Redis 源码分析 RESP 协议

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Redis 通讯协议（RESP）

Redis 基于 RESP （Redis Serialization Protocal）协议来完成客户端和服务端通讯。RESP 本质是一种文本协议，实现简单、易于解析。 特征和描述如下表所示：

类型	协议描述
网络层	客户端和服务端通过 tcp/ 流式套接字来进行通讯，为了 防止粘包 因此命令或数据均以 \r\n (CRLF) 结尾
请求	*<参数数量>CR LF $<参数 1 的字节数量 >CR LF<br/><参数 1 的数据> CR LF<br/>... <br>$<参数 N 的字节数量 >CR LF <参数 N 的数据> CR LF	*2\r\n3\nget\r\n$13\r\nusername:1234\r\n。见 callSendCommond -> redis AppendConnadnArgv -> redisFromatCommandArgv
简单字符串回复	第一个字节+	+ok\r\n
错误回复	第一个字节-	-ERR unknown command 'sa' \r\n
整数回复	第一个字节:	:0\r\n
批量回复	第一个字节$	6\r\nfoobar\r\n，空回复 -1
多条批量回复	第一个字节*	*5\r\n:1\r\n:2\r\n:3\r\n:4\r\n$6\r\nfoobar\r\n, 空回复 *0\r\n

特别说明：如果客户端和服务端在一台机器上。那么会对通讯协议进行优化，直接走本地回环

我们以通过 tcpdump 工具来帮助我们进行网络抓包。

# linux
tcpdump -i lo part 6379 -Ann

# mac 
tcpdump -i lo0 port 6379 -Ann
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测试一下 ( 我本机是 mac 环境)：

# 客户端 A
127.0.0.1:6379> set msg100 1
OK
  
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服务端抓包结果，如下：

➜  ~ sudo tcpdump -i lo0 port 6379 -Ann
Password:
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on lo0, link-type NULL (BSD loopback), capture size 262144 bytes
21:52:53.447885 IP 127.0.0.1.51645 > 127.0.0.1.6379: Flags [P.], seq 1564111974:1564112006, ack 169183468, win 6272, options [nop,nop,TS val 774447713 ecr 772455554], length 32: RESP "set" "msg100" "1"
E..T..@.@...............]:tf
........H.....
.)"a.
..*3
$3
set
$6
msg100
$1
1

21:52:53.447912 IP 127.0.0.1.6379 > 127.0.0.1.51645: Flags [.], ack 32, win 6376, options [nop,nop,TS val 774447713 ecr 774447713], length 0
E..4..@.@...............
...]:t......(.....
.)"a.)"a
21:52:53.528935 IP 127.0.0.1.6379 > 127.0.0.1.51645: Flags [P.], seq 1:6, ack 32, win 6376, options [nop,nop,TS val 774447793 ecr 774447713], length 5: RESP "OK"
E..9..@.@...............
...]:t......-.....
.)"..)"a+OK

21:52:53.528966 IP 127.0.0.1.51645 > 127.0.0.1.6379: Flags [.], ack 6, win 6272, options [nop,nop,TS val 774447793 ecr 774447793], length 0
E..4..@.@...............]:t.
........(.....
.)"..)".
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客户端效果

客户端是对显示结果做了转化，在 redis-cli.c/cliFormatReplyTTY 中

static sds cliFormatReplyTTY(redisReply *r, char *prefix) {
    sds out = sdsempty();
    switch (r->type) {
    case REDIS_REPLY_ERROR:
        out = sdscatprintf(out,"(error) %s\n", r->str);
    break;
    case REDIS_REPLY_STATUS:
        out = sdscat(out,r->str);
        out = sdscat(out,"\n");
    break;
    case REDIS_REPLY_INTEGER:
        out = sdscatprintf(out,"(integer) %lld\n",r->integer);
    break;
    case REDIS_REPLY_DOUBLE:
        out = sdscatprintf(out,"(double) %s\n",r->str);
    break;
    case REDIS_REPLY_STRING:
    case REDIS_REPLY_VERB:
        /* If you are producing output for the standard output we want
        * a more interesting output with quoted characters and so forth,
        * unless it's a verbatim string type. */
        if (r->type == REDIS_REPLY_STRING) {
            out = sdscatrepr(out,r->str,r->len);
            out = sdscat(out,"\n");
        } else {
            out = sdscatlen(out,r->str,r->len);
            out = sdscat(out,"\n");
        }
    break;
    case REDIS_REPLY_NIL:
        out = sdscat(out,"(nil)\n");
    break;
    case REDIS_REPLY_BOOL:
        out = sdscat(out,r->integer ? "(true)\n" : "(false)\n");
    break;
    case REDIS_REPLY_ARRAY:
    case REDIS_REPLY_MAP:
    case REDIS_REPLY_SET:
    case REDIS_REPLY_PUSH:
        if (r->elements == 0) {
            if (r->type == REDIS_REPLY_ARRAY)
                out = sdscat(out,"(empty array)\n");
            else if (r->type == REDIS_REPLY_MAP)
                out = sdscat(out,"(empty hash)\n");
            else if (r->type == REDIS_REPLY_SET)
                out = sdscat(out,"(empty set)\n");
            else if (r->type == REDIS_REPLY_PUSH)
                out = sdscat(out,"(empty push)\n");
            else
                out = sdscat(out,"(empty aggregate type)\n");
        } else {
            unsigned int i, idxlen = 0;
            char _prefixlen[16];
            char _prefixfmt[16];
            sds _prefix;
            sds tmp;

