[External] : Re: My experience with Structured Concurrency

Mon Aug 18 10:14:17 UTC 2025

Trying to come up with an example on the spot is always challenging, so if the following doesn't make sense in your case(s) let me know (and, fair warning, I just typed it into the email client, so this is unlikely to compile as-is).

Now, imagine that you want to create the following logical structure of tasks (note that these are all concurrent): (A and B) or C

You could go down the path of creating a bespoke Joiner that handles this case specifically, and then when you have (A and B and C) or D or E you're likely to have to create yet another such custom thing (repeat for permutations thereof).

If you were to instead imagine you have the following two different joiners:

MyJoiners.<T>and() // takes tasks which produce a result of type T and joins them into a List<T> with all the results if all tasks are successful, and fails if any of them fail (see: Joiners.allSuccessfulOrThrow())
MyJoiners.<T>or() // takes tasks which produce a result of type T and returns the first result to complete successfully, and fails if all fail (see: Joiners.anySuccessfulResultOrThrow())

That would be equivalent to:

try (var orScope = StructuredTaskScope.open(MyJoiners.<List<String>>or())) {
    orScope.fork(() -> {
        try (var andScope = StructuredTaskScope.open(MyJoiners.<String>and())) {
            andScope.fork(() -> "A");
            andScope.fork(() -> "B");
            return andScope.join();
        }
    });

    orScope.fork(() -> List.of("C"));

    return orScope.join(); // either ["A", "B"] or ["C"]
}

So while this is merely a (possibly contrived) example, I hope that it illustrates the notion of composition of scopes rather than the composition of Joiners.

Cheers,
√

Viktor Klang
Software Architect, Java Platform Group
Oracle
________________________________
From: David Alayachew <davidalayachew at gmail.com>
Sent: Sunday, 17 August 2025 22:50
To: Viktor Klang <viktor.klang at oracle.com>
Cc: loom-dev <loom-dev at openjdk.org>
Subject: Re: [External] : Re: My experience with Structured Concurrency

Hey, I've been trying out your idea, but I can't find a single place where doing it would apply.

I understand the idea of nesting scopes well enough. For example, I could put a innerScope inside of a outerscope.fork() call. Alternatively, I could have another scope after the outerScope.join() call.

But doing it the first way would mean that I am creating an entire scope for each outerScope subtask. Is that what you are proposing? That makes sense if there are multiple tasks worth of processing that I want. For example, if the outerscope subtask would return a list of things, each of which, I could do further processing on. But that doesn't make sense if each outerScope subtask has only one "thing" that needs further downstream processing. The entire point of a scope is to do multi-threading. Having one task defeats the purpose of that.

And doing it the second way means I would have to wait for all of my tasks to complete from the outerScope before the innerScope could do anything.

In both cases, I wouldn't really be able to gain anything for my composed joiners.

And to be clear, most of my Joiner compositions were simply nesting and adding to an existing implementation. For example, I had a joiner that closed the scope after 3 counts of tasks failing with SomeException. Well, I then used composition to make it cancel AND return the tasks up to and including the 3 failed. In that case, I don't see how I would gain anything by adding another scope.

Could you help me see what you had in mind? Maybe I am just lacking creativity.

On Fri, Aug 15, 2025 at 8:04 PM David Alayachew <davidalayachew at gmail.com<mailto:davidalayachew at gmail.com>> wrote:
Oh, I'm already doing that. Lol, I love STS BECAUSE nesting scopes is so easy to do.

But that's an interesting idea. I kind of see what you mean -- maybe I am stuffing too much logic into a single Joiner. I can't tell yet, so I guess I'll just have to try it out and get back to you.

Thanks for the tip.

On Fri, Aug 15, 2025, 5:09 PM Viktor Klang <viktor.klang at oracle.com<mailto:viktor.klang at oracle.com>> wrote:
Hi David,

Thanks for the added detail, that really helps my understanding of your situation.

Did you try/consider/evaluate nested scopes (each with different Joiner strategies) over composing Joiners themselves?
And if you did, what were your findings when comparing those two different approaches?

Cheers,
√

Viktor Klang
Software Architect, Java Platform Group
Oracle
________________________________
From: David Alayachew <davidalayachew at gmail.com<mailto:davidalayachew at gmail.com>>
Sent: Friday, 15 August 2025 20:53
To: Viktor Klang <viktor.klang at oracle.com<mailto:viktor.klang at oracle.com>>
Cc: loom-dev <loom-dev at openjdk.org<mailto:loom-dev at openjdk.org>>
Subject: [External] : Re: My experience with Structured Concurrency

One other detail I'd like to highlight.

Much like Collectors and Gatherers, there are a handful of super useful ones that you use everywhere, and then the rest are ad-hoc, inline ones where you sort of just make your own to handle a custom scenario. If you use streams often, you will run into those frequently, and that's why those factory methods are fantastic.

Well, I have kind of found myself in the same position for Joiners. Joiners aren't as complex as Collectors and Gatherers, so there has certainly been less need for it. But I am also only a few weeks into using Joiners (though, I used STS for over a year). If I feel this strain now, then I feel like this experience is definitely worth sharing.

