聊聊redisson的分布式锁

序

本文主要研究一下redisson的分布式锁

maven

        <dependency>
            <groupId>org.redisson</groupId>
            <artifactId>redisson</artifactId>
            <version>3.8.1</version>
        </dependency>

实例

    @Test
    public void testDistributedLock(){
        Config config = new Config();
//        config.setTransportMode(TransportMode.EPOLL);
        config.useSingleServer()
                .setAddress("redis://192.168.99.100:6379");
        RedissonClient redisson = Redisson.create(config);


        IntStream.rangeClosed(1,5)
                .parallel()
                .forEach(i -> {
                    executeLock(redisson);
                });

        executeLock(redisson);
    }

    public void executeLock(RedissonClient redisson){
        RLock lock = redisson.getLock("myLock");
        boolean locked = false;
        try{
            LOGGER.info("try lock");
            locked = lock.tryLock();
//            locked = lock.tryLock(1,2,TimeUnit.MINUTES);
            LOGGER.info("get lock result:{}",locked);
            if(locked){
                TimeUnit.HOURS.sleep(1);
                LOGGER.info("get lock and finish");
            }
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            LOGGER.info("enter unlock");
            if(locked){
                lock.unlock();
            }
        }
    }

源码解析

RedissonLock.tryLock

redisson-3.8.1-sources.jar!/org/redisson/RedissonLock.java

    @Override
    public boolean tryLock() {
        return get(tryLockAsync());
    }

    @Override
    public RFuture<Boolean> tryLockAsync() {
        return tryLockAsync(Thread.currentThread().getId());
    }

    @Override
    public RFuture<Boolean> tryLockAsync(long threadId) {
        return tryAcquireOnceAsync(-1, null, threadId);
    }

    private RFuture<Boolean> tryAcquireOnceAsync(long leaseTime, TimeUnit unit, final long threadId) {
        if (leaseTime != -1) {
            return tryLockInnerAsync(leaseTime, unit, threadId, RedisCommands.EVAL_NULL_BOOLEAN);
        }
        RFuture<Boolean> ttlRemainingFuture = tryLockInnerAsync(commandExecutor.getConnectionManager().getCfg().getLockWatchdogTimeout(), TimeUnit.MILLISECONDS, threadId, RedisCommands.EVAL_NULL_BOOLEAN);
        ttlRemainingFuture.addListener(new FutureListener<Boolean>() {
            @Override
            public void operationComplete(Future<Boolean> future) throws Exception {
                if (!future.isSuccess()) {
                    return;
                }

                Boolean ttlRemaining = future.getNow();
                // lock acquired
                if (ttlRemaining) {
                    scheduleExpirationRenewal(threadId);
                }
            }
        });
        return ttlRemainingFuture;
    }

    <T> RFuture<T> tryLockInnerAsync(long leaseTime, TimeUnit unit, long threadId, RedisStrictCommand<T> command) {
        internalLockLeaseTime = unit.toMillis(leaseTime);

        return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, command,
                  "if (redis.call('exists', KEYS[1]) == 0) then " +
                      "redis.call('hset', KEYS[1], ARGV[2], 1); " +
                      "redis.call('pexpire', KEYS[1], ARGV[1]); " +
                      "return nil; " +
                  "end; " +
                  "if (redis.call('hexists', KEYS[1], ARGV[2]) == 1) then " +
                      "redis.call('hincrby', KEYS[1], ARGV[2], 1); " +
                      "redis.call('pexpire', KEYS[1], ARGV[1]); " +
                      "return nil; " +
                  "end; " +
                  "return redis.call('pttl', KEYS[1]);",
                    Collections.<Object>singletonList(getName()), internalLockLeaseTime, getLockName(threadId));
    }

    protected String getLockName(long threadId) {
        return id + ":" + threadId;
    }

    private void scheduleExpirationRenewal(final long threadId) {
        if (expirationRenewalMap.containsKey(getEntryName())) {
            return;
        }

        Timeout task = commandExecutor.getConnectionManager().newTimeout(new TimerTask() {
            @Override
            public void run(Timeout timeout) throws Exception {
                
                RFuture<Boolean> future = renewExpirationAsync(threadId);
                
                future.addListener(new FutureListener<Boolean>() {
                    @Override
                    public void operationComplete(Future<Boolean> future) throws Exception {
                        expirationRenewalMap.remove(getEntryName());
                        if (!future.isSuccess()) {
                            log.error("Can't update lock " + getName() + " expiration", future.cause());
                            return;
                        }
                        
                        if (future.getNow()) {
                            // reschedule itself
                            scheduleExpirationRenewal(threadId);
                        }
                    }
                });
            }

