redis源码分析Redis Sentinel 是如何实际解决分布式共识问题的
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开题
前几篇都偏离了轨道，方知写一个系列一定要先定指导思想。
那几篇过于注重“what”和“why”了，但是我写这个系列的初衷是“how”，怎么做！
所以，本篇将聚焦与以下几个问题：
1、哨兵是如何监视节点的？
2、哨兵是如何选举的？
3、从节点是如何上位的？
对于 分布式一致性算法raft 不了解的可以先了解一下：分布式一致性之raft算法，文中还有关于分布式事务的连接，想了解的也可以了解一下，后文出现专有名词我就不再解释了。
Sentinel 结构
每个 Sentinel 节点都维护一份自己视角下的当前 Sentinel 集群的状态，该状态信息存储在 SentinelState结构体中：
/* Main state. */
struct sentinelState
char myid[CONFIG_RUN_ID_SIZE+1]; /* This sentinel ID. */
uint64_t current_epoch; // 集群当前任期号，用于故障转移时使用 raft 算法选举 leader 节点
dict *masters; /* Dictionary of master sentinelRedisInstances.
Key is the instance name, value is the
sentinelRedisInstance structure pointer. */
......
sentinel;
这里要明确一点，集群节点会挂，哨兵也是会挂的。
sentinelRedisInstance 结构体负责存储Sentinel 集群中主从节点，以及其它 Sentinel 节点的实例数据。
typedef struct sentinelRedisInstance
int flags; // 节点标志，见下文
char *name; /* Master name from the point of view of this sentinel. */
char *runid; /* Run ID of this instance, or unique ID if is a Sentinel.*/
uint64_t config_epoch; /* Configuration epoch. */
sentinelAddr *addr; /* Master host. */
instanceLink *link; /* Link to the instance, may be shared for Sentinels. */
mstime_t last_pub_time; /* Last time we sent hello via Pub/Sub. */
mstime_t last_hello_time; /* Only used if SRI_SENTINEL is set. Last time
we received a hello from this Sentinel
via Pub/Sub. */
mstime_t last_master_down_reply_time; /* Time of last reply to
SENTINEL is-master-down command. */
mstime_t s_down_since_time; /* Subjectively down since time. */
mstime_t o_down_since_time; /* Objectively down since time. */
mstime_t down_after_period; /* Consider it down after that period. */
......
/* Role and the first time we observed it.
* This is useful in order to delay replacing what the instance reports
* with our own configuration. We need to always wait some time in order
* to give a chance to the leader to report the new configuration before
* we do silly things. */
int role_reported;
mstime_t role_reported_time;
mstime_t slave_conf_change_time; /* Last time slave master addr changed. */
/* Master specific. */
dict *sentinels; /* Other sentinels monitoring the same master. */
dict *slaves; /* Slaves for this master instance. */
unsigned int quorum;/* Number of sentinels that need to agree on failure. */
int parallel_syncs; /* How many slaves to reconfigure at same time. */
char *auth_pass; /* Password to use for AUTH against master & replica. */
char *auth_user; /* Username for ACLs AUTH against master & replica. */
/* Slave specific. */
mstime_t master_link_down_time; /* Slave replication link down time. */
int slave_priority; /* Slave priority according to its INFO output. */
mstime_t slave_reconf_sent_time; /* Time at which we sent SLAVE OF <new> */
struct sentinelRedisInstance *master; /* Master instance if it's slave. */
char *slave_master_host; /* Master host as reported by INFO */
int slave_master_port; /* Master port as reported by INFO */
int slave_master_link_status; /* Master link status as reported by INFO */
unsigned long long slave_repl_offset; /* Slave replication offset. */
/* Failover */
char *leader; /* If this is a master instance, this is the runid of
the Sentinel that should perform the failover. If
this is a Sentinel, this is the runid of the Sentinel
that this Sentinel voted as leader. */
uint64_t leader_epoch; /* Epoch of the 'leader' field. */
uint64_t failover_epoch; /* Epoch of the currently started failover. */
int failover_state; /* See SENTINEL_FAILOVER_STATE_* defines. */
mstime_t failover_state_change_time;
mstime_t failover_start_time; /* Last failover attempt start time. */
mstime_t failover_timeout; /* Max time to refresh failover state. */
mstime_t failover_delay_logged; /* For what failover_start_time value we
logged the failover delay. */
struct sentinelRedisInstance *promoted_slave; /* Promoted slave instance. */
......