            /* Calculate chars needed to represent the largest index */
            i = r->elements;
            if (r->type == REDIS_REPLY_MAP) i /= 2;
            do {
                idxlen++;
                i /= 10;
            } while(i);

            /* Prefix for nested multi bulks should grow with idxlen+2 spaces */
            memset(_prefixlen,' ',idxlen+2);
            _prefixlen[idxlen+2] = '\0';
            _prefix = sdscat(sdsnew(prefix),_prefixlen);

            /* Setup prefix format for every entry */
            char numsep;
            if (r->type == REDIS_REPLY_SET) numsep = '~';
            else if (r->type == REDIS_REPLY_MAP) numsep = '#';
            else numsep = ')';
            snprintf(_prefixfmt,sizeof(_prefixfmt),"%%s%%%ud%c ",idxlen,numsep);

            for (i = 0; i < r->elements; i++) {
                unsigned int human_idx = (r->type == REDIS_REPLY_MAP) ?
                                         i/2 : i;
                human_idx++; /* Make it 1-based. */

                /* Don't use the prefix for the first element, as the parent
                 * caller already prepended the index number. */
                out = sdscatprintf(out,_prefixfmt,i == 0 ? "" : prefix,human_idx);

                /* Format the multi bulk entry */
                tmp = cliFormatReplyTTY(r->element[i],_prefix);
                out = sdscatlen(out,tmp,sdslen(tmp));
                sdsfree(tmp);

                /* For maps, format the value as well. */
                if (r->type == REDIS_REPLY_MAP) {
                    i++;
                    sdsrange(out,0,-2);
                    out = sdscat(out," => ");
                    tmp = cliFormatReplyTTY(r->element[i],_prefix);
                    out = sdscatlen(out,tmp,sdslen(tmp));
                    sdsfree(tmp);
                }
            }
            sdsfree(_prefix);
        }
    break;
    default:
        fprintf(stderr,"Unknown reply type: %d\n", r->type);
        exit(1);
    }
    return out;
}
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我们也可以使用 nc 命令来替代 redis-cli 命令行：

➜  ~ sudo nc 127.0.0.1 6379
set a a
+OK
get a
$1
a
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其他说明

• Redis 常见的错误 (src/redis-cli.c)

#define REDIS_ERR -1
#define REDIS_OK 0

/* When an error occurs, the err flag in a context is set to hold the type of
 * error that occurred. REDIS_ERR_IO means there was an I/O error and you
 * should use the "errno" variable to find out what is wrong.
 * For other values, the "errstr" field will hold a description. */
#define REDIS_ERR_IO 1 /* Error in read or write */
#define REDIS_ERR_EOF 3 /* End of file */
#define REDIS_ERR_PROTOCOL 4 /* Protocol error */
#define REDIS_ERR_OOM 5 /* Out of memory */
#define REDIS_ERR_TIMEOUT 6 /* Timed out */
#define REDIS_ERR_OTHER 2 /* Everything else... */

#define REDIS_REPLY_STRING 1
#define REDIS_REPLY_ARRAY 2
#define REDIS_REPLY_INTEGER 3
#define REDIS_REPLY_NIL 4
#define REDIS_REPLY_STATUS 5
#define REDIS_REPLY_ERROR 6
#define REDIS_REPLY_DOUBLE 7
#define REDIS_REPLY_BOOL 8
#define REDIS_REPLY_MAP 9
#define REDIS_REPLY_SET 10
#define REDIS_REPLY_ATTR 11
#define REDIS_REPLY_PUSH 12
#define REDIS_REPLY_BIGNUM 13
#define REDIS_REPLY_VERB 14

/* Default max unused reader buffer. */
#define REDIS_READER_MAX_BUF (1024*16)

