对 health_check_period与healht_check_timeout，官方解释如下：
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 a case 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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两者到底是怎样的关系？
看看代码就可以知晓：
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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();
/** 着里面有根据结果进行failover处理的逻辑，省略*/
if ((sts > 0 || sys_sts < 0)
&& (errno != EINTR ||
(errno == EINTR && health_check_timer_expired)))
{
……
}
if (pool_config->health_check_timeout > 0)
{
/* seems ok. cancel health check timer */
pool_signal(SIGALRM, SIG_IGN);
}
/** 请注意这里的sleep处理 */
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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也就是说首先，health_check是否发生，要看 health_check_period是否大于0。
在这个前提下，如果health_check_timeout也大于零，就埋下一个定时器，
到达health_check_timeout的秒数时，从定时器激活 healht_check函数。
与此同时，
在主循环中进行了 health_check处理后，如果结果OK，那么进行一番整理后，要开始睡眠一段时间，睡眠的时间间隔就是: health_check_period。睡醒了，再回到循环起始处，继续循环。
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/2012/08/02/2620070.html，如需转载请自行联系原作者
