Redis（139）Redis的Cluster是如何实现的？

Redis Cluster 是 Redis 的一种分布式架构，允许将数据分布到多个节点上以实现数据的自动分片、负载均衡和高可用性。Redis Cluster 通过分片、复制、故障检测和自动故障转移等机制实现这些功能。以下是 Redis Cluster 的详细实现及其关键组件和代码示例。

核心概念

1. 数据分片

Redis Cluster 将数据键空间分为 16384 个哈希槽（hash slots）。每个键根据其 CRC16 校验和被映射到其中一个哈希槽。

2. 节点角色

Redis Cluster 中的节点分为主节点（master）和从节点（slave）。主节点负责处理写请求和数据分片，从节点负责备份主节点的数据并在主节点失效时进行故障转移。

3. 集群总线

Redis Cluster 节点之间通过集群总线（Cluster Bus）进行通信，该总线实现了节点间的故障检测、配置更新等功能。

4. 故障检测和转移

Redis Cluster 通过投票机制检测节点故障，并在主节点失效时触发自动故障转移。

数据分片实现

Redis Cluster 的数据分片通过哈希槽实现。以下是相关的代码示例：

1/* Redis Cluster hash function */
2unsigned int keyHashSlot(char *key, int keylen) {
3    return crc16(key, keylen) & 16383;
4}
5

节点间通信

Redis Cluster 节点通过集群总线进行通信。集群总线使用特殊的二进制协议进行消息传递。以下是节点通信的实现：

1/* Cluster message types */
2#define CLUSTERMSG_TYPE_PING 0
3#define CLUSTERMSG_TYPE_PONG 1
4
5/* Cluster message structure */
6typedef struct {
7    uint16_t type;     /* Message type */
8    uint16_t totlen;   /* Total length of the message */
9    char sender[40];   /* Node ID of the sender */
10    char myip[16];     /* Sender IP address */
11    uint16_t port;     /* Sender port */
12    // other fields...
13} clusterMsg;
14
15/* Function to send a message to a node */
16void clusterSendMessage(clusterNode *node, clusterMsg *msg) {
17    // Serialize the message and send it over the network
18}
19
20

故障检测和转移

Redis Cluster 通过投票机制实现故障检测和转移。当一个主节点被检测为失效后，集群中的其他主节点会进行投票决定是否进行故障转移。

故障检测

每个节点都会定期向其他节点发送 PING 消息，并期待收到 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失效。

1/* Cluster node structure */
2typedef struct clusterNode {
3    char name[40];      /* Node ID */
4    int flags;          /* Node flags: master, slave, fail, etc. */
5    int ping_sent;      /* Last time a ping was sent */
6    int pong_received;  /* Last time a pong was received */
7    // other fields...
8} clusterNode;
9
10/* Function to check for node failures */
11void clusterCron(void) {
12    mstime_t now = mstime();
13    dictIterator *di;
14    dictEntry *de;
15
16    di = dictGetSafeIterator(server.cluster->nodes);
17    while((de = dictNext(di)) != NULL) {
18        clusterNode *node = dictGetVal(de);
19
20        if (node->flags & CLUSTER_NODE_MYSELF) continue;
21
22        if (now - node->pong_received > server.cluster_node_timeout) {
23            // Mark the node as failing
24            node->flags |= CLUSTER_NODE_PFAIL;
25        }
26    }
27    dictReleaseIterator(di);
28}
29