On Fri, Aug 15, 2025, 2:44 PM David Alayachew <davidalayachew at gmail.com<mailto:davidalayachew at gmail.com>> wrote:
Sure.

Long story short, the biggest reason why STS is so useful for me is because it allows me to fire off a bunch of requests, and handle their failures and outcomes centrally. That is the single most useful feature of this library for me. It's also why Future.status was not so useful for me -- it calls get under the hood, and therefore might fail! Handling that was too much scaffolding.

So, when someone recently challenged me to use Joiners (rather than the old STS preview versions I was used to), I started creating Joiners to handle all sorts of failure and outcomes. At first, a lot of them could be handled by the Joiner.awaitUntil(), where I would just check and see if the task failed, then handle the error. But as I got further and further along, I started needing to add state to my Joiners in order to get the failure handling that I wanted. For example, if a certain number of timeouts occur, cancel the scope. Well, that necessitates an AtomicNumber.

Then, as the error-handling got more and more complex, I started finding myself making a whole bunch of copy paste, minor variations of similar Joiners. Which isn't bad or wrong, but started to feel some strain. Now, I need to jump through an inheritance chain just to see what my Joiner is really doing. It wasn't so bad, but I did start to feel a little uneasy. Bad memories.

So, the solution to a problem like this is to create a Joiner factory. Which is essentially what I started to write before I started remembering how Collectors and Gatherers worked. At that point, I kind of realized that this is worth suggesting, which prompted me to write my original email.

Like I said, not a big deal if you don't give it to me -- I can just make my own.

But yes, that is the surrounding context behind that quote. Let me know if you need more details.

On Fri, Aug 15, 2025, 9:25 AM Viktor Klang <viktor.klang at oracle.com<mailto:viktor.klang at oracle.com>> wrote:
Hi David,

First of all—thank you for your feedback!

I'm curious to learn more about why you ended up in the situation you describe below, specifically about what use-cases led you into wishing for an augmentation to Joiner to facilitate composition.

Are you able to share more details?

>Which, funnily enough, led to a slightly different problem -- I found myself wanting an easier way to create Joiners. Since I was leaning on Joiners so much more heavily than I was for STS, I ended up creating many Joiners that do almost the same thing, with just minor variations. And inheritance wasn't always the right answer, as I can't inherit from multiple classes. Plus, most of my joiners were stateful, but I only wanted the non-stateful parts of it. I could do composition, but it sort of felt weird to delegate to multiple other Joiners.

Cheers,
√

Viktor Klang
Software Architect, Java Platform Group
Oracle
________________________________
From: loom-dev <loom-dev-retn at openjdk.org<mailto:loom-dev-retn at openjdk.org>> on behalf of David Alayachew <davidalayachew at gmail.com<mailto:davidalayachew at gmail.com>>
Sent: Friday, 15 August 2025 11:52
To: loom-dev <loom-dev at openjdk.org<mailto:loom-dev at openjdk.org>>
Subject: My experience with Structured Concurrency

Hello @loom-dev<mailto:loom-dev at openjdk.org>,

I just wanted to share my experience with Structured Concurrency. I had actually been using it for a while now, but only recently got experience with the new Joiner. After trying it out, my previously stated opinion has changed.

Overall, Structured Concurrency has been a pleasure. I'll avoid repeating ALL my old thoughts and just highlight the KEY details.

* Structured Concurrency is excellent for complex error-handling. Receiving exceptions via the subtask makes all the error-handling less painful.
* Structured Concurrency makes nesting scopes a breeze, a task I historically found very painful to do.
* Inheritance allows me to take an existing Scope (now Joiner), and modify only what I need to in order to modify it for my use case. Great for reusing old strategies in new ways.

Now for the new stuff -- having Joiner be the point of extension definitely proved to be the right move imo. I didn't mention this in my original message, but while it was easy to get a scope set up using inheritance, it wasn't always clear what invariants needed to be maintained. For example, the ensureOwnerAndJoined method. Was that something we needed to call when inheriting? On which methods? Just join()?

The Joiner solution is comparatively simpler, which actually meant that I ended up creating way more Joiners, rather than only several STS'. Joiners invariants are obvious, and there is no ambiguity on what is expected from the implementor.

Which, funnily enough, led to a slightly different problem -- I found myself wanting an easier way to create Joiners. Since I was leaning on Joiners so much more heavily than I was for STS, I ended up creating many Joiners that do almost the same thing, with just minor variations. And inheritance wasn't always the right answer, as I can't inherit from multiple classes. Plus, most of my joiners were stateful, but I only wanted the non-stateful parts of it. I could do composition, but it sort of felt weird to delegate to multiple other Joiners.

Part of me kept wondering how well a factory method, similar to the ones for Collectors and Gatherers, might fare for Joiners.

Regardless, even if we don't get that factory method, this library has been a pleasure, and I can't wait to properly implement this once it goes live.

Thank you for your time and consideration.
David Alayachew

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