Redis（三）：set/get 命令解析教程
经过前两篇的介绍，我们对整个redis的动作流程已经有比较清晰的认识。
接下来就是到具体的命令处理方式的理解了，想来我们用这些工具的意义也是在此。虽然没有人觉得，一个set/get方法会有难度，但是我们毕竟不是很清楚，否则也不至于在谈到深处就懵逼了。
我觉得本文的一个重要意义就是: 让set/get还原成它本来样子，和写"hello world"一样简单。
框架性质的东西，我们前面已经讲解，就直接进入主题： set/get 的操作。
set/get 对应的两个处理函数 (redisCommand) 定义是这样的:
// rF 代表 getCommand 是只读命令，又快又准，时间复杂度 O(1)或者O(log(n))
// wm 代表 setCommand 是个写命令,当心空间问题
{"get",getCommand,2,"rF",0,NULL,1,1,1,0,0}
{"set",setCommand,-3,"wm",0,NULL,1,1,1,0,0}所以，我们只要理解了， setCommand,getCommand 之后，就可以完全自信的说，set/get 就是和 "hello world" 一样简单了。
零、hash 算法
很显然，kv型的存储一定是hash相关算法的实现。那么redis中如何使用这个hash算法的呢？
redis 中许多不同场景的hash算法，其原型是在 dictType 中定义的。
typedef struct dictType {
// hash 算法原型
unsigned int (*hashFunction)(const void *key);
void *(*keyDup)(void *privdata, const void *key);
void *(*valDup)(void *privdata, const void *obj);
int (*keyCompare)(void *privdata, const void *key1, const void *key2);
void (*keyDestructor)(void *privdata, void *key);
void (*valDestructor)(void *privdata, void *obj);
} dictType;针对大部分场景，我们的key一般都是 string 类型的，但是还是会稍微有不一样的。这里我们就两个场景来说明下：
1. 命令集构建的hash算法
即是 server.commands 中的key的hash算法，这里元素是有限的。其定义如下:
/* Command table. sds string -> command struct pointer. */
dictType commandTableDictType = {
// 即 dictSdsCaseHash 是 command 的hash算法实现
dictSdsCaseHash, /* hash function */
NULL, /* key dup */
NULL, /* val dup */
dictSdsKeyCaseCompare, /* key compare */
dictSdsDestructor, /* key destructor */
NULL /* val destructor */
};
// server.c, 不区分大小写的key hash
unsigned int dictSdsCaseHash(const void *key) {
return dictGenCaseHashFunction((unsigned char*)key, sdslen((char*)key));
}
// dict.c, 不区分大小写的key-hash算法: hash * 33 + c
/* And a case insensitive hash function (based on djb hash) */
unsigned int dictGenCaseHashFunction(const unsigned char *buf, int len) {
// static uint32_t dict_hash_function_seed = 5381;
unsigned int hash = (unsigned int)dict_hash_function_seed;
while (len--)
hash = ((hash << 5) + hash) + (tolower(*buf++)); /* hash * 33 + c */
return hash;
}2. 针对普通kv查询的hash算法
整个nosql就是kv的增删改查，所以这是个重要的算法。
/* Db->dict, keys are sds strings, vals are Redis objects. */
dictType dbDictType = {
// k 的hash算法实现
dictSdsHash, /* hash function */
NULL, /* key dup */
NULL, /* val dup */
dictSdsKeyCompare, /* key compare */
dictSdsDestructor, /* key destructor */
dictObjectDestructor /* val destructor */
};
// server.c, 调用 dict 的实现
unsigned int dictSdsHash(const void *key) {
return dictGenHashFunction((unsigned char*)key, sdslen((char*)key));
}
// dict.c, 数据key的hash算法，或者通用的 string 的hashCode 算法
/* MurmurHash2, by Austin Appleby
* Note - This code makes a few assumptions about how your machine behaves -
* 1. We can read a 4-byte value from any address without crashing
* 2. sizeof(int) == 4
*
* And it has a few limitations -
*
* 1. It will not work incrementally.
* 2. It will not produce the same results on little-endian and big-endian
* machines.
