[I] Optimize DistributedPubSubMediator topic key construction with caching [pekko]

["He-Pin (via GitHub)" <gi...@apache.org>]

He-Pin opened a new issue, #3132:
URL: https://github.com/apache/pekko/issues/3132

   ### Motivation
   
   In `DistributedPubSubMediator`, topic name encoding (`URLEncoder.encode`) and key construction (`mkKey(self.path / encName(topic))`) are performed on every Publish, Subscribe, Unsubscribe, and CountSubscribers message. For high-throughput pub/sub workloads with a limited set of recurring topic names, this creates unnecessary allocations and CPU overhead on the hot path.
   
   The topic names and their encoded forms are typically stable — the same topics are published to and subscribed from repeatedly. Caching the encoded names and constructed keys avoids redundant string allocations and URL encoding operations.
   
   ### Proposed Enhancement
   
   Introduce a lightweight cache (e.g., a `HashMap` or similar structure) that stores:
   1. The URL-encoded form of topic names
   2. The fully constructed routing keys
   
   The cache should be:
   - Bounded to prevent unbounded memory growth if topic names are dynamic
   - Invalidated when topics are removed/pruned
   - Thread-safe within the mediator actor's single-threaded context (no locking needed since the mediator is an actor)
   
   ### Impact
   
   - Reduced allocations on the pub/sub hot path
   - Lower GC pressure in high-throughput scenarios
   - No API changes — purely internal optimization
   - The trade-off is a small amount of additional memory for the cache
   
   ### References
   
   This optimization was implemented in the Akka.NET project: https://github.com/akkadotnet/akka.net/issues/7642


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Re: [I] Optimize DistributedPubSubMediator topic key construction with caching [pekko]

["He-Pin (via GitHub)" <gi...@apache.org>]

He-Pin commented on issue #3132:
URL: https://github.com/apache/pekko/issues/3132#issuecomment-4771328818

   Closing as premature optimization. The proposed cache adds implementation complexity (bounded eviction, invalidation on topic pruning) without demonstrated benefit.
   
   `URLEncoder.encode` on short topic names and `ActorPath.toStringWithoutAddress` are not expensive operations — the GC pressure from a few string allocations per message is likely negligible compared to actor message dispatch, serialization, and network I/O costs.
   
   If this optimization is worth pursuing, please reopen with:
   - A JMH benchmark showing measurable throughput improvement (before vs. after) on a realistic high-throughput pub/sub workload
   - Evidence that the encoding/key-construction overhead is a meaningful fraction of total per-message cost


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Re: [I] Optimize DistributedPubSubMediator topic key construction with caching [pekko]

["He-Pin (via GitHub)" <gi...@apache.org>]

He-Pin closed issue #3132: Optimize DistributedPubSubMediator topic key construction with caching
URL: https://github.com/apache/pekko/issues/3132


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