对象池技术在服务器开发上应用广泛。在各种对象池的实现中,尤其以数据库的连接池最为明显,可以说是每个服务器必须实现的部分。
apache common pool 官方文档可以参考: https://commons.apache.org/proper/commons-pool/。
结合JedisPool看Commons Pool对象池技术
结合JedisPool,我们来了解一下commons pool的整体设计:
面向用户的往往是ObjectPool,用户看到的是一个对象池,对于使用Redis连接的用户来说,就是JedisPool。对象池ObjectPool提供了借用对象,返还对象,验证对象等API,需要具体的配置GenericObjectPoolConfig来确定池的大小,以及创建具体池化对象的工厂接口PooledObjectFactory来根据需要创建,销毁,激活,钝化每个对象。
PooledObjectFactory接口,用来创建池对象(makeObject),将不用的池对象进行钝化(passivateObject),对要使用的池对象进行激活(activateObject),对池对象进行验证(valiateObject),将有问题的池对象销毁(destroyObject)。
如果需要使用commons-pool,那么就需要提供一个PooledObjectFactory接口的具体实现,一个比较简单的办法是使用BasePooledObjectFactory这个抽象类,只需要实现两个方法:create()和wrap(T obj)。JedisFactory也就是用来创建每个Jedis连接的对象工厂类,其中直接实现了PooledObjectFactory,makeObject的过程中,直接创建了PooledObject。
当我们使用JedisPool.getResource(),用于返回jedis连接时,实际调用的是其中GenericObjectPool的borrowObject方法,在Jedis连接池中借用一个对象。
借用对象时,先去idleObjects(LinkedBlockingDeque>)列表中查看是否有空闲的对象,如果存在则直接使用;如果不存在,则需要考虑在没有超出连接池最大数量的情况下,使用PooledObjectFactory进行初始化,这里使用的是JedisFactory.makeObject来创建连接,并将其激活。
对于Jedis对象,不能总是重用同一个对象,在使用一段时间后其就会产生失效,连接出现异常。此时就需要使用JedisPool来获取资源,注意在最后要回收资源,实际上就是returnObject,以下面的代码作为实例:
Jedis jedis = jedisPool.getResource();
try {
while (true) {
String productCountString = jedis.get("product");
if (Integer.parseInt(productCountString) > 0) {
if (acquireLock(jedis, "abc")) {
int productCount = Integer.parseInt(jedis.get("product"));
System.out.println(String.format("%tT --- Get product: %s", new Date(), productCount));
// System.out.println(productCount);
jedis.decr("product");
releaseLock(jedis, "abc");
return "Success";
}
Thread.sleep(1000L);
} else {
return "Over";
}
}
} finally {
jedis.close();
}
JedisCluster的连接/执行源码研究
我们使用的JedisCluster(Redis集群模式)需要初始化并使用JedisCluster对象,通过该对象来进行Redis的相关操作,下面就列举出了JedisCluster的基本类图结构:
在执行任务BinaryJedisCluster的相关命令 set/get/exist 等redis命令时,都采用回调的方式:
@Override
public String set(final byte[] key, final byte[] value) {
return new JedisClusterCommand(connectionHandler, maxRedirections) {
@Override
public String execute(Jedis connection) {
return connection.set(key, value);
}
}.runBinary(key);
}
初始化一个JedisClusterCommand对象,执行runBinary方法,进行execute(Jedis connection)回调,其实可以看出执行回调之前的作用是将使用到的Jedis连接在内部统一管理起来。
可以猜想使用了JedisSlotBasedConnectionHandler中实现了父类定义的getConnection()获取Redis连接的方法:
@Override
public Jedis getConnection() {
// In antirez's redis-rb-cluster implementation,
// getRandomConnection always return valid connection (able to
// ping-pong)
// or exception if all connections are invalid
List pools = getShuffledNodesPool();
for (JedisPool pool : pools) {
Jedis jedis = null;
try {
jedis = pool.getResource();
if (jedis == null) {
continue;
}
String result = jedis.ping();
if (result.equalsIgnoreCase("pong")) return jedis;
pool.returnBrokenResource(jedis);
} catch (JedisConnectionException ex) {
if (jedis != null) {
pool.returnBrokenResource(jedis);
}
}
}
throw new JedisConnectionException("no reachable node in cluster");
}
其中调用的方法 getShuffledNodesPool(),就是从JedisClusterInfoCache中包含的所有JedisPool,执行shuffle操作,随机拿到对应的JedisPool,去其中getResource拿到连接。
这属于随机去获取connection,但事实上并不是这样处理的,我们可以通过slot来获得其对应的Connection,在JedisClusterCommand.run方法的最后一行中,其中第三个参数为是否为tryRandomMode,调用方式显示为非random Mode。
return runWithRetries(SafeEncoder.encode(keys[0]), this.redirections, false, false);
