聊聊hikari链接池的fixed pool design

序

本文主要研究一下hikari链接池的fixed pool design

fixed pool design

hikari的做者比较倾向于fixed pool design的理念，即建议minimumIdle与maximumPoolSize设置成同样，当作固定链接大小的链接池。做者认为minimumIdle小于maximumPoolSize的话，在流量激增的时候须要额外的链接，此时在请求方法里头再去处理新建链接会形成性能损失，即会致使数据库一方面下降链接创建的速度，另外一方面也会影响既有的链接事务的完成，间接影响了这些既有链接归还到链接池的速度。

做者认为minimumIdle与maximumPoolSize设置成同样，多余的空闲链接不会对总体的性能有什么严重影响，若是说设置minimumIdle小于maximumPoolSize是为了在必要的时候能够释放链接以释放内存给其余功能用，可是在高峰时期，链接池可能也会到达maximumPoolSize，于是这个目的彷佛没起到效果。

HikariPool.houseKeeperTask

HikariCP-2.7.6-sources.jar!/com/zaxxer/hikari/pool/HikariPool.java

private final long HOUSEKEEPING_PERIOD_MS = Long.getLong("com.zaxxer.hikari.housekeeping.periodMs", SECONDS.toMillis(30));

   /**
    * Construct a HikariPool with the specified configuration.
    *
    * @param config a HikariConfig instance
    */
   public HikariPool(final HikariConfig config)
   {
      super(config);

      this.connectionBag = new ConcurrentBag<>(this);
      this.suspendResumeLock = config.isAllowPoolSuspension() ? new SuspendResumeLock() : SuspendResumeLock.FAUX_LOCK;

      this.houseKeepingExecutorService = initializeHouseKeepingExecutorService();

      checkFailFast();

      if (config.getMetricsTrackerFactory() != null) {
         setMetricsTrackerFactory(config.getMetricsTrackerFactory());
      }
      else {
         setMetricRegistry(config.getMetricRegistry());
      }

      setHealthCheckRegistry(config.getHealthCheckRegistry());

      registerMBeans(this);

      ThreadFactory threadFactory = config.getThreadFactory();

      LinkedBlockingQueue<Runnable> addConnectionQueue = new LinkedBlockingQueue<>(config.getMaximumPoolSize());
      this.addConnectionQueue = unmodifiableCollection(addConnectionQueue);
      this.addConnectionExecutor = createThreadPoolExecutor(addConnectionQueue, poolName + " connection adder", threadFactory, new ThreadPoolExecutor.DiscardPolicy());
      this.closeConnectionExecutor = createThreadPoolExecutor(config.getMaximumPoolSize(), poolName + " connection closer", threadFactory, new ThreadPoolExecutor.CallerRunsPolicy());

      this.leakTaskFactory = new ProxyLeakTaskFactory(config.getLeakDetectionThreshold(), houseKeepingExecutorService);

      this.houseKeeperTask = houseKeepingExecutorService.scheduleWithFixedDelay(new HouseKeeper(), 100L, HOUSEKEEPING_PERIOD_MS, MILLISECONDS);
   }

在初始化HikariPool的时候会初始化houseKeeperTask

HikariPool.HouseKeeper

HikariCP-2.7.6-sources.jar!/com/zaxxer/hikari/pool/HikariPool.java

/**
    * The house keeping task to retire and maintain minimum idle connections.
    */
   private final class HouseKeeper implements Runnable
   {
      private volatile long previous = plusMillis(currentTime(), -HOUSEKEEPING_PERIOD_MS);

      @Override
      public void run()
      {
         try {
            // refresh timeouts in case they changed via MBean
            connectionTimeout = config.getConnectionTimeout();
            validationTimeout = config.getValidationTimeout();
            leakTaskFactory.updateLeakDetectionThreshold(config.getLeakDetectionThreshold());

            final long idleTimeout = config.getIdleTimeout();
            final long now = currentTime();