/* Default multi-bulk element limit */
#define REDIS_READER_MAX_ARRAY_ELEMENTS ((1LL<<32) - 1)
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Redis 命令对象

redis 命令是使用的是 redisCommand 数据结构来管理的。

数据结构

typedef void redisCommandProc(client *c);
// 函数指针类型，指向命令实现函数
typedef int redisGetKeysProc(struct redisCommand *cmd, robj **argv, int argc, getKeysResult *result);
struct redisCommand {
    char *name;
    redisCommandProc *proc;
    // 限制命令的个数。-N 表示至少 N 个参数，包含命令本身
    int arity;
    // 字符串方式设置命令的属性之间运用 ｜ 运算，程序内部自动解析，函数 populateCommandTable
    char *sflags;   /* Flags as string representation, one char per flag. */
    // 将 flags 字符串类型转换成整数，多个属性
    uint64_t flags; /* The actual flags, obtained from the 'sflags' field. */
    /* Use a function to determine keys arguments in a command line.
     * Used for Redis Cluster redirect. */
    redisGetKeysProc *getkeys_proc;
    /* What keys should be loaded in background when calling this command? */
    int firstkey; /* The first argument that's a key (0 = no keys) */
    int lastkey;  /* The last argument that's a key */
    int keystep;  /* The step between first and last key */
    long long microseconds, calls, rejected_calls, failed_calls;
    int id;     /* Command ID. This is a progressive ID starting from 0 that
                   is assigned at runtime, and is used in order to check
                   ACLs. A connection is able to execute a given command if
                   the user associated to the connection has this command
                   bit set in the bitmap of allowed commands. */
};
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针对 sflag 标示，这里可以看看 《redis 设计与实现》

flag 记录的是 flag 值与 sflag 进行运算的结果，见 populateCommandTable 函数 (src/server.c/populateCommandTable)

    for (int j = 0; j < argc; j++) {
        char *flag = argv[j];
        if (!strcasecmp(flag,"write")) {
            c->flags |= CMD_WRITE|CMD_CATEGORY_WRITE;
        } else if (!strcasecmp(flag,"read-only")) {
            c->flags |= CMD_READONLY|CMD_CATEGORY_READ;
        } else if (!strcasecmp(flag,"use-memory")) {
            c->flags |= CMD_DENYOOM;
        } else if (!strcasecmp(flag,"admin")) {
            c->flags |= CMD_ADMIN|CMD_CATEGORY_ADMIN|CMD_CATEGORY_DANGEROUS;
        } else if (!strcasecmp(flag,"pub-sub")) {
            c->flags |= CMD_PUBSUB|CMD_CATEGORY_PUBSUB;
        } else if (!strcasecmp(flag,"no-script")) {
            c->flags |= CMD_NOSCRIPT;
        } else if (!strcasecmp(flag,"random")) {
            c->flags |= CMD_RANDOM;
        } else if (!strcasecmp(flag,"to-sort")) {
            c->flags |= CMD_SORT_FOR_SCRIPT;
        } else if (!strcasecmp(flag,"ok-loading")) {
            c->flags |= CMD_LOADING;
        } else if (!strcasecmp(flag,"ok-stale")) {
            c->flags |= CMD_STALE;
        } else if (!strcasecmp(flag,"no-monitor")) {
            c->flags |= CMD_SKIP_MONITOR;
        } else if (!strcasecmp(flag,"no-slowlog")) {
            c->flags |= CMD_SKIP_SLOWLOG;
        } else if (!strcasecmp(flag,"cluster-asking")) {
            c->flags |= CMD_ASKING;
        } else if (!strcasecmp(flag,"fast")) {
            c->flags |= CMD_FAST | CMD_CATEGORY_FAST;
        } else if (!strcasecmp(flag,"no-auth")) {
            c->flags |= CMD_NO_AUTH;
        } else if (!strcasecmp(flag,"may-replicate")) {
            c->flags |= CMD_MAY_REPLICATE;
        } else {
            /* Parse ACL categories here if the flag name starts with @. */
            uint64_t catflag;
            if (flag[0] == '@' &&
                (catflag = ACLGetCommandCategoryFlagByName(flag+1)) != 0)
            {
                c->flags |= catflag;
            } else {
                sdsfreesplitres(argv,argc);
                return C_ERR;
            }
        }
    }
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具体命令比较多

struct redisCommand redisCommandTable[] = {
    {"module",moduleCommand,-2,
     "admin no-script",
     0,NULL,0,0,0,0,0,0},

    {"get",getCommand,2,
     "read-only fast @string",
     0,NULL,1,1,1,0,0,0},
     // .....   
}
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以 set 为例子 {"set",setCommand,-3, "write use-memory @string", 0,NULL,1,1,1,0,0,0}

参考资料

• 《Redis 设计与实现》 黄健宏