*/
unsigned int dictGenHashFunction(const void *key, int len) {
/* 'm' and 'r' are mixing constants generated offline.
They're not really 'magic', they just happen to work well. */
uint32_t seed = dict_hash_function_seed;
const uint32_t m = 0x5bd1e995;
const int r = 24;
/* Initialize the hash to a 'random' value */
uint32_t h = seed ^ len;
/* Mix 4 bytes at a time into the hash */
const unsigned char *data = (const unsigned char *)key;
// 核心算法: step1. *m, ^k>>r, *m, *m, ^k, 每4位做一次运算
while(len >= 4) {
uint32_t k = *(uint32_t*)data;
k *= m;
k ^= k >> r;
k *= m;
h *= m;
h ^= k;
data += 4;
len -= 4;
}
/* Handle the last few bytes of the input array */
// step2. 倒数第三位 ^<<16, 第二位 ^<<8, 第一位 ^, 然后 *m
switch(len) {
case 3: h ^= data[2] << 16;
case 2: h ^= data[1] << 8;
case 1: h ^= data[0]; h *= m;
};
/* Do a few final mixes of the hash to ensure the last few
* bytes are well-incorporated. */
// step3. 再混合 ^>>13, *m, ^>>15
h ^= h >> 13;
h *= m;
h ^= h >> 15;
return (unsigned int)h;
}可以看到，针对普通的字符串的hash可是要复杂许多呢，因为这里数据远比 command 的数据多，情况更复杂，这样的算法唯一的目标就是尽量避免hash冲突。(虽然不知道为啥这么干，但它就是牛逼)
redis中还有其他的hash算法，比如dictObjHash,dictEncObjHash, 后续有接触我们再聊。
接下来，我们正式来看看 set/get 到底如何？
一、getCommand 解析
很显然，get 会是个最简单的命令，自然要检软柿子捏了。
// t_string.c
void getCommand(client *c) {
getGenericCommand(c);
}
int getGenericCommand(client *c) {
robj *o;
// 如果在kv里找不到，则直接响应空，shared.nullbulk 作为全局常量的优势就体现出来了
// shared.nullbulk = createObject(OBJ_STRING,sdsnew("$-1\r\n"));
if ((o = lookupKeyReadOrReply(c,c->argv[1],shared.nullbulk)) == NULL)
return C_OK;
// 找到对应的数据，但是类型不匹配，说明不能使用 get 命令，响应错误信息
// shared.wrongtypeerr = "-WRONGTYPE Operation against a key holding the wrong kind of value\r\n"
if (o->type != OBJ_STRING) {
addReply(c,shared.wrongtypeerr);
return C_ERR;
} else {
// 正常情况则直接响应结果即可
addReplyBulk(c,o);
return C_OK;
}
}整个处理流程果然是异常简单，感觉人生已经达到了巅峰！但是，我们还没有看到关键，那就是查找 key 的过程。我们通过之前的介绍，知道有个叫做 redisDb 的东西，看起来它是负责所有的数据管理。它应该不会因为简单而不存储某些数据吧。
// db.c, 查找某个key对应的元素或者直接响应客户端
robj *lookupKeyReadOrReply(client *c, robj *key, robj *reply) {
// 使用 c->db 对应的数据库进行查询，所以要求客户端必须针对某db进行操作，且不能跨库操作是原理决定
robj *o = lookupKeyRead(c->db, key);
// 如果没有查到数据就直接使用默认的 reply, 响应客户端了
if (!o) addReply(c,reply);
return o;
}
// db.c, 读取key 对应值
robj *lookupKeyRead(redisDb *db, robj *key) {
robj *val;
// 检查过期情况，如果过期，则不用再查了
if (expireIfNeeded(db,key) == 1) {
/* Key expired. If we are in the context of a master, expireIfNeeded()
* returns 0 only when the key does not exist at all, so it's save
* to return NULL ASAP. */
if (server.masterhost == NULL) return NULL;
/* However if we are in the context of a slave, expireIfNeeded() will
* not really try to expire the key, it only returns information
* about the "logical" status of the key: key expiring is up to the
* master in order to have a consistent view of master's data set.
