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本文主要研究一下hikari连接池的fixed pool design
fixed pool design
hikari的作者比较倾向于fixed pool design的理念，即建议minimumIdle与maximumPoolSize设置成一样，当做固定连接大小的连接池。作者认为minimumIdle小于maximumPoolSize的话，在流量激增的时候需要额外的连接，此时在请求方法里头再去处理新建连接会造成性能损失，即会导致数据库一方面降低连接建立的速度，另一方面也会影响既有的连接事务的完成，间接影响了这些既有连接归还到连接池的速度。
作者认为minimumIdle与maximumPoolSize设置成一样，多余的空闲连接不会对整体的性能有什么严重影响，如果说设置minimumIdle小于maximumPoolSize是为了在必要的时候可以释放连接以释放内存给其他功能用，但是在高峰时期，连接池可能也会到达maximumPoolSize，因而这个目的似乎没起到效果。
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设置成一样，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值一样，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大小连接
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连接，有min-idle及max-idle来控制空闲连接的最小值及最大值，有max-active来控制连接池总大小。min-evictable-idle-time-millis用来指定空闲连接的时长，time-between-eviction-runs-millis用来指定清理空闲连接的任务的调度时间间隔。
- hikari connection pool
有minIdle来指定空闲连接的最小数量，maxPoolSize指定连接池连接最大值，默认初始化的时候，是初始化minIdle大小的连接，如果minIdle与maxPoolSize值相等那就是初始化时把连接池填满。idleTimeout用来指定空闲连接的时长，maxLifetime用来指定所有连接的时长。com.zaxxer.hikari.housekeeping.periodMs用来指定连接池空闲连接处理及连接池数补充的HouseKeeper任务的调度时间间隔。
也就是说hikari比tomcat jdbc pool多了个maxLifetime，也就是所有的连接在maxLifetime之后都得重连一次，保证连接池的活性。
