codis3.2升级redis3.11到redis6.0.10调研

codis升级redis3.11到redis6.0.10背景

当前codis最新版本为3.2对应的redis的版本为3.2.11，针对以往的redis在使用过程中当内存碎片率过高时只能重启节点，无法动态释放故调研升级到redis6版本。

codis调研

从codis的架构可以知道，codis中的jodis-client（通过zk）或者redis-client直连的方式来访问codis-proxy，codis-proxy通过计算对应的key的slot来转发到对应的codis-group中去，这样完成一个完整的请求访问。

当前有三种方式来使用redis中的数据：

通过codis提供的封装过zk的jodis客户端，来访问这样通过key就能直接获取内容而无需做其他修改，该客户端将proxy的服务发现等操作都已经封装完成，使用方便快捷。
通过proxy来访问redis实例信息，但是直连proxy带来的风险是无法再proxy层做负载均衡和服务发现，对后期运维不太友好。
直连redis实例，如果直连redis实例相当于把所有slot的操作系统都放在了业务端来实现基本没有使用上codis的分布式可扩展的天然优势，虽然codis-server提供计算slot的命令但是该方案的意义不大。

开发的思路整理

当前主要是为了升级redis3，针对官方的版本有过升级版本的redis4分支，通过该patch可以看出所有的开发的代码与修复的内容。

首先对比查看该patch与当前codis3.2稳定版本的patch对比，发现修改的内容并不算多但是为啥没有并入主分支原因不得而知。

redis改造的核心

slot信息的添加维护

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redis的内存的存储的逻辑如上所述，基本上就是在初始化的时候就初始化16个DB结构来存储对应的数据。那对应的codis为了将每个Key都加上slot信息，就采用了如下的结果来存储。

codis为了实现slot信息的维护修改为了如下结构；

为每一个DB都新增加一个slot数组，每个slot数组保持对应slot的数据，这样就快速的完成了slot信息的维护管理，并且不需要过多的修改redis其他的源码流程；但是该方案的一个问题就是会加大内存的使用。为了保证slot数组的信息的同步只需要再添加和删除key和value的过程中，将对应的标记删除掉就行，即在源码中的dbAdd等函数和dbSyncDelete等函数中；

void dbAdd(redisDb *db, robj *key, robj *val) {sds copy = sdsdup(key->ptr);int retval = dictAdd(db->dict, copy, val);do {uint32_t crc;int hastag;int slot = slots_num(key->ptr, &crc, &hastag);dictAdd(db->hash_slots[slot], copy, (void *)(long)crc);if (hastag) {zslInsert(db->tagged_keys, (double)crc, sdsdup(key->ptr));}} while (0);serverAssertWithInfo(NULL,key,retval == DICT_OK);if (val->type == OBJ_LIST ||val->type == OBJ_ZSET ||val->type == OBJ_STREAM)signalKeyAsReady(db, key);if (server.cluster_enabled) slotToKeyAdd(key->ptr);
}...int dbSyncDelete(redisDb *db, robj *key) {/* Deleting an entry from the expires dict will not free the sds of* the key, because it is shared with the main dictionary. */if (dictSize(db->expires) > 0) dictDelete(db->expires,key->ptr);do {uint32_t crc;int hastag;int slot = slots_num(key->ptr, &crc, &hastag);if (dictDelete(db->hash_slots[slot], key->ptr) == DICT_OK) {if (hastag) {zslDelete(db->tagged_keys, (double)crc, key->ptr, NULL);}}} while (0);if (dictDelete(db->dict,key->ptr) == DICT_OK) {if (server.cluster_enabled) slotToKeyDel(key->ptr);return 1;} else {return 0;}
}

slot信息的迁移

在codis中，有关slot节点在各个组中的迁移主要是通过扩展redis的命令来实现的最主要的命令就是slotsmgrtslot-async和slotsrestore-async来实现的，该方案的主要逻辑如下；

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.edge-pattern-dashed{stroke-dasharray:3}#mermaid-svg-CCeHZM1R6QzGZl9e .edge-pattern-dotted{stroke-dasharray:2}#mermaid-svg-CCeHZM1R6QzGZl9e .marker{fill:#333}#mermaid-svg-CCeHZM1R6QzGZl9e .marker.cross{stroke:#333}:root { --mermaid-font-family: "trebuchet ms", verdana, arial;}#mermaid-svg-CCeHZM1R6QzGZl9e {color: rgba(0, 0, 0, 0.75);font: ;}dashboardcodis_server_acodis_server_b迁移slots操作通过slotsmgrtslot-async命令来连接codis_server_b通过slotsrestore-async将对应的数据结构发生给adashboardcodis_server_acodis_server_b

通过在codis_server_a中创建一个client连接到codis_server_b，然后通过该client一直将数据通过命令发送给codis_server_b然后通过恢复数据来达到slot在各个server直接的迁移功能。

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.edge-pattern-dashed{stroke-dasharray:3}#mermaid-svg-khjQYZALxhYMNhFR .edge-pattern-dotted{stroke-dasharray:2}#mermaid-svg-khjQYZALxhYMNhFR .marker{fill:#333}#mermaid-svg-khjQYZALxhYMNhFR .marker.cross{stroke:#333}:root { --mermaid-font-family: "trebuchet ms", verdana, arial;}#mermaid-svg-khjQYZALxhYMNhFR {color: rgba(0, 0, 0, 0.75);font: ;}clientcodis_proxycodis_server_a通过zk或者直连proxy连接到codis_proxy计算slot的值然后根据分配信息连接a响应数据返回返回客户端clientcodis_proxycodis_server_a

总结

当前的codis3.2基本上处于维护的状态并无新功能迭代开发的计划，一方面在kv领域也出现了更多好的分布式的技术方案如tidb和redis本身支持实现的分布式方案，并且codis在适用场景上面越来越不容易满足当今多样化的需求，升级redis3到redis6这个方案个人感觉还是只是一个过度方案，并没有在新场景下提供解决方案。由于本人才疏学浅，如有错误请批评指正。