lhotari commented on code in PR #21681:
URL: https://github.com/apache/pulsar/pull/21681#discussion_r1425279118


##########
pulsar-common/src/main/java/org/apache/pulsar/common/util/AsyncTokenBucket.java:
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@@ -0,0 +1,483 @@
+/*
+ * 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.
+ */
+
+package org.apache.pulsar.common.util;
+
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicLongFieldUpdater;
+import java.util.concurrent.atomic.LongAdder;
+import java.util.function.LongSupplier;
+
+/**
+ * An asynchronous token bucket algorithm implementation that is optimized for 
performance with highly concurrent
+ * use. CAS (compare-and-swap) operations are used and multiple levels of CAS 
fields are used to minimize contention
+ * when using CAS fields. The {@link LongAdder} class is used in the hot path 
to hold the sum of consumed tokens.
+ * It is eventually consistent, meaning that the tokens are not updated on 
every call to the "consumeTokens" method.
+ * <p>Main usage flow:
+ * 1. tokens are consumed by calling the "consumeTokens" method.
+ * 2. the "calculateThrottlingDuration" method is called to calculate the 
duration of a possible needed pause when the
+ * tokens
+ * are fully consumed.

Review Comment:
   @Shawyeok PTAL. I have also updated the [PIP-322 
document](https://github.com/lhotari/pulsar/blob/lh-ratelimiter-refactoring-pip/pip/pip-322.md)
 with better explanations.



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