Redis（140）Redis的Cluster的故障转移是如何实现的？

Redis Cluster 的故障转移机制确保在主节点（Master）出现故障时，集群中的从节点（Slave）能够自动接管主节点的角色，从而保证高可用性。故障转移的实现涉及节点状态监控、故障检测、选举新主节点及更新集群状态等步骤。下面我们详细解析这些步骤，并结合代码示例进行深入探讨。

1. 节点状态监控和故障检测

每个 Redis 集群节点都会定期向其他节点发送 PING 消息，并期望收到 PONG 回复。如果在一定时间内未收到回复，该节点将认为目标节点可能失效。

代码示例

/* Cluster node structure */
typedef struct clusterNode {
    char name[40];      /* Node ID */
    int flags;          /* Node flags: master, slave, fail, etc. */
    mstime_t ping_sent; /* Last time a ping was sent */
    mstime_t pong_received; /* Last time a pong was received */
    // other fields...
} clusterNode;

/* Function to check for node failures */
void clusterCron(void) {
    mstime_t now = mstime();
    dictIterator *di;
    dictEntry *de;

    di = dictGetSafeIterator(server.cluster->nodes);
    while((de = dictNext(di)) != NULL) {
        clusterNode *node = dictGetVal(de);

        if (node->flags & CLUSTER_NODE_MYSELF) continue;

        if (now - node->pong_received > server.cluster_node_timeout) {
            // Mark the node as failing
            node->flags |= CLUSTER_NODE_PFAIL;
            printf("Node %s is in PFAIL state\n", node->name);
        }
    }
    dictReleaseIterator(di);
}

2. 故障确认和投票

当一个节点被标记为 PFAIL（疑似失败）状态后，如果多数主节点（Master）也标记该节点为 FAIL（确定失败）状态，那么该节点将被认为真的失效。

代码示例

/* Function to confirm node failure */
void clusterSendFail(clusterNode *node) {
    dictIterator *di;
    dictEntry *de;

    di = dictGetSafeIterator(server.cluster->nodes);
    while ((de = dictNext(di)) != NULL) {
        clusterNode *peer = dictGetVal(de);

        if (peer->flags & CLUSTER_NODE_MYSELF) continue;

        // Send FAIL message to other nodes
        clusterSendMessage(peer, node, CLUSTERMSG_TYPE_FAIL);
    }
    dictReleaseIterator(di);
}

/* Function to process FAIL messages */
void clusterProcessFail(clusterMsg *msg) {
    clusterNode *node = clusterLookupNode(msg->data.fail.nodeid);

    if (node == NULL) return;

    // Mark the node as FAIL
    node->flags |= CLUSTER_NODE_FAIL;
    printf("Node %s marked as FAIL\n", node->name);
}

3. 选举新的主节点

当主节点被确定为失效后，其从节点将参与选举。参与选举的从节点会向其他主节点发送投票请求，并根据收到的投票数量决定是否成为新的主节点。

代码示例

/* Function to initiate failover */
void clusterHandleSlaveFailover(clusterNode *slave) {
    if (!(slave->flags & CLUSTER_NODE_SLAVE)) return;

    // Promote the slave to master
    slave->flags &= ~CLUSTER_NODE_SLAVE;
    slave->flags |= CLUSTER_NODE_MASTER;

    // Send update to the cluster
    clusterBroadcastSlaveOf(slave, NULL);
    printf("Slave %s promoted to master\n", slave->name);
}

/* Function to broadcast SLAVEOF message */
void clusterBroadcastSlaveOf(clusterNode *node, clusterNode *master) {
    dictIterator *di;
    dictEntry *de;

    di = dictGetSafeIterator(server.cluster->nodes);
    while ((de = dictNext(di)) != NULL) {
        clusterNode *peer = dictGetVal(de);

        if (peer->flags & CLUSTER_NODE_MYSELF) continue;

        clusterSendMessage(peer, node, CLUSTERMSG_TYPE_SLAVEOF, master);
    }
    dictReleaseIterator(di);
}

/* Function to process SLAVEOF messages */
void clusterProcessSlaveOf(clusterMsg *msg) {
    clusterNode *node = clusterLookupNode(msg->data.slaveof.nodeid);

    if (node == NULL) return;

    // Update the node's master
    node->slaveof = clusterLookupNode(msg->data.slaveof.masterid);
    printf("Node %s updated to follow master %s\n", node->name, node->slaveof->name);
}