        }, internalLockLeaseTime / 3, TimeUnit.MILLISECONDS);

        if (expirationRenewalMap.putIfAbsent(getEntryName(), new ExpirationEntry(threadId, task)) != null) {
            task.cancel();
        }
    }

    protected RFuture<Boolean> renewExpirationAsync(long threadId) {
        return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,
                "if (redis.call('hexists', KEYS[1], ARGV[2]) == 1) then " +
                    "redis.call('pexpire', KEYS[1], ARGV[1]); " +
                    "return 1; " +
                "end; " +
                "return 0;",
            Collections.<Object>singletonList(getName()), 
            internalLockLeaseTime, getLockName(threadId));
    }

    private <T> RFuture<Long> tryAcquireAsync(long leaseTime, TimeUnit unit, final long threadId) {
        if (leaseTime != -1) {
            return tryLockInnerAsync(leaseTime, unit, threadId, RedisCommands.EVAL_LONG);
        }
        RFuture<Long> ttlRemainingFuture = tryLockInnerAsync(commandExecutor.getConnectionManager().getCfg().getLockWatchdogTimeout(), TimeUnit.MILLISECONDS, threadId, RedisCommands.EVAL_LONG);
        ttlRemainingFuture.addListener(new FutureListener<Long>() {
            @Override
            public void operationComplete(Future<Long> future) throws Exception {
                if (!future.isSuccess()) {
                    return;
                }

                Long ttlRemaining = future.getNow();
                // lock acquired
                if (ttlRemaining == null) {
                    scheduleExpirationRenewal(threadId);
                }
            }
        });
        return ttlRemainingFuture;
    }

• 这里leaseTime没有设置的话，默认是-1，使用的是commandExecutor.getConnectionManager().getCfg().getLockWatchdogTimeout()，默认为30秒
• tryLockInnerAsync使用的是一段lua脚本，该脚本有3个参数，第一个参数为KEYS数组，后面几个参数为ARGV数组的元素
• 这里key的值为调用方指定的这个redissonLock的名称，两个变量，第一个为leaseTime，第二个为锁的名称，使用redissonLock的id+线程id
• lua脚本第一个方法判断redissonLock的hashmap是否存在，如果不存在则创建，该hashmap有一个entry的key为锁名称，valude为1，之后设置该hashmap失效时间为leaseTime
• lua脚本第二个方法是在redissonLock的hashmap存在的情况下，将该锁名的value增1，同时设置失效时间为leaseTime
• 最后返回该redissonLock名称的key的ttl
• 执行成功之后判断ttl是否还有值，有的话则调用scheduleExpirationRenewal，防止lock未执行完就失效
• scheduleExpirationRenewal是注册一个延时任务，在internalLockLeaseTime / 3的时候触发，执行的方法是renewExpirationAsync，将该锁失效时间重置回internalLockLeaseTime
• scheduleExpirationRenewal里头给scheduleExpirationRenewal任务增加listener，如果设置成功之后还会再次递归调用scheduleExpirationRenewal重新注册延时任务
• tryAcquireAsync(long leaseTime, TimeUnit unit, final long threadId)方法是指定自动解锁时间时调用的方法，它与tryAcquireOnceAsync的区别在于，它对ttl的方回值采用long值来判断，如果是null，才执行延长失效时间的定时任务，而tryAcquireOnceAsync方法采用的是BooleanNullReplayConvertor，只要返回不是null，则返回true

RedissonLock.unlock

redisson-3.8.1-sources.jar!/org/redisson/RedissonLock.java

    @Override
    public void unlock() {
        try {
            get(unlockAsync(Thread.currentThread().getId()));
        } catch (RedisException e) {
            if (e.getCause() instanceof IllegalMonitorStateException) {
                throw (IllegalMonitorStateException)e.getCause();
            } else {
                throw e;
            }
        }
        
//        Future<Void> future = unlockAsync();
//        future.awaitUninterruptibly();
//        if (future.isSuccess()) {
//            return;
//        }
//        if (future.cause() instanceof IllegalMonitorStateException) {
//            throw (IllegalMonitorStateException)future.cause();
//        }
//        throw commandExecutor.convertException(future);
    }

    @Override
    public RFuture<Void> unlockAsync(final long threadId) {
        final RPromise<Void> result = new RedissonPromise<Void>();
        RFuture<Boolean> future = unlockInnerAsync(threadId);

        future.addListener(new FutureListener<Boolean>() {
            @Override
            public void operationComplete(Future<Boolean> future) throws Exception {
                if (!future.isSuccess()) {
                    cancelExpirationRenewal(threadId);
                    result.tryFailure(future.cause());
                    return;
                }

