My experience with Structured Concurrency

[David Alayachew]

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>
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>
> 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> on behalf of David
>> Alayachew <davidalayachew at gmail.com>
>> *Sent:* Friday, 15 August 2025 11:52
>> *To:* loom-dev <loom-dev at openjdk.org>
>> *Subject:* My experience with Structured Concurrency
>>
>> Hello @loom-dev <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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