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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