聊聊hikari链接池的idleTimeout及minimumIdle属性

序

本文主要研究一个hikari链接池的idleTimeout及minimumIdle属性

idleTimeout

默认是600000毫秒，即10分钟。若是idleTimeout+1秒>maxLifetime 且 maxLifetime>0，则会被重置为0；若是idleTimeout!=0且小于10秒，则会被重置为10秒。若是idleTimeout=0则表示空闲的链接在链接池中永远不被移除。

只有当minimumIdle小于maximumPoolSize时，这个参数才生效，当空闲链接数超过minimumIdle，并且空闲时间超过idleTimeout，则会被移除。

minimumIdle

控制链接池空闲链接的最小数量，当链接池空闲链接少于minimumIdle，并且总共链接数不大于maximumPoolSize时，HikariCP会尽力补充新的链接。为了性能考虑，不建议设置此值，而是让HikariCP把链接池当作固定大小的处理，默认minimumIdle与maximumPoolSize同样。

当minIdle<0或者minIdle>maxPoolSize,则被重置为maxPoolSize，该值默认为10。

HikariPool.HouseKeeper

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));

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

   /**
    * 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);
         }
      }
   }

这个HouseKeeper是一个定时任务，在HikariPool构造器里头初始化，默认的是初始化后100毫秒执行，以后每执行完一次以后隔HOUSEKEEPING_PERIOD_MS( 30秒)时间执行。
这个定时任务的做用就是根据idleTimeout的值，移除掉空闲超时的链接。
首先检测时钟是否倒退，若是倒退了则当即对过时的链接进行标记evict；以后当idleTimeout>0且配置的minimumIdle<maximumPoolSize时才开始处理超时的空闲链接。
取出状态是STATE_NOT_IN_USE的链接数，若是大于minimumIdle，则遍历STATE_NOT_IN_USE的链接的链接，将空闲超时达到idleTimeout的链接从connectionBag移除掉，若移除成功则关闭该链接，而后toRemove--。
在空闲链接移除以后，再调用fillPool，尝试补充空间链接数到minimumIdle值

HikariPool.fillPool

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

private final PoolEntryCreator POOL_ENTRY_CREATOR = new PoolEntryCreator(null /*logging prefix*/);
      private final PoolEntryCreator POST_FILL_POOL_ENTRY_CREATOR = new PoolEntryCreator("After adding ");
      LinkedBlockingQueue<Runnable> addConnectionQueue = new LinkedBlockingQueue<>(config.getMaximumPoolSize());
      this.addConnectionQueue = unmodifiableCollection(addConnectionQueue);
      this.addConnectionExecutor = createThreadPoolExecutor(addConnectionQueue, poolName + " connection adder", threadFactory, new ThreadPoolExecutor.DiscardPolicy());

   /**
    * 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);
      }
   }

PoolEntryCreator

/**
    * Creating and adding poolEntries (connections) to the pool.
    */
   private final class PoolEntryCreator implements Callable<Boolean>
   {
      private final String loggingPrefix;

      PoolEntryCreator(String loggingPrefix)
      {
         this.loggingPrefix = loggingPrefix;
      }

      @Override
      public Boolean call() throws Exception
      {
         long sleepBackoff = 250L;
         while (poolState == POOL_NORMAL && shouldCreateAnotherConnection()) {
            final PoolEntry poolEntry = createPoolEntry();
            if (poolEntry != null) {
               connectionBag.add(poolEntry);
               LOGGER.debug("{} - Added connection {}", poolName, poolEntry.connection);
               if (loggingPrefix != null) {
                  logPoolState(loggingPrefix);
               }
               return Boolean.TRUE;
            }

            // failed to get connection from db, sleep and retry
            quietlySleep(sleepBackoff);
            sleepBackoff = Math.min(SECONDS.toMillis(10), Math.min(connectionTimeout, (long) (sleepBackoff * 1.5)));
         }
         // Pool is suspended or shutdown or at max size
         return Boolean.FALSE;
      }

      /**
       * We only create connections if we need another idle connection or have threads still waiting
       * for a new connection.  Otherwise we bail out of the request to create.
       *
       * @return true if we should create a connection, false if the need has disappeared
       */
      private boolean shouldCreateAnotherConnection() {
         return getTotalConnections() < config.getMaximumPoolSize() &&
            (connectionBag.getWaitingThreadCount() > 0 || getIdleConnections() < config.getMinimumIdle());
      }
   }

shouldCreateAnotherConnection方法决定了是否须要添加新的链接

createPoolEntry

/**
    * Creating new poolEntry.  If maxLifetime is configured, create a future End-of-life task with 2.5% variance from
    * the maxLifetime time to ensure there is no massive die-off of Connections in the pool.
    */
   private PoolEntry createPoolEntry()
   {
      try {
         final PoolEntry poolEntry = newPoolEntry();

         final long maxLifetime = config.getMaxLifetime();
         if (maxLifetime > 0) {
            // variance up to 2.5% of the maxlifetime
            final long variance = maxLifetime > 10_000 ? ThreadLocalRandom.current().nextLong( maxLifetime / 40 ) : 0;
            final long lifetime = maxLifetime - variance;
            poolEntry.setFutureEol(houseKeepingExecutorService.schedule(
               () -> {
                  if (softEvictConnection(poolEntry, "(connection has passed maxLifetime)", false /* not owner */)) {
                     addBagItem(connectionBag.getWaitingThreadCount());
                  }
               },
               lifetime, MILLISECONDS));
         }

         return poolEntry;
      }
      catch (Exception e) {
         if (poolState == POOL_NORMAL) { // we check POOL_NORMAL to avoid a flood of messages if shutdown() is running concurrently
            LOGGER.debug("{} - Cannot acquire connection from data source", poolName, (e instanceof ConnectionSetupException ? e.getCause() : e));
         }
         return null;
      }
   }

createPoolEntry方法建立一个poolEntry，同时给它的lifetime过时设定了一个延时任务。

小结

• HouseKeeper是一个定时任务，在HikariPool构造器里头初始化，默认的是初始化后100毫秒执行，以后每执行完一次以后隔HOUSEKEEPING_PERIOD_MS(30秒)时间执行。
• 若是发现时钟倒退，则当即标记evict链接，而后退出；不然都会执行fillPool，来试图维持空闲链接到minimumIdle的数值
• 当idleTimeout>0且配置的minimumIdle<maximumPoolSize时，会移除超过idleTimeout的空闲链接，不然不作操做，继续往下执行fillPool
• 当minIdle<0或者minIdle>maxPoolSize,则minIdle被重置为maxPoolSize，该值默认为10，官方建议设置为一致，当作固定大小的链接池处理提升性能

idleTimeout有点相似tomcat jdbc pool里头的min-evictable-idle-time-millis参数。不一样的是tomcat jdbc pool的链接泄露检测以及空闲链接清除的工做都放在一个名为PoolCleaner的timerTask中处理，该任务的执行间隔为timeBetweenEvictionRunsMillis，默认为5秒；而hikari的链接泄露是每次getConnection的时候单独触发一个延时任务来处理，而空闲链接的清除则是使用HouseKeeper定时任务来处理，其运行间隔由com.zaxxer.hikari.housekeeping.periodMs环境变量控制，默认为30秒。

doc

• configuration-knobs-baby