TimeBucketCounter.java
/*
* 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.catalina.util;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.atomic.AtomicInteger;
/**
* this class maintains a thread safe hash map that has timestamp-based buckets followed by a string for a key, and a
* counter for a value. each time the increment() method is called it adds the key if it does not exist, increments its
* value and returns it. a maintenance thread cleans up keys that are prefixed by previous timestamp buckets.
*/
public class TimeBucketCounter {
/**
* Map to hold the buckets
*/
private final ConcurrentHashMap<String,AtomicInteger> map = new ConcurrentHashMap<>();
/**
* Milliseconds bucket size as a Power of 2 for bit shift math, e.g. 16 for 65_536ms which is about 1:05 minute
*/
private final int numBits;
/**
* ratio of actual duration to config duration
*/
private final double ratio;
/**
* flag for the maintenance thread
*/
volatile boolean isRunning = false;
/**
* Creates a new TimeBucketCounter with the specified lifetime.
*
* @param bucketDuration duration in seconds, e.g. for 1 minute pass 60
*/
public TimeBucketCounter(int bucketDuration) {
int durationMillis = bucketDuration * 1000;
int bits = 0;
int pof2 = nextPowerOf2(durationMillis);
int bitCheck = pof2;
while (bitCheck > 1) {
bitCheck = pof2 >> ++bits;
}
this.numBits = bits;
this.ratio = ratioToPowerOf2(durationMillis);
int cleanupsPerBucketDuration = (durationMillis >= 60_000) ? 6 : 3;
Thread mt = new MaintenanceThread(durationMillis / cleanupsPerBucketDuration);
mt.start();
}
/**
* increments the counter for the passed identifier in the current time bucket and returns the new value
*
* @param identifier an identifier for which we want to maintain count, e.g. IP Address
*
* @return the count within the current time bucket
*/
public final int increment(String identifier) {
String key = getCurrentBucketPrefix() + "-" + identifier;
AtomicInteger ai = map.get(key);
if (ai == null) {
// there is a small chance of a benign data race where we might not count a request or
// two but as a tradeoff in favor of performance we do not synchronize this operation
ai = new AtomicInteger();
AtomicInteger currentValue = map.putIfAbsent(key, ai);
if (currentValue != null) {
ai = currentValue;
}
}
return ai.incrementAndGet();
}
/**
* calculates the current time bucket prefix by shifting bits for fast division, e.g. shift 16 bits is the same as
* dividing by 65,536 which is about 1:05m
*
* @return The current bucket prefix.
*/
public final int getCurrentBucketPrefix() {
return (int) (System.currentTimeMillis() >> this.numBits);
}
public int getNumBits() {
return numBits;
}
/**
* the actual duration may differ from the configured duration because it is set to the next power of 2 value in
* order to perform very fast bit shift arithmetic
*
* @return the actual bucket duration in milliseconds
*/
public int getActualDuration() {
return (int) Math.pow(2, getNumBits());
}
/**
* returns the ratio between the configured duration param and the actual duration which will be set to the next
* power of 2. we then multiply the configured requests param by the same ratio in order to compensate for the added
* time, if any
*
* @return the ratio, e.g. 1.092 if the actual duration is 65_536 for the configured duration of 60_000
*/
public double getRatio() {
return ratio;
}
/**
* returns the ratio to the next power of 2 so that we can adjust the value
*/
static double ratioToPowerOf2(int value) {
double nextPO2 = nextPowerOf2(value);
return Math.round((1000 * nextPO2 / value)) / 1000d;
}
/**
* returns the next power of 2 given a value, e.g. 256 for 250, or 1024, for 1000
*/
static int nextPowerOf2(int value) {
int valueOfHighestBit = Integer.highestOneBit(value);
if (valueOfHighestBit == value) {
return value;
}
return valueOfHighestBit << 1;
}
/**
* when we want to test a full bucket duration we need to sleep until the next bucket starts
*
* @return the number of milliseconds until the next bucket
*/
public long getMillisUntilNextBucket() {
long millis = System.currentTimeMillis();
long nextTimeBucketMillis = ((millis + (long) Math.pow(2, numBits)) >> numBits) << numBits;
long delta = nextTimeBucketMillis - millis;
return delta;
}
/**
* sets isRunning to false to terminate the maintenance thread
*/
public void destroy() {
this.isRunning = false;
}
/**
* this class runs a background thread to clean up old keys from the map
*/
class MaintenanceThread extends Thread {
final long sleeptime;
MaintenanceThread(long sleeptime) {
super.setDaemon(true);
this.sleeptime = sleeptime;
}
@SuppressWarnings("sync-override")
@Override
public void start() {
isRunning = true;
super.start();
}
@Override
public void run() {
while (isRunning) {
String currentBucketPrefix = String.valueOf(getCurrentBucketPrefix());
Set<String> keys = map.keySet();
for (String k : keys) {
if (!k.startsWith(currentBucketPrefix)) {
// the key is obsolete, remove it
keys.remove((k));
}
}
try {
Thread.sleep(sleeptime);
} catch (InterruptedException e) {
}
}
}
}
}