java 36 lines · 6 steps

Coalescing duplicate requests in Java

How a ConcurrentHashMap of in-flight futures collapses concurrent calls for the same key into one load.

Explained by highlit

Advanced concurrency caching completablefuture deduplication async

1public class RequestCoalescer<K, V> {
2 
  private final ConcurrentHashMap<K, CompletableFuture<V>> inFlight = new ConcurrentHashMap<>();
  private final Function<K, V> loader;
  private final Executor executor;
6 
  public RequestCoalescer(Function<K, V> loader, Executor executor) {
      this.loader = loader;
      this.executor = executor;
  }
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  public CompletableFuture<V> get(K key) {
      CompletableFuture<V> existing = inFlight.get(key);
      if (existing != null) {
          return existing;
      }
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      CompletableFuture<V> created = new CompletableFuture<>();
      CompletableFuture<V> raced = inFlight.putIfAbsent(key, created);
      if (raced != null) {
          return raced;
      }
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      CompletableFuture.supplyAsync(() -> loader.apply(key), executor)
          .whenComplete((value, error) -> {
              inFlight.remove(key, created);
              if (error != null) {
                  created.completeExceptionally(error);
              } else {
                  created.complete(value);
              }
          });
33 
      return created;
  }
36}

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Sharing a future per key lets many callers await one computation instead of triggering redundant work.
2putIfAbsent turns a check-then-act race into a single atomic claim, so exactly one caller starts the load.
3Removing the entry on completion keeps the map bounded and lets future callers trigger a fresh load.

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