一、简介
从HBase-1.1.0起，HBase可以使用memcached作为外部BlockCache，这是一个在设备失效或者升级时不会发生完全的冷缓存的很好的特性。用句通俗的话讲，就是HBase出现故障或者升级时，缓存轻易不会丢失。
二、启动
通过配置以下两个参数实现Allow block cache to be external的启动：
配置hbase.blockcache.use.external为true，并且通过配置hbase.cache.memcached.servers来指明memcached servers.
三、实现分析
1、构造
实现HBase external BlockCache的主要是MemcachedBlockCache这个类，它同LruBlockCache一样，实现了BlockCache接口。在其构造函数中，主要是实例化了一个Memcached客户端MemcachedClient，通过这个客户端完成缓存的相关操作，如下：
public MemcachedBlockCache(Configuration c) throws IOException {
LOG.info("Creating MemcachedBlockCache");
long opTimeout = c.getLong(MEMCACHED_OPTIMEOUT_KEY, MEMCACHED_DEFAULT_TIMEOUT);
long queueTimeout = c.getLong(MEMCACHED_TIMEOUT_KEY, opTimeout + MEMCACHED_DEFAULT_TIMEOUT);
boolean optimize = c.getBoolean(MEMCACHED_OPTIMIZE_KEY, MEMCACHED_OPTIMIZE_DEFAULT);
ConnectionFactoryBuilder builder = new ConnectionFactoryBuilder()
.setOpTimeout(opTimeout)
.setOpQueueMaxBlockTime(queueTimeout) // Cap the max time before anything times out
.setFailureMode(FailureMode.Redistribute)
.setShouldOptimize(optimize)
.setDaemon(true) // Don't keep threads around past the end of days.
.setUseNagleAlgorithm(false) // Ain't nobody got time for that
.setReadBufferSize(HConstants.DEFAULT_BLOCKSIZE * 4 * 1024); // Much larger just in case
// Assume only the localhost is serving memecached.
// A la mcrouter or co-locating memcached with split regionservers.
//
// If this config is a pool of memecached servers they will all be used according to the
// default hashing scheme defined by the memcache client. Spy Memecache client in this
// case.
String serverListString = c.get(MEMCACHED_CONFIG_KEY,"localhost:11211");
String[] servers = serverListString.split(",");
List<InetSocketAddress> serverAddresses = new ArrayList<InetSocketAddress>(servers.length);
for (String s:servers) {
serverAddresses.add(Addressing.createInetSocketAddressFromHostAndPortStr(s));
}
client = new MemcachedClient(builder.build(), serverAddresses);
} 2、缓存
缓存还是通过cacheBlock()方法来实现，如下：
@Override
public void cacheBlock(BlockCacheKey cacheKey, Cacheable buf) {
if (buf instanceof HFileBlock) {
client.add(cacheKey.toString(), MAX_SIZE, (HFileBlock) buf, tc);
} else {
if (LOG.isDebugEnabled()) {
LOG.debug("MemcachedBlockCache can not cache Cacheable's of type "
+ buf.getClass().toString());
}
}
} 需要缓存的对象Cacheable必须是继承HFileBlock的子类才行，然后通过上述client将其add入Memcached。
3、回收缓存
缓存的回收，则是通过client的delete操作完成的，如下：
@Override
public boolean evictBlock(BlockCacheKey cacheKey) {
try {
cacheStats.evict();
return client.delete(cacheKey.toString()).get();
} catch (InterruptedException e) {
LOG.warn("Error deleting " + cacheKey.toString(), e);
Thread.currentThread().interrupt();
} catch (ExecutionException e) {
if (LOG.isDebugEnabled()) {
LOG.debug("Error deleting " + cacheKey.toString(), e);
}
}
return false;
}
4、获取缓存
获取缓存则是通过getBlock()方法实现的，它是通过client的get()操作来实现，如下：
public Cacheable getBlock(BlockCacheKey cacheKey, boolean caching,
boolean repeat, boolean updateCacheMetrics) {
// Assume that nothing is the block cache
HFileBlock result = null;
try (TraceScope traceScope = Trace.startSpan("MemcachedBlockCache.getBlock")) {
result = client.get(cacheKey.toString(), tc);
} catch (Exception e) {
// Catch a pretty broad set of exceptions to limit any changes in the memecache client
// and how it handles failures from leaking into the read path.
if (LOG.isDebugEnabled()) {
LOG.debug("Exception pulling from memcached [ "
+ cacheKey.toString()
+ " ]. Treating as a miss.", e);
}
result = null;
} finally {
// Update stats if this request doesn't have it turned off 100% of the time
if (updateCacheMetrics) {
if (result == null) {
cacheStats.miss(caching, cacheKey.isPrimary());
} else {
cacheStats.hit(caching, cacheKey.isPrimary());
}
}
}
return result;
} 5、初始化BlockCache
BlockCache的初始化是在类CacheConfig的静态方法instantiateBlockCache()中完成的，代码如下：
/**
* Returns the block cache or <code>null</code> in case none should be used.
