Redis（141）Redis的Cluster的槽分配是如何实现的？

Redis Cluster 使用哈希槽（hash slots）来进行数据分片和分配。整个键空间被分为 16384 个哈希槽，每个键根据其 CRC16 校验和被映射到其中一个哈希槽上。每个节点负责一部分哈希槽，从而实现数据的分布式存储。

1. 哈希槽计算

Redis Cluster 使用 CRC16 算法计算键的哈希值，并将其映射到 16384 个哈希槽之一。具体步骤如下：

代码示例

/* CRC16 implementation to hash the key */
unsigned int crc16(const char *buf, int len) {
    static const unsigned short crc16tab[256] = {
        /* Table of CRC16 values for high–order byte */
        /* ... (省略表格数据) ... */
    };

    int counter;
    unsigned short crc = 0;
    for (counter = 0; counter < len; counter++)
        crc = (crc << 8) ^ crc16tab[((crc >> 8) ^ *buf++) & 0x00FF];
    return crc;
}

/* Function to get the hash slot for a given key */
unsigned int keyHashSlot(const char *key, int keylen) {
    return crc16(key, keylen) % 16384;
}

2. 哈希槽分配

哈希槽的分配是指将 16384 个哈希槽分配给不同的集群节点。通常，初始的哈希槽分配是在集群配置和初始化时完成的。以下是示例代码，展示如何在集群节点之间分配哈希槽。

代码示例

#include <stdio.h>
#include <string.h>

#define HASH_SLOTS 16384

/* Cluster node structure */
typedef struct clusterNode {
    char name[40];      /* Node ID */
    int start_slot;     /* Start hash slot */
    int end_slot;       /* End hash slot */
    // other fields...
} clusterNode;

/* Function to assign hash slots to nodes */
void assignHashSlots(clusterNode *nodes, int num_nodes) {
    int slots_per_node = HASH_SLOTS / num_nodes;
    int remaining_slots = HASH_SLOTS % num_nodes;

    int current_slot = 0;
    for (int i = 0; i < num_nodes; i++) {
        nodes[i].start_slot = current_slot;
        current_slot += slots_per_node;
        if (remaining_slots > 0) {
            current_slot++;
            remaining_slots--;
        }
        nodes[i].end_slot = current_slot - 1;
    }
}

/* Example usage */
int main() {
    clusterNode nodes[3] = {
        {"node1", 0, 0},
        {"node2", 0, 0},
        {"node3", 0, 0}
    };
    int num_nodes = 3;

    assignHashSlots(nodes, num_nodes);

    for (int i = 0; i < num_nodes; i++) {
        printf("Node %s: slots %d - %d\n", nodes[i].name, nodes[i].start_slot, nodes[i].end_slot);
    }

    return 0;
}

3. 集群节点之间的槽迁移

在实际运行过程中，可能需要在集群节点之间迁移哈希槽以实现负载均衡或进行节点的增加/删除操作。下面的代码示例展示了简单的哈希槽迁移。

代码示例

/* Function to move hash slots from one node to another */
void migrateHashSlots(clusterNode *src, clusterNode *dst, int start_slot, int end_slot) {
    if (start_slot < src->start_slot || end_slot > src->end_slot) {
        printf("Invalid slot range\n");
        return;
    }

    // Adjust source node's slot range
    if (start_slot == src->start_slot) {
        src->start_slot = end_slot + 1;
    } else if (end_slot == src->end_slot) {
        src->end_slot = start_slot - 1;
    } else {
        printf("Source node's slot range cannot be split\n");
        return;
    }

    // Adjust destination node's slot range
    if (dst->start_slot == -1 && dst->end_slot == -1) {
        dst->start_slot = start_slot;
        dst->end_slot = end_slot;
    } else {
        if (start_slot < dst->start_slot) {
            dst->start_slot = start_slot;
        }
        if (end_slot > dst->end_slot) {
            dst->end_slot = end_slot;
        }
    }
    
    printf("Migrated slots %d-%d from node %s to node %s\n", start_slot, end_slot, src->name, dst->name);
}