sentinelRedisInstance;
/* A Sentinel Redis Instance object is monitoring. */
#define SRI_MASTER (1<<0)
#define SRI_SLAVE (1<<1)
#define SRI_SENTINEL (1<<2)
#define SRI_S_DOWN (1<<3) //该节点已主观下线
#define SRI_O_DOWN (1<<4) //该节点已客观下线
#define SRI_MASTER_DOWN (1<<5) /* A Sentinel with this flag set thinks that
its master is down. */
#define SRI_FAILOVER_IN_PROGRESS (1<<6) //节点正在进行故障迁移
#define SRI_PROMOTED (1<<7) //节点被选为前一种的晋升节点
在 Sentinel 集群没有执行故障转移时，集群中所有 sentinel 节点都是平等的。当执行故障转移时，会选出一个leader节点，由leader节点完成故障转移。
Sentinel 利用了频道订阅功能，每个Sentinel节点都订阅了主从节点的一个特定频道，并将自身节点信息发送到该频道，这样每个Sentinel节点自身信息就会被广播给集群其他Sentinel节点。
Sentinel故障与安全模式：TITL模式
Sentinel 机制非常依赖系统时间，举个栗子：基于某个节点上次响应 PING 命令的时间与当前系统时间之差来判断该节点是否下线。如果系统时间被修改或者进程由于繁忙而阻塞，那么Sentinel机制可能出现运行不正常的情况。
为了结局这种情况，Sentinel 机制中定义了TITL 模式。每次执行 sentinelTimer 函数都会检查上次执行该函数的时间与当前系统时间之差，如果出现负数或时间差特别大，则Sentinel进入TITL模式：
1、它不再执行任何操作，如故障转移
2、当其他Sentinel节点询问它对于某个主节点主观下线的判定结果时，它将返回节点未下线的判定结果
3、如果TITL模式下Sentinel机制可以正常运行30秒，则该节点退出TITL模式
故障转移主逻辑
/* Perform scheduled operations for all the instances in the dictionary.
* Recursively call the function against dictionaries of slaves. */
void sentinelHandleDictOfRedisInstances(dict *instances)
dictIterator *di;
dictEntry *de;
sentinelRedisInstance *switch_to_promoted = NULL;
/* There are a number of things we need to perform against every master. */
di = dictGetIterator(instances);
while((de = dictNext(di)) != NULL)
sentinelRedisInstance *ri = dictGetVal(de);
sentinelHandleRedisInstance(ri); //调用主逻辑函数
if (ri->flags & SRI_MASTER)
//如果当前处理的是主节点，还需要递归处理主节点实例下的slaves 和 sentinels
sentinelHandleDictOfRedisInstances(ri->slaves);
sentinelHandleDictOfRedisInstances(ri->sentinels);
if (ri->failover_state == SENTINEL_FAILOVER_STATE_UPDATE_CONFIG)
switch_to_promoted = ri;
//完成故障转移的最后一步
if (switch_to_promoted)
sentinelFailoverSwitchToPromotedSlave(switch_to_promoted);
dictReleaseIterator(di);
/* ======================== SENTINEL timer handler ==========================
* This is the "main" our Sentinel, being sentinel completely non blocking
* in design. The function is called every second.
* -------------------------------------------------------------------------- */
/* Perform scheduled operations for the specified Redis instance. */
void sentinelHandleRedisInstance(sentinelRedisInstance *ri)
/* ========== MONITORING HALF ============ */
/* Every kind of instance */
sentinelReconnectInstance(ri); //建立网络连接
sentinelSendPeriodicCommands(ri);
/* ============== ACTING HALF ============= */
/* We don't proceed with the acting half if we are in TILT mode.