                Boolean opStatus = future.getNow();
                if (opStatus == null) {
                    IllegalMonitorStateException cause = new IllegalMonitorStateException("attempt to unlock lock, not locked by current thread by node id: "
                            + id + " thread-id: " + threadId);
                    result.tryFailure(cause);
                    return;
                }
                if (opStatus) {
                    cancelExpirationRenewal(null);
                }
                result.trySuccess(null);
            }
        });

        return result;
    }

    protected RFuture<Boolean> unlockInnerAsync(long threadId) {
        return commandExecutor.evalWriteAsync(getName(), LongCodec.INSTANCE, RedisCommands.EVAL_BOOLEAN,
                "if (redis.call('exists', KEYS[1]) == 0) then " +
                    "redis.call('publish', KEYS[2], ARGV[1]); " +
                    "return 1; " +
                "end;" +
                "if (redis.call('hexists', KEYS[1], ARGV[3]) == 0) then " +
                    "return nil;" +
                "end; " +
                "local counter = redis.call('hincrby', KEYS[1], ARGV[3], -1); " +
                "if (counter > 0) then " +
                    "redis.call('pexpire', KEYS[1], ARGV[2]); " +
                    "return 0; " +
                "else " +
                    "redis.call('del', KEYS[1]); " +
                    "redis.call('publish', KEYS[2], ARGV[1]); " +
                    "return 1; "+
                "end; " +
                "return nil;",
                Arrays.<Object>asList(getName(), getChannelName()), LockPubSub.unlockMessage, internalLockLeaseTime, getLockName(threadId));

    }

    String getChannelName() {
        return prefixName("redisson_lock__channel", getName());
    }

    void cancelExpirationRenewal(Long threadId) {
        ExpirationEntry task = expirationRenewalMap.get(getEntryName());
        if (task != null && (threadId == null || task.getThreadId() == threadId)) {
            expirationRenewalMap.remove(getEntryName());
            task.getTimeout().cancel();
        }
    }

• unlockInnerAsync通过lua脚本来释放锁，该lua使用两个key，一个是redissonLock名称，一个是channelName
• 该lua使用的变量有三个，一个是pubSub的unlockMessage，默认为0，一个是internalLockLeaseTime，默认为commandExecutor.getConnectionManager().getCfg().getLockWatchdogTimeout()，一个是锁名称
• 如果该redissonLock不存在，则直接发布unlock消息返回1；如果该锁不存在则返回nil；
• 如果该锁存在则将其计数-1，如果counter大于0，则重置下失效时间，返回0；如果counter不大于0，则删除该redissonLock锁，发布unlockMessage，返回1；如果上面条件都没有命中返回nil
• unlockAsync里头对unlockInnerAsync注册了FutureListener，主要是调用cancelExpirationRenewal，取消掉scheduleExpirationRenewal任务

LockPubSub

redisson-3.8.1-sources.jar!/org/redisson/pubsub/LockPubSub.java

public class LockPubSub extends PublishSubscribe<RedissonLockEntry> {

    public static final Long unlockMessage = 0L;

    @Override
    protected RedissonLockEntry createEntry(RPromise<RedissonLockEntry> newPromise) {
        return new RedissonLockEntry(newPromise);
    }

    @Override
    protected void onMessage(RedissonLockEntry value, Long message) {
        if (message.equals(unlockMessage)) {
            Runnable runnableToExecute = value.getListeners().poll();
            if (runnableToExecute != null) {
                runnableToExecute.run();
            }

            value.getLatch().release();
        }
    }

}

• 接收到unlockMessage的时候，会调用RedissonLockEntry的listener，然后触发latch的release
• tryAcquireOnceAsync这个方法默认没有创建LockPubSub，而且没有指定自动解锁时间，则定时任务会一直延长失效时间，这个可能存在锁一直没释放的风险

小结

加锁有如下注意事项：

• 加锁需要设置超时时间，防止出现死锁
• 加锁以及设置超时时间的时候，需要保证两个操作的原子性，因而最好使用lua脚本或者使用支持NX以及EX的set方法
• 加锁的时候需要把加锁的调用方信息，比如线程id给记录下来，这个在解锁的时候需要使用
• 对于加锁时长不确定的任务，为防止任务未执行完导致超时被释放，需要对尚未运行完的任务延长失效时间

解锁有如下注意事项：

• 解锁一系列操作(判断key是否存在，存在的话删除key等)需要保证原子性，因而最好使用lua脚本
• 解锁需要判断调用方是否与加锁时记录的是否一致，防止锁被误删
• 如果有延续失效时间的延时任务，在解锁的时候，需要终止掉该任务

doc

• 一分钟实现分布式锁
• 这才是真正的分布式锁
• 漫画：什么是分布式锁？
• Redis分布式锁的正确实现方式（Java版）
• 基于Redis的分布式锁到底安全吗（上）？
• 基于Redis的分布式锁到底安全吗（下）？
• Redis分布式锁的正确实现姿势
• Redisson分布式锁浅析