*
* However, if the command caller is not the master, and as additional
* safety measure, the command invoked is a read-only command, we can
* safely return NULL here, and provide a more consistent behavior
* to clients accessign expired values in a read-only fashion, that
* will say the key as non exisitng.
*
* Notably this covers GETs when slaves are used to scale reads. */
if (server.current_client &&
server.current_client != server.master &&
server.current_client->cmd &&
server.current_client->cmd->flags & CMD_READONLY)
{
return NULL;
}
}
// 然后从db中查找对应的key值，其实就是一个 hash 查找
// 缓存命中统计
val = lookupKey(db,key);
if (val == NULL)
server.stat_keyspace_misses++;
else
server.stat_keyspace_hits++;
return val;
}
// db.c, 过期的处理有点复杂，我们稍后再看，先看 db 的查找key过程
robj *lookupKey(redisDb *db, robj *key) {
// 直接在 db->dict 中进行hash查找即可，前面已经介绍完成，关键优化点在增量rehash
dictEntry *de = dictFind(db->dict,key->ptr);
if (de) {
robj *val = dictGetVal(de);
/* Update the access time for the ageing algorithm.
* Don't do it if we have a saving child, as this will trigger
* a copy on write madness. */
if (server.rdb_child_pid == -1 && server.aof_child_pid == -1)
val->lru = LRU_CLOCK();
return val;
} else {
return NULL;
}
}
// db.c, 接下来看下，检查过期情况
int expireIfNeeded(redisDb *db, robj *key) {
mstime_t when = getExpire(db,key);
mstime_t now;
if (when < 0) return 0; /* No expire for this key */
/* Don't expire anything while loading. It will be done later. */
if (server.loading) return 0;
/* If we are in the context of a Lua script, we claim that time is
* blocked to when the Lua script started. This way a key can expire
* only the first time it is accessed and not in the middle of the
* script execution, making propagation to slaves / AOF consistent.
* See issue #1525 on Github for more information. */
now = server.lua_caller ? server.lua_time_start : mstime();
/* If we are running in the context of a slave, return ASAP:
* the slave key expiration is controlled by the master that will
* send us synthesized DEL operations for expired keys.
*
* Still we try to return the right information to the caller,
* that is, 0 if we think the key should be still valid, 1 if
* we think the key is expired at this time. */
if (server.masterhost != NULL) return now > when;
/* Return when this key has not expired */
// 如果还没到期就直接返回
if (now <= when) return 0;
/* Delete the key */
server.stat_expiredkeys++;
// key过期，是一个写动作，需要传播到 AOF 或者 slaves...
propagateExpire(db,key,server.lazyfree_lazy_expire);
// pub/sub 监控通知
notifyKeyspaceEvent(NOTIFY_EXPIRED,
"expired",key,db->id);
// 同步删除或者异步删除, 稍后讨论
return server.lazyfree_lazy_expire ? dbAsyncDelete(db,key) :
dbSyncDelete(db,key);
}
/* Return the expire time of the specified key, or -1 if no expire
* is associated with this key (i.e. the key is non volatile) */
long long getExpire(redisDb *db, robj *key) {
dictEntry *de;
/* No expire? return ASAP */
// 查找过期队列, 数据量小
if (dictSize(db->expires) == 0 ||
(de = dictFind(db->expires,key->ptr)) == NULL) return -1;
/* The entry was found in the expire dict, this means it should also
* be present in the main dict (safety check). */
serverAssertWithInfo(NULL,key,dictFind(db->dict,key->ptr) != NULL);
// 返回到期时间戳, union 的应用
return dictGetSignedIntegerVal(de);
}
// 删除过期数据key的两种方式，同步+异步
// db.c, 同步删除, 删除 expires 队列和 dict 数据
/* Delete a key, value, and associated expiration entry if any, from the DB */
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);
if (dictDelete(db->dict,key->ptr) == DICT_OK) {
if (server.cluster_enabled) slotToKeyDel(key);
return 1;
} else {
return 0;
}
}
// lazyfree.c, 异步删除过期数据, 一看就很复杂
/* Delete a key, value, and associated expiration entry if any, from the DB.