            // Detect retrograde time, allowing +128ms as per NTP spec.
            if (plusMillis(now, 128) < plusMillis(previous, HOUSEKEEPING_PERIOD_MS)) {
               LOGGER.warn("{} - Retrograde clock change detected (housekeeper delta={}), soft-evicting connections from pool.",
                           poolName, elapsedDisplayString(previous, now));
               previous = now;
               softEvictConnections();
               return;
            }
            else if (now > plusMillis(previous, (3 * HOUSEKEEPING_PERIOD_MS) / 2)) {
               // No point evicting for forward clock motion, this merely accelerates connection retirement anyway
               LOGGER.warn("{} - Thread starvation or clock leap detected (housekeeper delta={}).", poolName, elapsedDisplayString(previous, now));
            }

            previous = now;

            String afterPrefix = "Pool ";
            if (idleTimeout > 0L && config.getMinimumIdle() < config.getMaximumPoolSize()) {
               logPoolState("Before cleanup ");
               afterPrefix = "After cleanup  ";

               final List<PoolEntry> notInUse = connectionBag.values(STATE_NOT_IN_USE);
               int toRemove = notInUse.size() - config.getMinimumIdle();
               for (PoolEntry entry : notInUse) {
                  if (toRemove > 0 && elapsedMillis(entry.lastAccessed, now) > idleTimeout && connectionBag.reserve(entry)) {
                     closeConnection(entry, "(connection has passed idleTimeout)");
                     toRemove--;
                  }
               }
            }

            logPoolState(afterPrefix);

            fillPool(); // Try to maintain minimum connections
         }
         catch (Exception e) {
            LOGGER.error("Unexpected exception in housekeeping task", e);
         }
      }
   }

假设minimumIdle与maximumPoolSize设置成同样，那么这个task在第一次执行的时候，直接执行fillPool

fillPool

/**
    * Fill pool up from current idle connections (as they are perceived at the point of execution) to minimumIdle connections.
    */
   private synchronized void fillPool()
   {
      final int connectionsToAdd = Math.min(config.getMaximumPoolSize() - getTotalConnections(), config.getMinimumIdle() - getIdleConnections())
                                   - addConnectionQueue.size();
      for (int i = 0; i < connectionsToAdd; i++) {
         addConnectionExecutor.submit((i < connectionsToAdd - 1) ? POOL_ENTRY_CREATOR : POST_FILL_POOL_ENTRY_CREATOR);
      }
   }

这个fillPool，在初始化时刻，minimumIdle与maximumPoolSize值同样，totalConnections与idleConnections都为0，那么connectionsToAdd的值就是maximumPoolSize
也就是说这个task会添加maximumPoolSize大小链接

小结

• tomcat jdbc pool

有个initial-size参数来指定最开始的时候初始化多少个链接，有min-idle及max-idle来控制空闲链接的最小值及最大值，有max-active来控制链接池总大小。min-evictable-idle-time-millis用来指定空闲链接的时长，time-between-eviction-runs-millis用来指定清理空闲链接的任务的调度时间间隔。

• hikari connection pool

有minIdle来指定空闲链接的最小数量，maxPoolSize指定链接池链接最大值，默认初始化的时候，是初始化minIdle大小的链接，若是minIdle与maxPoolSize值相等那就是初始化时把链接池填满。idleTimeout用来指定空闲链接的时长，maxLifetime用来指定全部链接的时长。com.zaxxer.hikari.housekeeping.periodMs用来指定链接池空闲链接处理及链接池数补充的HouseKeeper任务的调度时间间隔。

也就是说hikari比tomcat jdbc pool多了个maxLifetime，也就是全部的链接在maxLifetime以后都得重连一次，保证链接池的活性。

doc

• About Pool Sizing
• Re: Connection pooling - Number of connections
• Hikari connection pool grows to maximum size at start