Module cache 

Response cache with TTL, LRU eviction, and in-flight deduplication.

Overview

ResponseCache stores the full ServerStatus result for each (protocol, address) cache key with a configurable TTL. On a cache hit the stored status is returned instantly with latency = 0.0 and cached = true.

The cache is disabled by default (TTL = Duration::ZERO). Enable it via McClientBuilder::response_cache.

In-flight deduplication (thundering herd protection)

When many tasks concurrently miss the cache for the same key, only one real ping is performed. The rest subscribe via a tokio::sync::broadcast channel and receive the result as soon as it arrives.

task-1  ──► cache miss ──► starts real ping ──────────────► insert + broadcast
task-2  ──► cache miss ──► sees in-flight   ──► subscribe ──────────────────────► receive
task-3  ──► cache miss ──► sees in-flight   ──► subscribe ──────────────────────► receive

The caller must call insert or insert_error after the ping completes — this broadcasts the result to all subscribers and removes the in-flight entry. If the sender is dropped without broadcasting, subscribers receive Err(RecvError::Closed) and fall through to a direct ping.

Structs

ResponseCache

Response cache: LRU + TTL + in-flight deduplication.