* If there are enough allocations to free the value object may be put into
* a lazy free list instead of being freed synchronously. The lazy free list
* will be reclaimed in a different bio.c thread. */
#define LAZYFREE_THRESHOLD 64
int dbAsyncDelete(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);
/* If the value is composed of a few allocations, to free in a lazy way
* is actually just slower... So under a certain limit we just free
* the object synchronously. */
dictEntry *de = dictFind(db->dict,key->ptr);
if (de) {
robj *val = dictGetVal(de);
// 判断删除的数据的影响范围，与 数据类型有关，string为1，hash/set则计算count，list计算length
size_t free_effort = lazyfreeGetFreeEffort(val);
/* If releasing the object is too much work, let's put it into the
* lazy free list. */
if (free_effort > LAZYFREE_THRESHOLD) {
// 将相关的数据放入队列中，后台任务慢慢删除
atomicIncr(lazyfree_objects,1,&lazyfree_objects_mutex);
bioCreateBackgroundJob(BIO_LAZY_FREE,val,NULL,NULL);
// 自身则立即设置为 NULL
dictSetVal(db->dict,de,NULL);
}
}
/* Release the key-val pair, or just the key if we set the val
* field to NULL in order to lazy free it later. */
if (dictDelete(db->dict,key->ptr) == DICT_OK) {
if (server.cluster_enabled) slotToKeyDel(key);
return 1;
} else {
return 0;
}
}怎么样？是不是有一首歌叫凉凉~
可以说，get操作本身是相当简单的，在无hash冲突前提下，O(1)的复杂度搞定。然而它还要处理过期的数据问题，就不那么简单了。
我们用一个时序图整体体会下get的流程:
二、setCommand
setCommand 是个写操作，就不是 get 那么简单了。
// t_string.c, set 的所有用法都统一 setCommand, 多个参数共同解析为 flags
/* SET key value [NX] [XX] [EX <seconds>] [PX <milliseconds>] */
void setCommand(client *c) {
int j;
robj *expire = NULL;
int unit = UNIT_SECONDS;
int flags = OBJ_SET_NO_FLAGS;
for (j = 3; j < c->argc; j++) {
char *a = c->argv[j]->ptr;
robj *next = (j == c->argc-1) ? NULL : c->argv[j+1];
// NX 与 XX 互斥
if ((a[0] == 'n' || a[0] == 'N') &&
(a[1] == 'x' || a[1] == 'X') && a[2] == '\0' &&
!(flags & OBJ_SET_XX))
{
flags |= OBJ_SET_NX;
} else if ((a[0] == 'x' || a[0] == 'X') &&
(a[1] == 'x' || a[1] == 'X') && a[2] == '\0' &&
!(flags & OBJ_SET_NX))
{
flags |= OBJ_SET_XX;
}
// PX 与 EX 互斥
else if ((a[0] == 'e' || a[0] == 'E') &&
(a[1] == 'x' || a[1] == 'X') && a[2] == '\0' &&
!(flags & OBJ_SET_PX) && next)
{
flags |= OBJ_SET_EX;
unit = UNIT_SECONDS;
expire = next;
j++;
} else if ((a[0] == 'p' || a[0] == 'P') &&
(a[1] == 'x' || a[1] == 'X') && a[2] == '\0' &&
!(flags & OBJ_SET_EX) && next)
{
flags |= OBJ_SET_PX;
unit = UNIT_MILLISECONDS;
expire = next;
j++;
} else {
addReply(c,shared.syntaxerr);
return;
}
}
// 尝试压缩 value 值以节省空间 (原始命令: set key value)
c->argv[2] = tryObjectEncoding(c->argv[2]);
setGenericCommand(c,flags,c->argv[1],c->argv[2],expire,unit,NULL,NULL);
}
// object.c, 压缩字符串
/* Try to encode a string object in order to save space */
robj *tryObjectEncoding(robj *o) {
long value;
sds s = o->ptr;
size_t len;
/* Make sure this is a string object, the only type we encode
* in this function. Other types use encoded memory efficient
* representations but are handled by the commands implementing
* the type. */
serverAssertWithInfo(NULL,o,o->type == OBJ_STRING);
/* We try some specialized encoding only for objects that are
* RAW or EMBSTR encoded, in other words objects that are still
* in represented by an actually array of chars. */
if (!sdsEncodedObject(o)) return o;
/* It's not safe to encode shared objects: shared objects can be shared
* everywhere in the "object space" of Redis and may end in places where
* they are not handled. We handle them only as values in the keyspace. */
if (o->refcount > 1) return o;
/* Check if we can represent this string as a long integer.