可以根据slot来定位到具体的JedisPool,getResource拿到对应的Jedis Connection,但该方法也标明了不能保证一定能够拿到可用的连接。
@Override
public Jedis getConnectionFromSlot(int slot) {
JedisPool connectionPool = cache.getSlotPool(slot);
if (connectionPool != null) {
// It can't guaranteed to get valid connection because of node
// assignment
return connectionPool.getResource();
} else {
return getConnection();
}
}
在JedisClusterInfoCache缓存了Map(host:port->JedisPool)和Map(slot->JedisPool),用于查询连接,那么这两个缓存是如何查询出来的,这就需要用到Jedis.clusterNodes,它可以通过该Redis连接找到其他连接的相关配置,例如可以发现整个集群的配置,其中三个master,三个slave,并且能够识别出自身连接,可参考文档: http://redis.io/commands/cluster-nodes:
5974ed7dd81c112d9a2354a0a985995913b4702c 192.168.1.137:6389 master - 0 1468809898374 26 connected 0-5640
d08dc883ee4fcb90c4bb47992ee03e6474398324 192.168.1.137:6390 master - 0 1468809898875 25 connected 5641-11040
ffb4db4e1ced0f91ea66cd2335f7e4eadc29fd56 192.168.1.138:6390 slave 5974ed7dd81c112d9a2354a0a985995913b4702c 0 1468809899376 26 connected
c69b521a30336caf8bce078047cf9bb5f37363ee 192.168.1.137:6388 master - 0 1468809897873 28 connected 11041-16383
532e58842d001f8097fadc325bdb5541b788a360 192.168.1.138:6389 slave c69b521a30336caf8bce078047cf9bb5f37363ee 0 1468809899876 28 connected
aa52c7810e499d042e94e0aa4bc28c57a1da74e3 192.168.1.138:6388 myself,slave d08dc883ee4fcb90c4bb47992ee03e6474398324 0 0 19 connected
分配slot只可能在master节点上发生,而不会在slave节点上发生,这意味着Redis集群并未进行类似读写分离的形式。当Redis集群的slot发生改变时,会重新初始化该Cache,重置slot。
而执行每个get/set等Redis操作时,真正的核心入口,其实是JedisClusterCommand.runWithRetries方法:
private T runWithRetries(byte[] key, int redirections, boolean tryRandomNode, boolean asking) {
if (redirections <= 0) {
throw new JedisClusterMaxRedirectionsException("Too many Cluster redirections?");
}
Jedis connection = null;
try {
if (asking) {
// TODO: Pipeline asking with the original command to make it
// faster....
connection = askConnection.get();
connection.asking();
// if asking success, reset asking flag
asking = false;
} else {
if (tryRandomNode) {
connection = connectionHandler.getConnection();
} else {
connection = connectionHandler.getConnectionFromSlot(JedisClusterCRC16.getSlot(key));
}
}
return execute(connection);
} catch (JedisConnectionException jce) {
if (tryRandomNode) {
// maybe all connection is down
throw jce;
}
// release current connection before recursion
releaseConnection(connection);
connection = null;
// retry with random connection
return runWithRetries(key, redirections - 1, true, asking);
} catch (JedisRedirectionException jre) {
// if MOVED redirection occurred,
if (jre instanceof JedisMovedDataException) {
// it rebuilds cluster's slot cache
// recommended by Redis cluster specification
this.connectionHandler.renewSlotCache(connection);
}
// release current connection before recursion or renewing
releaseConnection(connection);
connection = null;
if (jre instanceof JedisAskDataException) {
asking = true;
askConnection.set(this.connectionHandler.getConnectionFromNode(jre.getTargetNode()));
} else if (jre instanceof JedisMovedDataException) {
} else {
throw new JedisClusterException(jre);
}
return runWithRetries(key, redirections - 1, false, asking);
} finally {
releaseConnection(connection);
}
}
出现的Redis Retries问题
可以参考: http://carlosfu.iteye.com/blog/2251034,讲的非常好。同样,我们的出现的异常堆栈:
- 2016-06-04 00:02:51,911 [// - - ] ERROR xxx - Too many Cluster redirections?
