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Posted to notifications@pekko.apache.org by "He-Pin (via GitHub)" <gi...@apache.org> on 2026/06/20 12:05:47 UTC
[I] GroupBy should support infinite substreams with maxSubstreams=-1 [pekko]
He-Pin opened a new issue, #3102:
URL: https://github.com/apache/pekko/issues/3102
### Motivation
`GroupBy` requires a fixed `maxSubstreams` limit. When the number of distinct keys exceeds this limit, the stream fails with `TooManySubstreamsOpenException`. Some use cases (e.g., event routing by tenant/user ID) need to handle an unbounded number of keys without a predefined limit.
### Current behavior
`StreamOfStreams.scala:485`:
```scala
} else if (activeSubstreamsMap.size + closedSubstreams.size == maxSubstreams) {
throw tooManySubstreamsOpenException
}
```
Passing `maxSubstreams = -1` technically bypasses the check (sizes can never equal -1), but this is undocumented and not an official API contract.
### Proposed fix
1. Add explicit guard: `if (maxSubstreams >= 0 && activeSubstreamsMap.size + closedSubstreams.size == maxSubstreams)`
2. Update Scaladoc in `Flow.scala:groupBy` to document `-1` as "unlimited substreams"
3. Add `require(maxSubstreams >= -1, ...)` validation
4. Add tests for `maxSubstreams = -1` with many distinct keys
5. Document the memory implication: with `allowClosedSubstreamRecreation = false`, `closedSubstreams` grows unbounded. Recommend using `allowClosedSubstreamRecreation = true` with infinite substreams.
### References
- Inspired by Akka.NET [#7607](https://github.com/akkadotnet/akka.net/pull/7607)
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Re: [I] GroupBy should support infinite substreams with maxSubstreams=-1 [pekko]
Posted by "He-Pin (via GitHub)" <gi...@apache.org>.
He-Pin closed issue #3102: GroupBy should support infinite substreams with maxSubstreams=-1
URL: https://github.com/apache/pekko/issues/3102
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Re: [I] GroupBy should support infinite substreams with maxSubstreams=-1 [pekko]
Posted by "He-Pin (via GitHub)" <gi...@apache.org>.
He-Pin commented on issue #3102:
URL: https://github.com/apache/pekko/issues/3102#issuecomment-4758050847
wdyt, I think this may cause some memory safe issue .
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