* Note that we are sure that a string larger than 21 chars is not
* representable as a 32 nor 64 bit integer. */
len = sdslen(s);
// 针对小于21个字符串的字符，尝试转为 long 型
if (len <= 21 && string2l(s,len,&value)) {
/* This object is encodable as a long. Try to use a shared object.
* Note that we avoid using shared integers when maxmemory is used
* because every object needs to have a private LRU field for the LRU
* algorithm to work well. */
if ((server.maxmemory == 0 ||
(server.maxmemory_policy != MAXMEMORY_VOLATILE_LRU &&
server.maxmemory_policy != MAXMEMORY_ALLKEYS_LRU)) &&
value >= 0 &&
value < OBJ_SHARED_INTEGERS)
{
decrRefCount(o);
incrRefCount(shared.integers[value]);
return shared.integers[value];
} else {
if (o->encoding == OBJ_ENCODING_RAW) sdsfree(o->ptr);
o->encoding = OBJ_ENCODING_INT;
o->ptr = (void*) value;
return o;
}
}
/* If the string is small and is still RAW encoded,
* try the EMBSTR encoding which is more efficient.
* In this representation the object and the SDS string are allocated
* in the same chunk of memory to save space and cache misses. */
// 44
if (len <= OBJ_ENCODING_EMBSTR_SIZE_LIMIT) {
robj *emb;
if (o->encoding == OBJ_ENCODING_EMBSTR) return o;
// 使用 EMBSTR 编码转换，实际就是同一个 s 返回
emb = createEmbeddedStringObject(s,sdslen(s));
decrRefCount(o);
return emb;
}
/* We can't encode the object...
*
* Do the last try, and at least optimize the SDS string inside
* the string object to require little space, in case there
* is more than 10% of free space at the end of the SDS string.
*
* We do that only for relatively large strings as this branch
* is only entered if the length of the string is greater than
* OBJ_ENCODING_EMBSTR_SIZE_LIMIT. */
if (o->encoding == OBJ_ENCODING_RAW &&
sdsavail(s) > len/10)
{
o->ptr = sdsRemoveFreeSpace(o->ptr);
}
/* Return the original object. */
return o;
}
// sds.c, 去除无用空间占用
/* Reallocate the sds string so that it has no free space at the end. The
* contained string remains not altered, but next concatenation operations
* will require a reallocation.
*
* After the call, the passed sds string is no longer valid and all the
* references must be substituted with the new pointer returned by the call. */
sds sdsRemoveFreeSpace(sds s) {
void *sh, *newsh;
// s[-1] 指针不越界, 它是在新建一个 sds 对象时，在该指针前一位写入的值，确定sds类型
char type, oldtype = s[-1] & SDS_TYPE_MASK;
int hdrlen;
size_t len = sdslen(s);
// sdsHdrSize: sds头部大小
sh = (char*)s-sdsHdrSize(oldtype);
type = sdsReqType(len);
hdrlen = sdsHdrSize(type);
if (oldtype==type) {
newsh = s_realloc(sh, hdrlen+len+1);
if (newsh == NULL) return NULL;
s = (char*)newsh+hdrlen;
} else {
newsh = s_malloc(hdrlen+len+1);
if (newsh == NULL) return NULL;
memcpy((char*)newsh+hdrlen, s, len+1);
s_free(sh);
s = (char*)newsh+hdrlen;
s[-1] = type;
sdssetlen(s, len);
}
sdssetalloc(s, len);
return s;
}
// t_string.c, expire 为超时时间设置
void setGenericCommand(client *c, int flags, robj *key, robj *val, robj *expire, int unit, robj *ok_reply, robj *abort_reply) {
long long milliseconds = 0; /* initialized to avoid any harmness warning */
if (expire) {
