使用redisson时关于订阅数的问题

在使用redisson消息订阅时，我针对门店商品库存减扣进行订阅的操作(在这里一个商品一个监听队列)，当正式投入生产时，发现一直再报Subscribe timeout: (" + timeout + "ms). Increase 'subscriptionsPerConnection' and/or 'subscriptionConnectionPoolSize' parameters.的错误，索性根据提示翻了翻源码看看原因：

在redisson里先关注一个类：RedisPubSubConnection该类继承自RedisConnection，根据名字我们可知它是一个典型的发布与订阅的类。那么在redisson使用时，会使用PubSubConnectionEntry进行一次包装：

    public class PubSubConnectionEntry {
    
        private final AtomicInteger subscribedChannelsAmount;
        private final RedisPubSubConnection conn;
    
        private final ConcurrentMap subscribeChannelListeners = new ConcurrentHashMap();
        private final ConcurrentMap>> channelListeners = new ConcurrentHashMap>>();
    
        public PubSubConnectionEntry(RedisPubSubConnection conn, int subscriptionsPerConnection) {
            super();
            this.conn = conn;
            this.subscribedChannelsAmount = new AtomicInteger(subscriptionsPerConnection);
        }
      
     //.....省略其他代码 
    }

在这里我们可以看到其有一个比较重要的属性 subscribedChannelsAmount，而这个值就是通过PublishSubscribeService进行调用的：

        private void connect(final Codec codec, final ChannelName channelName,
                final RPromise promise, final PubSubType type, final AsyncSemaphore lock, final RedisPubSubListener... listeners) {
          //....
          
          
             RedisPubSubConnection conn = future.getNow();
                    
                    final PubSubConnectionEntry entry = new PubSubConnectionEntry(conn, config.getSubscriptionsPerConnection());
                    entry.tryAcquire();
          //....
        }

那么此属性就是根据config的subscriptionsPerConnection里设置的，那么此值就代表了每个连接的最大订阅数。当tryAcqcurie的时候会减少这个数量：

      public int tryAcquire() {
            while (true) {
                int value = subscribedChannelsAmount.get();
                if (value == 0) {
                    return -1;
                }
                
                if (subscribedChannelsAmount.compareAndSet(value, value - 1)) {
                    return value - 1;
                }
            }
        }

如果当此值为0时，那么会重新获取一个可用的连接，代码如下：

     int remainFreeAmount = freeEntry.tryAcquire();
                    if (remainFreeAmount == -1) {
                        throw new IllegalStateException();
                    }
                    
                    final PubSubConnectionEntry oldEntry = name2PubSubConnection.putIfAbsent(channelName, freeEntry);
                    if (oldEntry != null) {
                        freeEntry.release();
                        freePubSubLock.release();
                        subscribe(channelName, promise, type, lock, oldEntry, listeners);
                        return;
                    }
                    
                    if (remainFreeAmount == 0) {
                        freePubSubConnections.poll();
                    }
                    freePubSubLock.release();

如果此时没有可用的连接的话，恐怕此次操作就会等待新的连接直至超时，超时了就报上述的错误了，不过根据提示。我们此时的解决办法是增大subscriptionsPerConnection或者subscriptionConnectionPoolSize的值。当我们使用springboot时可以通过设置spring.redis.redisson.config(具体设置请参考官网)来指定redisson的配置文件或者重新创建RedissonClient:

            @Bean(destroyMethod = "shutdown")
            public RedissonClient redisson(RedissonProperties redissonProperties, RedisProperties redisProperties) throws IOException {
    
                Config config = new Config();
                String prefix = "redis://";
                Method method = ReflectionUtils.findMethod(RedisProperties.class, "isSsl");
                if (method != null && (Boolean) ReflectionUtils.invokeMethod(method, redisProperties)) {
                    prefix = "rediss://";
                }
    
                config.useSingleServer()
                        .setAddress(prefix + redisProperties.getHost() + ":" + redisProperties.getPort())
                        .setConnectTimeout(30000).setSubscriptionsPerConnection(5000) //在这里指定数目
                        .setDatabase(redisProperties.getDatabase())
                        .setPassword(redisProperties.getPassword());
    
                return Redisson.create(config);
            }