* TILT happens when we find something odd with the time, like a
* sudden change in the clock. */
if (sentinel.tilt)
if (mstime()-sentinel.tilt_start_time < SENTINEL_TILT_PERIOD) return;
sentinel.tilt = 0;
sentinelEvent(LL_WARNING,"-tilt",NULL,"#tilt mode exited");
/* Every kind of instance */
sentinelCheckSubjectivelyDown(ri); //检查是否存在主观下线的节点
/* Masters and slaves */
if (ri->flags & (SRI_MASTER|SRI_SLAVE))
/* Nothing so far. */
/* Only masters */
if (ri->flags & SRI_MASTER) //只对主节点执行
sentinelCheckObjectivelyDown(ri); //检查是否存在客观下线的节点
if (sentinelStartFailoverIfNeeded(ri)) //判断是够可以进行故障转移
sentinelAskMasterStateToOtherSentinels(ri,SENTINEL_ASK_FORCED); //发送投票请求
sentinelFailoverStateMachine(ri); //实现一个故障转移状态机，实现故障转移逻辑
sentinelAskMasterStateToOtherSentinels(ri,SENTINEL_NO_FLAGS); //询问其他 Sentinel 节点对该节点主观下线的判定结果
主观下线：我个人认为你下线了
客观下线：超过半数的人都认为你下线了
心跳监测
Sentinel 会定时发送消息给主从节点和其他 Sentinel 节点，看它们还活着不：
/* Send periodic PING, INFO, and PUBLISH to the Hello channel to
* the specified master or slave instance. */
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->link->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->link->pending_commands >=
SENTINEL_MAX_PENDING_COMMANDS * ri->link->refcount) 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.
*
* Similarly we monitor the INFO output more often if the slave reports
* to be disconnected from the master, so that we can have a fresh
* disconnection time figure. */
if ((ri->flags & SRI_SLAVE) &&
((ri->master->flags & (SRI_O_DOWN|SRI_FAILOVER_IN_PROGRESS)) ||
(ri->master_link_down_time != 0)))
info_period = 1000;
else
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;
/* Send INFO to masters and slaves, not sentinels. */
if ((ri->flags & SRI_SENTINEL) == 0 &&
(ri->info_refresh == 0 ||
(now - ri->info_refresh) > info_period))
retval = redisAsyncCommand(ri->link->cc,
sentinelInfoReplyCallback, ri, "%s",
sentinelInstanceMapCommand(ri,"INFO"));
if (retval == C_OK) ri->link->pending_commands++;
/* Send PING to all the three kinds of instances. */
if ((now - ri->link->last_pong_time) > ping_period &&
(now - ri->link->last_ping_time) > ping_period/2)
sentinelSendPing(ri);
/* PUBLISH hello messages to all the three kinds of instances. */
if ((now - ri->last_pub_time) > SENTINEL_PUBLISH_PERIOD)
sentinelSendHello(ri);
判断下线 及 投票表决
首先，这种事情需要我自己先说服我自己，他已经挂了，所以：
/* ===================== SENTINEL availability checks ======================= */
/* Is this instance down from our point of view? */
void sentinelCheckSubjectivelyDown(sentinelRedisInstance *ri)
mstime_t elapsed = 0;
//计算目标节点上次响应后过去的时间
if (ri->link->act_ping_time)
elapsed = mstime() - ri->link->act_ping_time;
else if (ri->link->disconnected)
elapsed = mstime() - ri->link->last_avail_time;
/* Check if we are in need for a reconnection of one of the
* links, because we are detecting low activity.
*
* 1) Check if the command link seems connected, was connected not less
* than SENTINEL_MIN_LINK_RECONNECT_PERIOD, but still we have a
* pending ping for more than half the timeout. */
if (ri->link->cc &&
(mstime() - ri->link->cc_conn_time) >
SENTINEL_MIN_LINK_RECONNECT_PERIOD &&
ri->link->act_ping_time != 0 && /* There is a pending ping... */
/* The pending ping is delayed, and we did not receive
* error replies as well. */
(mstime() - ri->link->act_ping_time) > (ri->down_after_period/2) &&
(mstime() - ri->link->last_pong_time) > (ri->down_after_period/2))
instanceLinkCloseConnection(ri->link,ri->link->cc);
/* 2) Check if the pubsub link seems connected, was connected not less
* than SENTINEL_MIN_LINK_RECONNECT_PERIOD, but still we have no
* activity in the Pub/Sub channel for more than
* SENTINEL_PUBLISH_PERIOD * 3.
*/
if (ri->link->pc &&
(mstime() - ri->link->pc_conn_time) >
SENTINEL_MIN_LINK_RECONNECT_PERIOD &&
(mstime() - ri->link->pc_last_activity) > (SENTINEL_PUBLISH_PERIOD*3))
instanceLinkCloseConnection(ri->link,ri->link->pc);
/* Update the SDOWN flag. We believe the instance is SDOWN if:
*
* 1) It is not replying.