4. 更新集群状态

一旦新的主节点被选定，集群中所有节点需要更新自己的状态信息，以便正确地处理客户端请求。

代码示例

/* Function to update cluster state */
void clusterUpdateState(void) {
    dictIterator *di;
    dictEntry *de;

    di = dictGetSafeIterator(server.cluster->nodes);
    while((de = dictNext(di)) != NULL) {
        clusterNode *node = dictGetVal(de);

        if (node->flags & CLUSTER_NODE_MASTER) {
            // Update master's state
        } else if (node->flags & CLUSTER_NODE_SLAVE) {
            // Update slave's state
        }
    }
    dictReleaseIterator(di);
}

综合示例

以下是一个综合性的代码示例，展示了 Redis Cluster 故障转移的完整流程。

#include <stdio.h>
#include <string.h>
#include <stdint.h>
#include <time.h>
#include "crc16.h"  // Assume crc16.h provides crc16 function
#include "dict.h"   // Assume dict.h provides a dictionary implementation
#include "mstime.h" // Assume mstime.h provides mstime function

#define CLUSTER_NODE_TIMEOUT 15000  // 15 seconds
#define CLUSTER_NODE_MYSELF 0x01
#define CLUSTER_NODE_MASTER 0x02
#define CLUSTER_NODE_SLAVE 0x04
#define CLUSTER_NODE_PFAIL 0x08
#define CLUSTER_NODE_FAIL  0x10

typedef struct clusterNode {
    char name[40];      /* Node ID */
    int flags;          /* Node flags: master, slave, fail, etc. */
    mstime_t ping_sent; /* Last time a ping was sent */
    mstime_t pong_received; /* Last time a pong was received */
    struct clusterNode *slaveof; /* Master of this node if it's a slave */
    // other fields...
} clusterNode;

typedef struct clusterState {
    dict *nodes;        /* All known nodes in the cluster */
} clusterState;

clusterState *server.cluster;

/* Function to hash a key to a slot */
unsigned int keyHashSlot(char *key, int keylen) {
    return crc16(key, keylen) & 16383;
}

/* Function to send a message to a node */
void clusterSendMessage(clusterNode *node, const char *msg) {
    // In a real implementation, this would send the message over a network
    printf("Sending message to node %s: %s\n", node->name, msg);
}

/* Function to check for node failures */
void clusterCron(void) {
    mstime_t now = mstime();
    dictIterator *di;
    dictEntry *de;

    di = dictGetSafeIterator(server.cluster->nodes);
    while((de = dictNext(di)) != NULL) {
        clusterNode *node = dictGetVal(de);

        if (node->flags & CLUSTER_NODE_MYSELF) continue;

        if (now - node->pong_received > CLUSTER_NODE_TIMEOUT) {
            // Mark the node as failing
            node->flags |= CLUSTER_NODE_PFAIL;
            printf("Node %s is in PFAIL state\n", node->name);
        }
    }
    dictReleaseIterator(di);
}

/* Function to confirm node failure */
void clusterSendFail(clusterNode *node) {
    dictIterator *di;
    dictEntry *de;

    di = dictGetSafeIterator(server.cluster->nodes);
    while ((de = dictNext(di)) != NULL) {
        clusterNode *peer = dictGetVal(de);

        if (peer->flags & CLUSTER_NODE_MYSELF) continue;

        // Send FAIL message to other nodes
        clusterSendMessage(peer, "FAIL");
    }
    dictReleaseIterator(di);
}

/* Function to process FAIL messages */
void clusterProcessFail(clusterMsg *msg) {
    clusterNode *node = clusterLookupNode(msg->data.fail.nodeid);

    if (node == NULL) return;

    // Mark the node as FAIL
    node->flags |= CLUSTER_NODE_FAIL;
    printf("Node %s marked as FAIL\n", node->name);
}

/* Function to initiate failover */
void clusterHandleSlaveFailover(clusterNode *slave) {
    if (!(slave->flags & CLUSTER_NODE_SLAVE)) return;

    // Promote the slave to master
    slave->flags &= ~CLUSTER_NODE_SLAVE;
    slave->flags |= CLUSTER_NODE_MASTER;

    // Send update to the cluster
    clusterBroadcastSlaveOf(slave, NULL);
    printf("Slave %s promoted to master\n", slave->name);
}

/* Function to broadcast SLAVEOF message */
void clusterBroadcastSlaveOf(clusterNode *node, clusterNode *master) {
    dictIterator *di;
    dictEntry *de;

    di = dictGetSafeIterator(server.cluster->nodes);