最近看kafka源码，着实被它的客户端缓冲池技术优雅到了。忍不住要写篇文章赞美一下（哈哈）。
注：本文用到的源码来自kafka2.2.2版本。
背景
当我们应用程序调用kafka客户端 producer发送消息的时候，在kafka客户端内部，会把属于同一个topic分区的消息先汇总起来，形成一个batch。真正发往kafka服务器的消息都是以batch为单位的。如下图所示：
这么做的好处显而易见。客户端和服务端通过网络通信，这样批量发送可以减少网络带来的性能开销，提高吞吐量。
这个Batch的管理就非常值得探讨了。可能有人会说，这不简单吗？用的时候分配一个块内存，发送完了释放不就行了吗。
kafka是用java语言编写的（新版本大部分都是用java实现的了），用上面的方案就是使用的时候new一个空间然后赋值给一个引用，释放的时候把引用置为null等JVM GC处理就可以了。
看起来似乎也没啥问题。但是在并发量比较高的时候就会频繁的进行GC。我们都知道GC的时候有个stop the world，尽管最新的GC技术这个时间已经非常短，依然有可能成为生产环境的性能瓶颈。
kafka的设计者当然能考虑到这一层。下面我们就来学习下kafka是如何对batch进行管理的。
缓冲池技术原理解析
kafka客户端使用了缓冲池的概念，预先分配好真实的内存块，放在池子里。
每个batch其实都对应了缓冲池中的一个内存空间，发送完消息之后，batch不再使用了，就把内存块归还给缓冲池。
听起来是不是很耳熟啊？不错，数据库连接池，线程池等池化技术其实差不多都是这样的原理。通过池化技术降低创建和销毁带来的开销，提升执行效率。
代码是最好的文档，，下面我们就来撸下源码。
我们撸代码的步骤采用的是从上往下的原则，先带你看看缓冲池在哪里使用，然后再深入到缓存池内部深入分析。
下面的代码做了一些删减，值保留了跟本文相关的部分便于分析。
public class KafkaProducer<K, V> implements Producer<K, V> {private final Logger log;private static final AtomicInteger PRODUCER_CLIENT_ID_SEQUENCE = new AtomicInteger(1);private static final String JMX_PREFIX = "kafka.producer";public static final String NETWORK_THREAD_PREFIX = "kafka-producer-network-thread";public static final String PRODUCER_METRIC_GROUP_NAME = "producer-metrics";@Overridepublic Future<RecordMetadata> send(ProducerRecord<K, V> record, Callback callback) {// intercept the record, which can be potentially modified; this method does not throw exceptionsProducerRecord<K, V> interceptedRecord = this.interceptors.onSend(record);return doSend(interceptedRecord, callback);}private Future<RecordMetadata> doSend(ProducerRecord<K, V> record, Callback callback) {RecordAccumulator.RecordAppendResult result = accumulator.append(tp, timestamp, serializedKey,serializedValue, headers, interceptCallback, remainingWaitMs);...}
当我们调用客户端的发送消息的时候，底层会调用doSend，然后里面使用一个记录累计器RecordAccumulator把消息append进来。我们继续往下看看，
public final class RecordAccumulator {private final Logger log;private volatile boolean closed;private final AtomicInteger flushesInProgress;private final AtomicInteger appendsInProgress;private final int batchSize;private final CompressionType compression;private final int lingerMs;private final long retryBackoffMs;private final int deliveryTimeoutMs;private final BufferPool free;private final Time time;private final ApiVersions apiVersions;private final ConcurrentMap<TopicPartition, Deque<ProducerBatch>> batches;private final IncompleteBatches incomplete;// The following variables are only accessed by the sender thread, so we don't need to protect them.private final Map<TopicPartition, Long> muted;private int drainIndex;private final TransactionManager transactionManager;private long nextBatchExpiryTimeMs = Long.MAX_VALUE; // the earliest time (absolute) a batch will expire.public RecordAppendResult append(TopicPartition tp,long timestamp,byte[] key,byte[] value,Header[] headers,Callback callback,long maxTimeToBlock) throws InterruptedException {// We keep track of the number of appending thread to make sure we do not miss batches in// abortIncompleteBatches().appendsInProgress.incrementAndGet();ByteBuffer buffer = null;buffer = free.allocate(size, maxTimeToBlock);synchronized (dq) {// Need to check if producer is closed again after grabbing the dequeue lock.if (closed)throw new KafkaException("Producer closed while send in progress");RecordAppendResult appendResult = tryAppend(timestamp, key, value, headers, callback, dq);if (appendResult != null) {// Somebody else found us a batch, return the one we waited for! Hopefully this doesn't happen often...return appendResult;}MemoryRecordsBuilder recordsBuilder = recordsBuilder(buffer, maxUsableMagic);ProducerBatch batch = new ProducerBatch(tp, recordsBuilder, time.milliseconds());FutureRecordMetadata future = Utils.notNull(batch.tryAppend(timestamp, key, value, headers, callback, time.milliseconds()));dq.addLast(batch);...
