Monitor half：
Sentinel通过两种方式获取信息：
- INFO命令（通过命令连接）向其他主服务器或从服务器获取信息；
- 订阅hello频道，获取其他Sentinels发布的信息；
一、建立连接：
Sentinel在连接主服务器或者从服务器时，会同时创建命令连接和订阅连接，但是在连接其他Sentinel时，却只会创建命令连接，而不创建订阅连接。
为什么需要订阅连接？ todo
void sentinelHandleRedisInstance(sentinelRedisInstance *ri) {
/* ========== MONITORING HALF ============ */
/* Every kind of instance */
/* 建立命令连接(cc)，和订阅连接(pc) */
sentinelReconnectInstance(ri);
/* 对实例进行定期操作：INFO（10s | 1s）, PUBLISH (2s), PING(1s) */
sentinelSendPeriodicCommands(ri);
/* ============== ACTING HALF ============= */
// ...
// ...
}
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L5：建立连接过程中使用了hiredis的异步通信API[1]，以后补充。
建立订阅连接会执行SUBSCRIBE命令，设置回调sentinelReceiveHelloMessages，这个函数会通过频道接收其他同样监视该服务器（主从？）的Sentinel更新服务器状态，如果是新的Sentinel，会被新建并添加到该服务器实例结构的sentinels字典中。
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L7： 第二节分析一下周期操作sentinelSendPeriodicCommands；
二、周期操作sentinelSendPeriodicCommands：
void sentinelSendPeriodicCommands(sentinelRedisInstance *ri) {
mstime_t now = mstime();
mstime_t info_period, ping_period;
int retval;
/* Return ASAP if we have already a PING or INFO already pending, or
* in the case the instance is not properly connected. */
if (ri->flags & SRI_DISCONNECTED) return;
/* For INFO, PING, PUBLISH that are not critical commands to send we
* also have a limit of SENTINEL_MAX_PENDING_COMMANDS. We don't
* want to use a lot of memory just because a link is not working
* properly (note that anyway there is a redundant protection about this,
* that is, the link will be disconnected and reconnected if a long
* timeout condition is detected. */
if (ri->pending_commands >= SENTINEL_MAX_PENDING_COMMANDS) return;
/* If this is a slave of a master in O_DOWN condition we start sending
* it INFO every second, instead of the usual SENTINEL_INFO_PERIOD
* period. In this state we want to closely monitor slaves in case they
* are turned into masters by another Sentinel, or by the sysadmin. */
if ((ri->flags & SRI_SLAVE) &&
(ri->master->flags & (SRI_O_DOWN|SRI_FAILOVER_IN_PROGRESS))) {
/* 当前实例是slave，而且它的master已客观下线并且故障转移正在进行中，那么本机sentinel 1s发一次INFO */
info_period = 1000;
} else {
/* 正常情况下是10s发一次INFO */
info_period = SENTINEL_INFO_PERIOD;
}
/* We ping instances every time the last received pong is older than
* the configured 'down-after-milliseconds' time, but every second
* anyway if 'down-after-milliseconds' is greater than 1 second. */
ping_period = ri->down_after_period;
if (ping_period > SENTINEL_PING_PERIOD) ping_period = SENTINEL_PING_PERIOD;
if ((ri->flags & SRI_SENTINEL) == 0 &&
(ri->info_refresh == 0 ||
(now - ri->info_refresh) > info_period)) // 10s or 1s
{
/* Send INFO to masters and slaves, not sentinels. */
/* 实例不是sentinel，并且超过INFO周期，发送INFO */
retval = redisAsyncCommand(ri->cc,
sentinelInfoReplyCallback, NULL, "INFO");
if (retval == REDIS_OK) ri->pending_commands++;
} else if ((now - ri->last_pong_time) > ping_period) { // 1s
/* Send PING to all the three kinds of instances. */
sentinelSendPing(ri);
} else if ((now - ri->last_pub_time) > SENTINEL_PUBLISH_PERIOD) { // 2s
/* PUBLISH hello messages to all the three kinds of instances. */
sentinelSendHello(ri);
}
}
- L22~L25：注意发INFO周期的改变
- L43：发送INFO命令，回调为sentinelInfoReplyCallback；本机sentinel通过这个回调获取主服务器和从服务器的信息，故障转移的role变化也在这个函数执行，以后详解。
- L48：发送PING命令，回调函数只是更新time信息，不细说。
- L51：发送PUBLISH到_sentinel_:hello频道，回调函数只是更新time信息，不细说。