* 2) We believe it is a master, it reports to be a slave for enough time
* to meet the down_after_period, plus enough time to get two times
* INFO report from the instance. */
if (elapsed > ri->down_after_period ||
(ri->flags & SRI_MASTER &&
ri->role_reported == SRI_SLAVE &&
mstime() - ri->role_reported_time >
(ri->down_after_period+SENTINEL_INFO_PERIOD*2)))
/* Is subjectively down */
if ((ri->flags & SRI_S_DOWN) == 0)
sentinelEvent(LL_WARNING,"+sdown",ri,"%@");
ri->s_down_since_time = mstime();
ri->flags |= SRI_S_DOWN;
else
/* Is subjectively up */
if (ri->flags & SRI_S_DOWN)
sentinelEvent(LL_WARNING,"-sdown",ri,"%@");
ri->flags &= ~(SRI_S_DOWN|SRI_SCRIPT_KILL_SENT);
我说服了自己之后，为了避免决策失误，我便开始问询身边同频的朋友的意见：
//该函数内含选举逻辑
//其他 Sentinel 节点会回复一个标志位，如果为 true，则代表他也认为那个节点下线了
void sentinelAskMasterStateToOtherSentinels(sentinelRedisInstance *master, int flags)
dictIterator *di;
dictEntry *de;
di = dictGetIterator(master->sentinels);
while((de = dictNext(di)) != NULL)
sentinelRedisInstance *ri = dictGetVal(de);
mstime_t elapsed = mstime() - ri->last_master_down_reply_time;
char port[32];
int retval;
/* If the master state from other sentinel is too old, we clear it. */
if (elapsed > SENTINEL_ASK_PERIOD*5)
ri->flags &= ~SRI_MASTER_DOWN;
sdsfree(ri->leader);
ri->leader = NULL;
/* Only ask if master is down to other sentinels if:
*
* 1) We believe it is down, or there is a failover in progress.
* 2) Sentinel is connected.
* 3) We did not receive the info within SENTINEL_ASK_PERIOD ms. */
if ((master->flags & SRI_S_DOWN) == 0) continue;
if (ri->link->disconnected) continue;
if (!(flags & SENTINEL_ASK_FORCED) &&
mstime() - ri->last_master_down_reply_time < SENTINEL_ASK_PERIOD)
continue;
/* Ask */
ll2string(port,sizeof(port),master->addr->port);
retval = redisAsyncCommand(ri->link->cc,
sentinelReceiveIsMasterDownReply, ri,
"%s is-master-down-by-addr %s %s %llu %s",
sentinelInstanceMapCommand(ri,"SENTINEL"),
master->addr->ip, port,
sentinel.current_epoch,
(master->failover_state > SENTINEL_FAILOVER_STATE_NONE) ?
sentinel.myid : "*");
if (retval == C_OK) ri->link->pending_commands++;
dictReleaseIterator(di);
投票选举 leader哨兵
现在认定他挂了，我们一群监视的要推举一个主事儿的来料理他的后事，由于是我先发现他不对劲儿的，也是我先获取了他最终挂掉的信息，所以我抢先发起了料理后事的请求，其他哨兵只能先给我投票，如果我落选了，他们才有机会发起选举：
1、拉票
void sentinelAskMasterStateToOtherSentinels(sentinelRedisInstance *master, int flags)
dictIterator *di;
dictEntry *de;
di = dictGetIterator(master->sentinels);
while((de = dictNext(di)) != NULL)
sentinelRedisInstance *ri = dictGetVal(de);
mstime_t elapsed = mstime() - ri->last_master_down_reply_time;
char port[32];
int retval;
/* If the master state from other sentinel is too old, we clear it. */
if (elapsed > SENTINEL_ASK_PERIOD*5)
ri->flags &= ~SRI_MASTER_DOWN;
sdsfree(ri->leader);
ri->leader = NULL;
/* Only ask if master is down to other sentinels if:
*
* 1) We believe it is down, or there is a failover in progress.
* 2) Sentinel is connected.
* 3) We did not receive the info within SENTINEL_ASK_PERIOD ms. */
if ((master->flags & SRI_S_DOWN) == 0) continue;
if (ri->link->disconnected) continue;
if (!(flags & SENTINEL_ASK_FORCED) &&
mstime() - ri->last_master_down_reply_time < SENTINEL_ASK_PERIOD)
continue;
/* Ask */
ll2string(port,sizeof(port),master->addr->port);
retval 以上是关于redis源码分析Redis Sentinel 是如何实际解决分布式共识问题的的主要内容，如果未能解决你的问题，请参考以下文章
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