redis.clients.jedis.exceptions.JedisClusterMaxRedirectionsException: Too many Cluster redirections?
at redis.clients.jedis.JedisClusterCommand.runWithRetries(JedisClusterCommand.java:97)
at redis.clients.jedis.JedisClusterCommand.runWithRetries(JedisClusterCommand.java:131)
at redis.clients.jedis.JedisClusterCommand.runWithRetries(JedisClusterCommand.java:152)
at redis.clients.jedis.JedisClusterCommand.runWithRetries(JedisClusterCommand.java:131)
直译过来就是出现过多的redirections异常,出现过JedisConnectionException,完整的堆栈内容:
redis.clients.jedis.exceptions.JedisConnectionException: Unexpected end of stream.
at redis.clients.util.RedisInputStream.ensureFill(RedisInputStream.java:198)
at redis.clients.util.RedisInputStream.readByte(RedisInputStream.java:40)
at redis.clients.jedis.Protocol.process(Protocol.java:141)
at redis.clients.jedis.Protocol.read(Protocol.java:205)
at redis.clients.jedis.Connection.readProtocolWithCheckingBroken(Connection.java:297)
at redis.clients.jedis.Connection.getBinaryBulkReply(Connection.java:216)
at redis.clients.jedis.Connection.getBulkReply(Connection.java:205)
at redis.clients.jedis.Jedis.get(Jedis.java:101)
at redis.clients.jedis.JedisCluster$3.execute(JedisCluster.java:79)
at redis.clients.jedis.JedisCluster$3.execute(JedisCluster.java:76)
at redis.clients.jedis.JedisClusterCommand.runWithRetries(JedisClusterCommand.java:119)
at redis.clients.jedis.JedisClusterCommand.run(JedisClusterCommand.java:30)
at redis.clients.jedis.JedisCluster.get(JedisCluster.java:81)
at redis.RedisClusterTest.main(RedisClusterTest.java:30)
调试状态下的异常信息:
jce = {redis.clients.jedis.exceptions.JedisConnectionException@1014} "redis.clients.jedis.exceptions.JedisConnectionException: Unexpected end of stream."
detailMessage = "Unexpected end of stream."
cause = {redis.clients.jedis.exceptions.JedisConnectionException@1014} "redis.clients.jedis.exceptions.JedisConnectionException: Unexpected end of stream."
stackTrace = {java.lang.StackTraceElement[0]@1017}
suppressedExceptions = {java.util.Collections$UnmodifiableRandomAccessList@1018} size = 0
关于这个问题,可以参考:http://blog.csdn.net/jiangguilong2000/article/details/45025355
客户端buffer控制。在客户端与server进行的交互中,每个连接都会与一个buffer关联,此buffer用来队列化等待被client接受的响应信息。如果client不能及时的消费响应信息,那么buffer将会被不断积压而给server带来内存压力.如果buffer中积压的数据达到阀值,将会导致连接被关闭,buffer被移除。
开发环境上执行查询该参数的命令:config get client-output-buffer-limit
1) "client-output-buffer-limit"
2) "normal 0 0 0 slave 268435456 67108864 60 pubsub 33554432 8388608 60"
关于Redis上的所有参数详解,可以参考:http://shift-alt-ctrl.iteye.com/blog/1882850
JedisMovedDataException
jre = {redis.clients.jedis.exceptions.JedisMovedDataException@2008} "redis.clients.jedis.exceptions.JedisMovedDataException: MOVED 8855 192.168.1.137:6390"
targetNode = {redis.clients.jedis.HostAndPort@2015} "192.168.1.137:6390"
slot = 8855
detailMessage = "MOVED 8855 192.168.1.137:6390"
cause = {redis.clients.jedis.exceptions.JedisMovedDataException@2008} "redis.clients.jedis.exceptions.JedisMovedDataException: MOVED 8855 192.168.1.137:6390"
stackTrace = {java.lang.StackTraceElement[0]@1978}
suppressedExceptions = {java.util.Collections$UnmodifiableRandomAccessList@1979} size = 0
日志中出现超时异常:
4851:S 18 Jul 11:05:38.005 * Asynchronous AOF fsync is taking too long (disk is busy?). Writing the AOF buffer without waiting for fsync to complete, this may slow down Redis.
可以参考github上关于redis的讨论: https://github.com/antirez/redis/issues/641,关闭AOF,可以暂时解决问题。JedisCluster中应用的Apache Commons Pool对象池技术