RecordAccumulator其实就是管理一个batch队列，我们看到append方法实现其实是调用BufferPool的free方法申请（allocate）了一块内存空间(ByteBuffer)， 然后把这个内存空空间包装成batch添加到队列后面。
当消息发送完成不在使用batch的时候，RecordAccumulator会调用deallocate方法归还内存，内部其实是调用BufferPool的deallocate方法。
public void deallocate(ProducerBatch batch) {incomplete.remove(batch);// Only deallocate the batch if it is not a split batch because split batch are allocated outside the// buffer pool.if (!batch.isSplitBatch())free.deallocate(batch.buffer(), batch.initialCapacity());}
很明显，BufferPool就是缓冲池管理的类，也是我们今天要讨论的重点。我们先来看看分配内存块的方法。
public class BufferPool {static final String WAIT_TIME_SENSOR_NAME = "bufferpool-wait-time";private final long totalMemory;private final int poolableSize;private final ReentrantLock lock;private final Deque<ByteBuffer> free;private final Deque<Condition> waiters;/** Total available memory is the sum of nonPooledAvailableMemory and the number of byte buffers in free * poolableSize. */private long nonPooledAvailableMemory;private final Metrics metrics;private final Time time;private final Sensor waitTime;public ByteBuffer allocate(int size, long maxTimeToBlockMs) throws InterruptedException {if (size > this.totalMemory)throw new IllegalArgumentException("Attempt to allocate " + size+ " bytes, but there is a hard limit of "+ this.totalMemory+ " on memory allocations.");ByteBuffer buffer = null;this.lock.lock();try {// check if we have a free buffer of the right size pooledif (size == poolableSize && !this.free.isEmpty())return this.free.pollFirst();// now check if the request is immediately satisfiable with the// memory on hand or if we need to blockint freeListSize = freeSize() * this.poolableSize;if (this.nonPooledAvailableMemory + freeListSize >= size) {// we have enough unallocated or pooled memory to immediately// satisfy the request, but need to allocate the bufferfreeUp(size);this.nonPooledAvailableMemory -= size;} else {// we are out of memory and will have to blockint accumulated = 0;Condition moreMemory = this.lock.newCondition();try {long remainingTimeToBlockNs = TimeUnit.MILLISECONDS.toNanos(maxTimeToBlockMs);this.waiters.addLast(moreMemory);// loop over and over until we have a buffer or have reserved// enough memory to allocate onewhile (accumulated < size) {long startWaitNs = time.nanoseconds();long timeNs;boolean waitingTimeElapsed;try {waitingTimeElapsed = !moreMemory.await(remainingTimeToBlockNs, TimeUnit.NANOSECONDS);} finally {long endWaitNs = time.nanoseconds();timeNs = Math.max(0L, endWaitNs - startWaitNs);recordWaitTime(timeNs);}if (waitingTimeElapsed) {throw new TimeoutException("Failed to allocate memory within the configured max blocking time " + maxTimeToBlockMs + " ms.");}remainingTimeToBlockNs -= timeNs;// check if we can satisfy this request from the free list,// otherwise allocate memoryif (accumulated == 0 && size == this.poolableSize && !this.free.isEmpty()) {// just grab a buffer from the free listbuffer = this.free.pollFirst();accumulated = size;} else {// we'll need to allocate memory, but we may only get// part of what we need on this iterationfreeUp(size - accumulated);int got = (int) Math.min(size - accumulated, this.nonPooledAvailableMemory);this.nonPooledAvailableMemory -= got;accumulated += got;}...
首先整个方法是加锁操作的，所以支持并发分配内存。
逻辑是这样的，当申请的内存大小等于poolableSize，则从缓存池中获取。这个poolableSize可以理解成是缓冲池的页大小，作为缓冲池分配的基本单位。从缓存池获取其实就是从ByteBuffer队列取出一个元素返回。
如果申请的内存不等于特定的数值，则向非缓存池申请。同时会从缓冲池中取一些内存并入到非缓冲池中。这个nonPooledAvailableMemory指的就是非缓冲池的可用内存大小。非缓冲池分配内存，其实就是调用ByteBuffer.allocat分配真实的JVM内存。
缓存池的内存一般都很少回收。而非缓存池的内存是使用后丢弃，然后等待GC回收。
继续来看看batch释放的代码，
public void deallocate(ByteBuffer buffer, int size) {lock.lock();try {if (size == this.poolableSize && size == buffer.capacity()) {buffer.clear();this.free.add(buffer);} else {this.nonPooledAvailableMemory += size;}Condition moreMem = this.waiters.peekFirst();if (moreMem != null)moreMem.signal();} finally {lock.unlock();}}
很简单，也是分为两种情况。要么直接归还缓冲池，要么就是更新非缓冲池部分的可以内存。然后通知等待队列里的第一个元素。
