pgpool-II中，与health check 相干的配置文件项有两个：

health_check_period

health_check_timeout

乍一看他们 文档的解释，看官方网站的说法：

http://pgpool.projects.postgresql.org/pgpool-II/doc/pgpool-en.html

health_check_period

This parameter specifies the interval between the health checks in seconds.

Default is 0, which means health check is disabled. You need to reload pgpool.conf if you change health_check_period.

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health_check_timeout

pgpool-II periodically tries to connect to the backends to detect any error on the servers or networks. This error check procedure is called "health check".

If an error is detected, pgpool-II tries to perform failover or degeneration.

This parameter serves to prevent the health check from waiting for a long time in acase such as un unplugged network cable. The timeout value is in seconds. Default value is 20.

0 disables timeout (waits until TCP/IP timeout).

This health check requires one extra connection to each backend,

so max_connections in the postgresql.conf needs to be incremented as needed. You need to reload pgpool.conf if you change this value.

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实际的情形如何呢，这里以 pgpool-II 3.1 为例(为了看着方便，去掉了一部分不重要的代码)：

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/*

* pgpool main program

*/

int main(int argc, char **argv)

{

……

/*

* This is the main loop

*/

for (;;)

{

CHECK_REQUEST;

/* do we need health checking for PostgreSQL? */

if (pool_config->health_check_period > 0)

{

……

if (pool_config->health_check_timeout > 0)

{

/*

* set health checker timeout. we want to detect

* communication path failure much earlier before

* TCP/IP stack detects it.

*/

pool_signal(SIGALRM, health_check_timer_handler);

alarm(pool_config->health_check_timeout);

}

/*

* do actual health check. trying to connect to the backend

*/

errno = 0;

health_check_timer_expired = 0;

POOL_SETMASK(&UnBlockSig);

sts = health_check();

POOL_SETMASK(&BlockSig);

if (pool_config->parallel_mode || pool_config->enable_query_cache)

sys_sts = system_db_health_check();

if ((sts > 0 || sys_sts < 0)

&& (errno != EINTR || (errno == EINTR && health_check_timer_expired)))

{

if (sts > 0)

{

sts--;

if (!pool_config->parallel_mode)

{

if (POOL_DISALLOW_TO_FAILOVER(BACKEND_INFO(sts).flag))

{

pool_log("health_check: %d failover is canceld

　　　　　　　　　　　　　　　　　　　　　　because failover is disallowed", sts);

}

else

{

pool_log("set %d th backend down status", sts);

Req_info->kind = NODE_DOWN_REQUEST;

Req_info->node_id[0] = sts;

failover();

/* need to distribute this info to children */

}

}

else

{

retrycnt++;

pool_signal(SIGALRM, SIG_IGN); /* Cancel timer */

if (retrycnt > NUM_BACKENDS)

{

/* retry count over */

pool_log("set %d th backend down status", sts);

Req_info->kind = NODE_DOWN_REQUEST;

Req_info->node_id[0] = sts;

failover();

retrycnt = 0;

}

else

{

/* continue to retry */

sleep_time = pool_config->health_check_period/

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　NUM_BACKENDS;

pool_debug("retry sleep time: %d seconds", sleep_time);

pool_sleep(sleep_time);

continue;

}

}

}

……

}

if (pool_config->health_check_timeout > 0)

{

/* seems ok. cancel health check timer */

pool_signal(SIGALRM, SIG_IGN);

}

sleep_time = pool_config->health_check_period;

pool_sleep(sleep_time);

}

else

{

for (;;)

{

int r;

struct timeval t = {3, 0};

POOL_SETMASK(&UnBlockSig);

r = pool_pause(&t);

POOL_SETMASK(&BlockSig);

if (r > 0)

break;

}

}

}

pool_shmem_exit(0);

}

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可以看得比较清楚了，

第一点，health_check_period的作用，如果不为零，则health_check可以发生。

其他非零值其实都是一样。

第二点，health_check_timeout的作用，如果>0，则会被设置timer,timer到时间后，激 活 health_check_timer_handler，对调用 health_check()函数的。

第三点，这里是最坑爹的部分了：

在主循环里面，只要 health_check_period不为零，则要不断地在循环里面作 health_check()动作。

这个一般而言比 缺省的 health_check_timeout 20秒可高多了。

实际运行 pgpool命令的时候，如果加入 -d 参数，就可以看到这一点：pgpool-II不断通过调用healt_check()来检查各节点状况。

可以说，有了这个主循环里面折腾 health_check以后，health_check_timeout就形同虚设了。

只是不知道从哪个版本开始变成这样的，或者可以说　pgpool-II的开发者很不负责，没有很好地协调代码和文档。也许这是很多开源项目的通病了。

本文转自健哥的数据花园博客园博客，原文链接：http://www.cnblogs.com/gaojian/archive/2012/07/27/2611935.html，如需转载请自行联系原作者