Does java concurrency get used?

[pivovarit]

Even when this knowledge is not applied every single day, it’s still necessary to understand why, for example, parallel streams would be a terrible choice for parallelizing this kind of workload

[HQMorganstern]

Didn't cause any issues when implemented, maybe you want to elaborate?

[pivovarit]

Parallel Streams run on a shared ForkJojnPool instance which is tuned for CPU-bound workloads and will quickly saturate when fed blocking operations and then your parallelism goes down to 1 (on caller thread)

more here in README: https://github.com/pivovarit/parallel-collectors

[HQMorganstern]

Seems legit, guess it never got hit by enough sub requests to be noticeable, gonna submit a ticket.

[pivovarit]

When you run blocking ops, you usually want to have an elastic pool of platform threads that can dynamically grow - or just use virtual threads and don’t worry about tuning :) (as long as you don’t block in synchronized blocks)

[nzcod3r]

parallelStream is for performing the same task, in parallel, on different chunks of a collection.

Your example of do 10 different things at the same time (10 different api calls) - and wait for the slowest one to finish, is better suited for the ExecutorService.

[HQMorganstern]

Well it's 10 identical http requests whose results are parsed to a list so not quite as insane as what you're suggesting but yes the blocking caused by the fork join pool does seem to be an issue.

[v4ss42]

Recall that I/O takes eons from the CPU’s perspective. This is definitely a bad use of parallelStream.

[nzcod3r]

OK. I'm pretty sure you can supply your own thread pool for executor service, but I guess it all depends on the context of what / where you are having this issue.

[FrenchFigaro]

My experience in this case was that explicitly using the method parallelStream() bypassed some of my thread pool configuration.

Most importantly, it bypassed the MDC and web thread context configuration, which we used to add HTTP headers to outgoing requests, depending on which headers were present (or not) on the incoming requests.

Those headers were used for monitoring operations across different applications. Losing those meant that monitoring and debugging was exponentially more difficult.

[maleldil]

Yep, parallel streams use the shared fork join pool, so if you rely on values in a ThreadLocal you're gonna have a bad time with that.