HTTPCLIENT-1165: Add optional request collapsing for async cache misses

httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/AsyncCachingExec.java

@@ -98,13 +98,23 @@ class AsyncCachingExec extends CachingExecBase implements AsyncExecChainHandler
     private final HttpAsyncCache responseCache;
     private final DefaultAsyncCacheRevalidator cacheRevalidator;
     private final ConditionalRequestBuilder<HttpRequest> conditionalRequestBuilder;
+    private final boolean requestCollapsingEnabled;
 
     AsyncCachingExec(final HttpAsyncCache cache, final DefaultAsyncCacheRevalidator cacheRevalidator, final CacheConfig config) {
+        this(cache, cacheRevalidator, config, false);
+    }
+
+    AsyncCachingExec(
+            final HttpAsyncCache cache,
+            final DefaultAsyncCacheRevalidator cacheRevalidator,
+            final CacheConfig config,
+            final boolean requestCollapsingEnabled) {
         super(config);
         this.responseCache = Args.notNull(cache, "Response cache");
         this.cacheRevalidator = cacheRevalidator;
         this.conditionalRequestBuilder = new ConditionalRequestBuilder<>(request ->
                 BasicRequestBuilder.copy(request).build());
+        this.requestCollapsingEnabled = requestCollapsingEnabled;
     }
 
     AsyncCachingExec(
@@ -274,6 +284,87 @@ public void completed(final CacheMatch result) {
                     final CacheHit hit = result != null ? result.hit : null;
                     final CacheHit root = result != null ? result.root : null;
                     if (hit == null) {
+                        if (requestCollapsingEnabled && root == null && entityProducer == null && !requestCacheControl.isOnlyIfCached()) {
+                            final String cacheKey = CacheKeyGenerator.INSTANCE.generateKey(target, request);
+                            final CacheRequestCollapser.Token token = CacheRequestCollapser.INSTANCE.enter(cacheKey);
+                            if (token.isLeader()) {
+                                handleCacheMiss(requestCacheControl, null, target, request, null, scope, chain, new AsyncExecCallback() {
+
+                                    @Override
+                                    public AsyncDataConsumer handleResponse(
+                                            final HttpResponse response,
+                                            final EntityDetails entityDetails) throws HttpException, IOException {
+                                        try {
+                                            return asyncExecCallback.handleResponse(response, entityDetails);
+                                        } catch (final HttpException | IOException ex) {
+                                            token.complete();
+                                            throw ex;
+                                        }
+                                    }
+
+                                    @Override
+                                    public void handleInformationResponse(final HttpResponse response) throws HttpException, IOException {
+                                        try {
+                                            asyncExecCallback.handleInformationResponse(response);
+                                        } catch (final HttpException | IOException ex) {
+                                            token.complete();
+                                            throw ex;
+                                        }
+                                    }
+
+                                    @Override
+                                    public void completed() {
+                                        try {
+                                            asyncExecCallback.completed();
+                                        } finally {
+                                            token.complete();
+                                        }
+                                    }
+
+                                    @Override
+                                    public void failed(final Exception cause) {
+                                        try {
+                                            asyncExecCallback.failed(cause);
+                                        } finally {
+                                            token.complete();
+                                        }
+                                    }
+
+                                });
+                            } else {
+                                operation.setDependency(CacheRequestCollapser.INSTANCE.await(token, () -> operation.setDependency(
+                                        responseCache.match(target, request, new FutureCallback<CacheMatch>() {
+
+                                            @Override
+                                            public void completed(final CacheMatch result) {
+                                                final CacheHit hit = result != null ? result.hit : null;
+                                                final CacheHit root = result != null ? result.root : null;
+                                                if (hit == null) {
+                                                    handleCacheMiss(requestCacheControl, root, target, request, entityProducer, scope, chain, asyncExecCallback);
+                                                } else {
+                                                    final ResponseCacheControl responseCacheControl = CacheControlHeaderParser.INSTANCE.parse(hit.entry);
+                                                    if (LOG.isDebugEnabled()) {
+                                                        LOG.debug("{} response cache control: {}", exchangeId, responseCacheControl);
+                                                    }
+                                                    context.setResponseCacheControl(responseCacheControl);
+                                                    handleCacheHit(requestCacheControl, responseCacheControl, hit, target, request, entityProducer, scope, chain, asyncExecCallback);
+                                                }
+                                            }
+
+                                            @Override
+                                            public void failed(final Exception cause) {
+                                                asyncExecCallback.failed(cause);
+                                            }
+
+                                            @Override
+                                            public void cancelled() {
+                                                asyncExecCallback.failed(new InterruptedIOException());
+                                            }
+
+                                        }))));
+                            }
+                            return;
+                        }
                         handleCacheMiss(requestCacheControl, root, target, request, entityProducer, scope, chain, asyncExecCallback);
                     } else {
                         final ResponseCacheControl responseCacheControl = CacheControlHeaderParser.INSTANCE.parse(hit.entry);

httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/CacheRequestCollapser.java

@@ -0,0 +1,155 @@
+/*
+ * ====================================================================
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ * ====================================================================
+ *
+ * This software consists of voluntary contributions made by many
+ * individuals on behalf of the Apache Software Foundation.  For more
+ * information on the Apache Software Foundation, please see
+ * <http://www.apache.org/>.
+ *
+ */
+package org.apache.hc.client5.http.impl.cache;
+
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import org.apache.hc.core5.annotation.Internal;
+import org.apache.hc.core5.concurrent.Cancellable;
+import org.apache.hc.core5.util.Args;
+
+/**
+ * Coordinates concurrent requests for the same cache key so that only one request
+ * goes to the backend while others wait for it to complete and then re-check the cache.
+ * <p>
+ * Internal helper used to implement request collapsing ({@code HTTPCLIENT-1165}).
+ */
+@Internal
+final class CacheRequestCollapser {
+
+    static final CacheRequestCollapser INSTANCE = new CacheRequestCollapser();
+
+    static final class Token {
+
+        private final ConcurrentHashMap<String, Entry> inflight;
+        private final String key;
+        private final Entry entry;
+        private final boolean leader;
+
+        private Token(final ConcurrentHashMap<String, Entry> inflight, final String key, final Entry entry, final boolean leader) {
+            this.inflight = inflight;
+            this.key = key;
+            this.entry = entry;
+            this.leader = leader;
+        }
+
+        boolean isLeader() {
+            return leader;
+        }
+
+        void complete() {
+            if (entry.completed.compareAndSet(false, true)) {
+                inflight.remove(key, entry);
+                entry.drain();
+            }
+        }
+
+    }
+
+    private static final class Waiter implements Cancellable {
+
+        private final AtomicBoolean cancelled;
+        private final Runnable task;
+
+        private Waiter(final Runnable task) {
+            this.cancelled = new AtomicBoolean(false);
+            this.task = task;
+        }
+
+        @Override
+        public boolean cancel() {
+            return cancelled.compareAndSet(false, true);
+        }
+
+        void runIfNotCancelled() {
+            if (!cancelled.get()) {
+                task.run();
+            }
+        }
+
+    }
+
+    private static final class Entry {
+
+        private final AtomicBoolean completed;
+        private final ConcurrentLinkedQueue<Waiter> waiters;
+
+        private Entry() {
+            this.completed = new AtomicBoolean(false);
+            this.waiters = new ConcurrentLinkedQueue<>();
+        }
+
+        private Cancellable await(final Runnable task) {
+            if (completed.get()) {
+                task.run();
+                return () -> false;
+            }
+            final Waiter waiter = new Waiter(task);
+            waiters.add(waiter);
+            if (completed.get()) {
+                drain();
+            }
+            return waiter;
+        }
+
+        private void drain() {
+            for (; ; ) {
+                final Waiter waiter = waiters.poll();
+                if (waiter == null) {
+                    return;
+                }
+                waiter.runIfNotCancelled();
+            }
+        }
+
+    }
+
+    private final ConcurrentHashMap<String, Entry> inflight;
+
+    private CacheRequestCollapser() {
+        this.inflight = new ConcurrentHashMap<>();
+    }
+
+    Token enter(final String key) {
+        Args.notEmpty(key, "Key");
+        final Entry created = new Entry();
+        final Entry existing = inflight.putIfAbsent(key, created);
+        if (existing == null) {
+            return new Token(inflight, key, created, true);
+        }
+        return new Token(inflight, key, existing, false);
+    }
+
+    Cancellable await(final Token token, final Runnable task) {
+        Args.notNull(token, "Token");
+        Args.notNull(task, "Task");
+        return token.entry.await(task);
+    }
+
+}

httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/CachingH2AsyncClientBuilder.java

@@ -56,6 +56,7 @@ public class CachingH2AsyncClientBuilder extends H2AsyncClientBuilder {
     private SchedulingStrategy schedulingStrategy;
     private CacheConfig cacheConfig;
     private boolean deleteCache;
+    private boolean requestCollapsingEnabled;
 
     public static CachingH2AsyncClientBuilder create() {
         return new CachingH2AsyncClientBuilder();
@@ -112,6 +113,16 @@ public CachingH2AsyncClientBuilder setDeleteCache(final boolean deleteCache) {
         return this;
     }
 
+    /**
+     * Enables request collapsing for cacheable requests ({@code HTTPCLIENT-1165}).
+     *
+     * @since 5.7
+     */
+    public CachingH2AsyncClientBuilder setRequestCollapsingEnabled(final boolean requestCollapsingEnabled) {
+        this.requestCollapsingEnabled = requestCollapsingEnabled;
+        return this;
+    }
+
     @Override
     protected void customizeExecChain(final NamedElementChain<AsyncExecChainHandler> execChainDefinition) {
         final CacheConfig config = this.cacheConfig != null ? this.cacheConfig : CacheConfig.DEFAULT;
@@ -156,7 +167,8 @@ protected void customizeExecChain(final NamedElementChain<AsyncExecChainHandler>
         final AsyncCachingExec cachingExec = new AsyncCachingExec(
                 httpCache,
                 cacheRevalidator,
-                config);
+                config,
+                requestCollapsingEnabled);
         execChainDefinition.addBefore(ChainElement.PROTOCOL.name(), cachingExec, ChainElement.CACHING.name());
     }
 

httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/CachingHttpAsyncClientBuilder.java

@@ -60,6 +60,7 @@ public class CachingHttpAsyncClientBuilder extends HttpAsyncClientBuilder {
     private SchedulingStrategy schedulingStrategy;
     private CacheConfig cacheConfig;
     private boolean deleteCache;
+    private boolean requestCollapsingEnabled;
 
     public static CachingHttpAsyncClientBuilder create() {
         return new CachingHttpAsyncClientBuilder();
@@ -116,6 +117,18 @@ public CachingHttpAsyncClientBuilder setDeleteCache(final boolean deleteCache) {
         return this;
     }
 
+    /**
+     * Enables request collapsing for cacheable requests ({@code HTTPCLIENT-1165}). When enabled,
+     * concurrent requests for the same cache key are coalesced so that only one request goes to
+     * the backend while the others wait and then re-check the cache.
+     *
+     * @since 5.7
+     */
+    public CachingHttpAsyncClientBuilder setRequestCollapsingEnabled(final boolean requestCollapsingEnabled) {
+        this.requestCollapsingEnabled = requestCollapsingEnabled;
+        return this;
+    }
+
     @Override
     protected void customizeExecChain(final NamedElementChain<AsyncExecChainHandler> execChainDefinition) {
         final CacheConfig config = this.cacheConfig != null ? this.cacheConfig : CacheConfig.DEFAULT;
@@ -160,7 +173,8 @@ protected void customizeExecChain(final NamedElementChain<AsyncExecChainHandler>
         final AsyncCachingExec cachingExec = new AsyncCachingExec(
                 httpCache,
                 cacheRevalidator,
-                config);
+                config,
+                requestCollapsingEnabled);
         execChainDefinition.addBefore(ChainElement.PROTOCOL.name(), cachingExec, ChainElement.CACHING.name());
     }