wrap the InterruptedException in a standard unchecked exception (StructuredConcurrencyInterruptedException?).

I have concerns with option 1 because it disables cancellation propagation when mapConcurrent() itself is used in a subtask of a parent mapConcurrent() or in a StructuredConcurrencyScope.

Both equivalent Future-composition async code, or C++'s fiber trees support cancellation propagation and imho it's a critical feature or else it's possible that a zombie thread is still sending RPCs long after the main thread has exited (failed, or falled back to some default action).

My arguments for option 2:

  1.  InterruptedException is more error prone than traditional checked exceptions for users to catch and handle. They can forget to re-interrupt the thread. It's so confusing that even seasoned programmers may not know they are supposed to re-interrupt the thread.
  2.  With Stream API using functional interfaces like Supplier, Function, the option of just tacking on "throws IE" isn't available to many users.
  3.  With Virtual Threads, it will be more acceptable, or even become common to do blocking calls from a stream operation (including but exclusive to mapConcurrent()). So the chance users are forced to deal with IE will become substantially higher.
  4.  Other APIs such as the Structured Concurrency API have already started wrapping system checked exceptions like ExecutionException, TimeoutException in unchecked exceptions ( join()<https://urldefense.com/v3/__https://download.java.net/java/early_access/loom/docs/api/java.base/java/util/concurrent/StructuredTaskScope.html*join()__;Iw!!ACWV5N9M2RV99hQ!L1LHRE2pnYPg43nM0J0dCoV4agscV_rybIV9jY97xn9XJi9d7VoPma4jhx4J4GBeoeQmVud8MxGG4HzA$> for example).
  5.  Imho, exceptions that we'd rather users not catch and handle but instead should mostly just propagate up as is, should be unchecked.

There is also a side discussion<https://urldefense.com/v3/__https://www.reddit.com/r/java/comments/1hr8xyu/comment/m4z4f8c/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button__;!!ACWV5N9M2RV99hQ!L1LHRE2pnYPg43nM0J0dCoV4agscV_rybIV9jY97xn9XJi9d7VoPma4jhx4J4GBeoeQmVud8MyZYl02k$> about whether mapConcurrent() is better off preserving input order or push to downstream as soon as an element is computed. I'd love to discuss that topic too but maybe it's better to start a separate thread?

Thank you and cheers!

Ben Yu

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