/* Example usage */
int main() {
    clusterNode nodes[3] = {
        {"node1", 0, 5460},
        {"node2", 5461, 10921},
        {"node3", 10922, 16383}
    };

    migrateHashSlots(&nodes[0], &nodes[2], 0, 1000);

    for (int i = 0; i < 3; i++) {
        printf("Node %s: slots %d - %d\n", nodes[i].name, nodes[i].start_slot, nodes[i].end_slot);
    }

    return 0;
}

4. 集成示例

以下是一个综合示例，展示了如何初始化集群节点、分配哈希槽以及在节点之间迁移哈希槽。

综合代码示例

#include <stdio.h>
#include <string.h>
#include <stdint.h>

#define HASH_SLOTS 16384

/* CRC16 implementation to hash the key */
unsigned int crc16(const char *buf, int len) {
    static const unsigned short crc16tab[256] = {
        // Table of CRC16 values for high–order byte
        // ... (省略表格数据) ...
    };

    int counter;
    unsigned short crc = 0;
    for (counter = 0; counter < len; counter++)
        crc = (crc << 8) ^ crc16tab[((crc >> 8) ^ *buf++) & 0x00FF];
    return crc;
}

/* Function to get the hash slot for a given key */
unsigned int keyHashSlot(const char *key, int keylen) {
    return crc16(key, keylen) % HASH_SLOTS;
}

/* Cluster node structure */
typedef struct clusterNode {
    char name[40];      /* Node ID */
    int start_slot;     /* Start hash slot */
    int end_slot;       /* End hash slot */
    // other fields...
} clusterNode;

/* Function to assign hash slots to nodes */
void assignHashSlots(clusterNode *nodes, int num_nodes) {
    int slots_per_node = HASH_SLOTS / num_nodes;
    int remaining_slots = HASH_SLOTS % num_nodes;

    int current_slot = 0;
    for (int i = 0; i < num_nodes; i++) {
        nodes[i].start_slot = current_slot;
        current_slot += slots_per_node;
        if (remaining_slots > 0) {
            current_slot++;
            remaining_slots--;
        }
        nodes[i].end_slot = current_slot - 1;
    }
}

/* Function to move hash slots from one node to another */
void migrateHashSlots(clusterNode *src, clusterNode *dst, int start_slot, int end_slot) {
    if (start_slot < src->start_slot || end_slot > src->end_slot) {
        printf("Invalid slot range\n");
        return;
    }

    // Adjust source node's slot range
    if (start_slot == src->start_slot) {
        src->start_slot = end_slot + 1;
    } else if (end_slot == src->end_slot) {
        src->end_slot = start_slot - 1;
    } else {
        printf("Source node's slot range cannot be split\n");
        return;
    }

    // Adjust destination node's slot range
    if (dst->start_slot == -1 && dst->end_slot == -1) {
        dst->start_slot = start_slot;
        dst->end_slot = end_slot;
    } else {
        if (start_slot < dst->start_slot) {
            dst->start_slot = start_slot;
        }
        if (end_slot > dst->end_slot) {
            dst->end_slot = end_slot;
        }
    }
    
    printf("Migrated slots %d-%d from node %s to node %s\n", start_slot, end_slot, src->name, dst->name);
}

/* Example usage */
int main() {
    clusterNode nodes[3] = {
        {"node1", 0, -1},
        {"node2", 0, -1},
        {"node3", 0, -1}
    };
    int num_nodes = 3;

    assignHashSlots(nodes, num_nodes);

    for (int i = 0; i < num_nodes; i++) {
        printf("Node %s: slots %d - %d\n", nodes[i].name, nodes[i].start_slot, nodes[i].end_slot);
    }

    printf("\nMigrating slots from node1 to node3...\n");
    migrateHashSlots(&nodes[0], &nodes[2], 0, 1000);