故障转移

当一个主节点失效后，由从节点中的一个进行接管。故障转移的过程包括选择一个新的主节点和更新集群状态。

1/* Function to trigger failover */
2void clusterHandleSlaveFailover(clusterNode *slave) {
3    // Promote the slave to master
4    slave->flags &= ~CLUSTER_NODE_SLAVE;
5    slave->flags |= CLUSTER_NODE_MASTER;
6
7    // Update the cluster state
8    clusterUpdateState();
9}
10
11/* Function to update cluster state */
12void clusterUpdateState(void) {
13    // Iterate over all nodes and update the state
14    dictIterator *di;
15    dictEntry *de;
16
17    di = dictGetSafeIterator(server.cluster->nodes);
18    while((de = dictNext(di)) != NULL) {
19        clusterNode *node = dictGetVal(de);
20
21        if (node->flags & CLUSTER_NODE_MASTER) {
22            // Update the master's state
23        } else if (node->flags & CLUSTER_NODE_SLAVE) {
24            // Update the slave's state
25        }
26    }
27    dictReleaseIterator(di);
28}
29

整合示例

以下是一个包含数据分片、通信和故障检测的完整示例：

1#include <stdio.h>
2#include <stdint.h>
3#include <string.h>
4#include "crc16.h"  // Assume crc16.h provides crc16 function
5#include "dict.h"   // Assume dict.h provides a dictionary implementation
6#include "mstime.h" // Assume mstime.h provides mstime function
7
8#define CLUSTER_NODE_TIMEOUT 15000  // 15 seconds
9
10typedef struct clusterNode {
11    char name[40];      /* Node ID */
12    int flags;          /* Node flags: master, slave, fail, etc. */
13    int ping_sent;      /* Last time a ping was sent */
14    int pong_received;  /* Last time a pong was received */
15    int port;           /* Node port */
16} clusterNode;
17
18typedef struct clusterState {
19    dict *nodes;        /* All known nodes in the cluster */
20} clusterState;
21
22clusterState *server.cluster;
23
24/* Function to hash a key to a slot */
25unsigned int keyHashSlot(char *key, int keylen) {
26    return crc16(key, keylen) & 16383;
27}
28
29/* Function to send a message to a node */
30void clusterSendMessage(clusterNode *node, const char *msg) {
31    // In a real implementation, this would send the message over a network
32    printf("Sending message to node %s: %s\n", node->name, msg);
33}
34
35/* Function to check for node failures */
36void clusterCron(void) {
37    mstime_t now = mstime();
38    dictIterator *di;
39    dictEntry *de;
40
41    di = dictGetSafeIterator(server.cluster->nodes);
42    while((de = dictNext(di)) != NULL) {
43        clusterNode *node = dictGetVal(de);
44
45        if (now - node->pong_received > CLUSTER_NODE_TIMEOUT) {
46            // Mark the node as failing
47            node->flags |= 0x01;  // Assume 0x01 represents the fail flag
48            printf("Node %s marked as failing\n", node->name);
49        }
50    }
51    dictReleaseIterator(di);
52}
53
54/* Entry point */
55int main(void) {
56    // Initialize the cluster state
57    server.cluster = malloc(sizeof(clusterState));
58    server.cluster->nodes = dictCreate();
59
60    // Create and add nodes to the cluster
61    clusterNode node1 = {"node1", 0, 0, mstime(), 6379};
62    clusterNode node2 = {"node2", 0, 0, mstime(), 6379};
63    dictAdd(server.cluster->nodes, node1.name, &node1);
64    dictAdd(server.cluster->nodes, node2.name, &node2);
65
66    // Simulate cron job
67    clusterCron();
68
69    // Simulate sending a message
70    clusterSendMessage(&node1, "PING");
71
72    // Cleanup
73    dictRelease(server.cluster->nodes);
74    free(server.cluster);
75
76    return 0;
77}
78

总结

Redis Cluster 通过数据分片、节点通信、故障检测和故障转移等机制实现了分布式架构。数据分片基于哈希槽进行，节点间通信通过集群总线实现，故障检测通过定期的 PING/PONG 消息完成，而故障转移则通过投票机制选择新的主节点并更新集群状态。上述代码示例展示了 Redis Cluster 的基本实现原理和关键组件。通过理解这些机制，可以更深入地了解 Redis Cluster 的工作原理，并根据需要进行扩展和优化。

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