* Sets GLOBAL_BLOCK_CACHE_INSTANCE
*
* @param conf The current configuration.
* @return The block cache or <code>null</code>.
*/
public static synchronized BlockCache instantiateBlockCache(Configuration conf) {
// 首选取静态成员变量GLOBAL_BLOCK_CACHE_INSTANCE，如果之前是否已经初始化过 ，立即返回
if (GLOBAL_BLOCK_CACHE_INSTANCE != null) return GLOBAL_BLOCK_CACHE_INSTANCE;
// 判断BlockCache是否被禁用
if (blockCacheDisabled) return null;
MemoryUsage mu = ManagementFactory.getMemoryMXBean().getHeapMemoryUsage();
// 通过getL1()方法获取LruBlockCache，即l1
LruBlockCache l1 = getL1(conf, mu);
// 因为getL1()的调用可能引起blockCacheDisabled的变化，再次检查
// blockCacheDisabled is set as a side-effect of getL1(), so check it again after the call.
if (blockCacheDisabled) return null;
// 通过getL2()方法获取BlockCache，即l2
BlockCache l2 = getL2(conf, mu);
// 如果l2为null，则GLOBAL_BLOCK_CACHE_INSTANCE定义为l1，下次直接获取GLOBAL_BLOCK_CACHE_INSTANCE
if (l2 == null) {
GLOBAL_BLOCK_CACHE_INSTANCE = l1;
} else {// 否则
// 判断是否启用外部缓存，通过参数hbase.blockcache.use.external判断，默认不启用
boolean useExternal = conf.getBoolean(EXTERNAL_BLOCKCACHE_KEY, EXTERNAL_BLOCKCACHE_DEFAULT);
// 判断是否启用综合缓存，通过参数hbase.bucketcache.combinedcache.enabled判断
boolean combinedWithLru = conf.getBoolean(BUCKET_CACHE_COMBINED_KEY,
DEFAULT_BUCKET_CACHE_COMBINED);
if (useExternal) {// 如果启动外部缓存，构造InclusiveCombinedBlockCache实例并赋值给GLOBAL_BLOCK_CACHE_INSTANCE
GLOBAL_BLOCK_CACHE_INSTANCE = new InclusiveCombinedBlockCache(l1, l2);
} else {
// 否则，如果启动综合缓存，构造CombinedBlockCache实例赋值给GLOBAL_BLOCK_CACHE_INSTANCE
if (combinedWithLru) {
GLOBAL_BLOCK_CACHE_INSTANCE = new CombinedBlockCache(l1, l2);
} else {
// L1 and L2 are not 'combined'. They are connected via the LruBlockCache victimhandler
// mechanism. It is a little ugly but works according to the following: when the
// background eviction thread runs, blocks evicted from L1 will go to L2 AND when we get
// a block from the L1 cache, if not in L1, we will search L2.
// 最后的一种情况则是l1
GLOBAL_BLOCK_CACHE_INSTANCE = l1;
}
}
l1.setVictimCache(l2);
}
return GLOBAL_BLOCK_CACHE_INSTANCE;
}
注释比较清晰，读者可自行阅读。
getL2()方法，在启用外部缓存的情况下，通过getExternalBlockcache()方法获取外部缓存对象，否则会尝试获取BucketCache，如下：
/**
* @param c Configuration to use.
* @param mu JMX Memory Bean
* @return Returns L2 block cache instance (for now it is BucketCache BlockCache all the time)
* or null if not supposed to be a L2.
*/
private static BlockCache getL2(final Configuration c, final MemoryUsage mu) {
final boolean useExternal = c.getBoolean(EXTERNAL_BLOCKCACHE_KEY, EXTERNAL_BLOCKCACHE_DEFAULT);
if (LOG.isDebugEnabled()) {
LOG.debug("Trying to use " + (useExternal?" External":" Internal") + " l2 cache");
}
// If we want to use an external block cache then create that.
if (useExternal) {
return getExternalBlockcache(c);
}
// otherwise use the bucket cache.
return getBucketCache(c, mu);
}
外部缓存对象通过hbase.blockcache.external.class反射构造而成，而BucketCache的是否能够构造，需要综合判断一些条件，这个后续再分析。
需要额外注意的是，上述代码中有一个地方，即l1.setVictimCache(l2)，这么做的目的是让l1和l2能够进行同步，在l1中获取不到缓存时，查询l2，并将可能的结果同步到l1中，这个下篇文章会有介绍。
下一篇博文会对上述缓存中的CombinedBlockCache和InclusiveCombinedBlockCache做一些简单介绍。
