[GitHub] [james-project] quantranhong1999 commented on a diff in pull request #1185: JAMES-3816 Limit IMAP concurrent requests

[GitBox <gi...@apache.org>]

quantranhong1999 commented on code in PR #1185:
URL: https://github.com/apache/james-project/pull/1185#discussion_r971485162


##########
server/protocols/protocols-imap4/src/main/java/org/apache/james/imapserver/netty/ReactiveThrottler.java:
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@@ -0,0 +1,91 @@
+/****************************************************************
+ * 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                 *
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+ * 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.                                           *
+ ****************************************************************/
+
+package org.apache.james.imapserver.netty;
+
+import java.util.Queue;
+import java.util.concurrent.ConcurrentLinkedQueue;
+import java.util.concurrent.atomic.AtomicInteger;
+
+import org.apache.james.metrics.api.GaugeRegistry;
+import org.reactivestreams.Publisher;
+
+import reactor.core.publisher.Mono;
+import reactor.core.publisher.Sinks;
+import reactor.core.scheduler.Schedulers;
+
+public class ReactiveThrottler {
+    public static class RejectedException extends RuntimeException {
+        public RejectedException(String message) {
+            super(message);
+        }
+    }
+
+    private final int maxConcurrentRequests;
+    private final int maxQueueSize;
+    // In flight + executing
+    private final AtomicInteger concurrentRequests = new AtomicInteger(0);
+    private final Queue<Publisher<Void>> queue = new ConcurrentLinkedQueue<>();
+
+    public ReactiveThrottler(GaugeRegistry gaugeRegistry, int maxConcurrentRequests, int maxQueueSize) {
+        gaugeRegistry.register("imap.request.queue.size", () -> Math.max(concurrentRequests.get() - maxConcurrentRequests, 0));
+
+        this.maxConcurrentRequests = maxConcurrentRequests;
+        this.maxQueueSize = maxQueueSize;
+    }
+
+    public Mono<Void> throttle(Publisher<Void> task) {
+        if (maxConcurrentRequests < 0) {
+            return Mono.from(task);
+        }
+        int requestNumber = concurrentRequests.incrementAndGet();
+
+        if (requestNumber <= maxConcurrentRequests) {
+            // We have capacity for one more concurrent request
+            return Mono.from(task)
+                .doFinally(any -> onRequestDone());
+        } else if (requestNumber <= maxQueueSize + maxConcurrentRequests) {
+            // Queue the request for later
+            Sinks.One<Void> one = Sinks.one();
+            queue.add(Mono.from(task)
+                .then(Mono.fromRunnable(() -> one.emitEmpty(Sinks.EmitFailureHandler.FAIL_FAST))));
+            // Let the caller await task completion
+            return one.asMono();
+        } else {
+            concurrentRequests.decrementAndGet();
+
+            return Mono.error(new RejectedException(
+                String.format(
+                    "The IMAP server has reached its maximum capacity "
+                        + "(concurrent requests: %d, queue size: %d)",
+                    maxConcurrentRequests, maxQueueSize)));
+        }
+    }
+
+    private void onRequestDone() {
+        concurrentRequests.decrementAndGet();
+        Publisher<Void> throttled = queue.poll();
+        if (throttled != null) {
+            Mono.from(throttled)
+                .doFinally(any -> onRequestDone())
+                .subscribeOn(Schedulers.parallel())

Review Comment:
   I have a concern here about the `Schedulers.parallel()`: the deeper layer of our code may contain some blocking code that is not friendly with the parallel scheduler.



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