// 解析 expire 到 milliseconds 中
if (getLongLongFromObjectOrReply(c, expire, &milliseconds, NULL) != C_OK)
return;
if (milliseconds <= 0) {
addReplyErrorFormat(c,"invalid expire time in %s",c->cmd->name);
return;
}
if (unit == UNIT_SECONDS) milliseconds *= 1000;
}
// 语法限制检测, NX 要求不存在, XX 要求存在
if ((flags & OBJ_SET_NX && lookupKeyWrite(c->db,key) != NULL) ||
(flags & OBJ_SET_XX && lookupKeyWrite(c->db,key) == NULL))
{
addReply(c, abort_reply ? abort_reply : shared.nullbulk);
return;
}
// 切实存储 kv
setKey(c->db,key,val);
server.dirty++;
// 设置超时
if (expire) setExpire(c->db,key,mstime()+milliseconds);
// 通知pub/sub变更
notifyKeyspaceEvent(NOTIFY_STRING,"set",key,c->db->id);
// 通知expire事件
if (expire) notifyKeyspaceEvent(NOTIFY_GENERIC,
"expire",key,c->db->id);
addReply(c, ok_reply ? ok_reply : shared.ok);
}
// object.c,
int getLongLongFromObjectOrReply(client *c, robj *o, long long *target, const char *msg) {
long long value;
if (getLongLongFromObject(o, &value) != C_OK) {
if (msg != NULL) {
addReplyError(c,(char*)msg);
} else {
addReplyError(c,"value is not an integer or out of range");
}
return C_ERR;
}
*target = value;
return C_OK;
}
// object.c,
int getLongLongFromObject(robj *o, long long *target) {
long long value;
if (o == NULL) {
value = 0;
} else {
serverAssertWithInfo(NULL,o,o->type == OBJ_STRING);
// 将字符串转换为 long 型，得到超时时间
if (sdsEncodedObject(o)) {
if (strict_strtoll(o->ptr,&value) == C_ERR) return C_ERR;
} else if (o->encoding == OBJ_ENCODING_INT) {
value = (long)o->ptr;
} else {
serverPanic("Unknown string encoding");
}
}
if (target) *target = value;
return C_OK;
}看完了超时及各标识位的解析，及set框架流程，我们来看下具体核心的kv存储: setKey(), setExpire();
// db.c, set kv
/* High level Set operation. This function can be used in order to set
* a key, whatever it was existing or not, to a new object.
*
* 1) The ref count of the value object is incremented.
* 2) clients WATCHing for the destination key notified.
* 3) The expire time of the key is reset (the key is made persistent). */
void setKey(redisDb *db, robj *key, robj *val) {
// 先查找，再更新
if (lookupKeyWrite(db,key) == NULL) {
// 新增 kv
dbAdd(db,key,val);
} else {
// 覆盖 kv
dbOverwrite(db,key,val);
}
// 增加 value 的引用计数
incrRefCount(val);
// 新增的元素，移出过期队列
removeExpire(db,key);
signalModifiedKey(db,key);
}
robj *lookupKeyWrite(redisDb *db, robj *key) {
// 先尝试过期处理，再查找db (hash 查找 db->dic)
expireIfNeeded(db,key);
return lookupKey(db,key);
}
// db.c, 添加kv
/* Add the key to the DB. It's up to the caller to increment the reference
* counter of the value if needed.
*
* The program is aborted if the key already exists. */
void dbAdd(redisDb *db, robj *key, robj *val) {
sds copy = sdsdup(key->ptr);
// 添加到 db->dict 中
int retval = dictAdd(db->dict, copy, val);
serverAssertWithInfo(NULL,key,retval == C_OK);
// list 类型的数据，进行特殊处理（阻塞）
if (val->type == OBJ_LIST) signalListAsReady(db, key);
// 集群添加
if (server.cluster_enabled) slotToKeyAdd(key);
}
// db.c
/* Slot to Key API. This is used by Redis Cluster in order to obtain in
* a fast way a key that belongs to a specified hash slot. This is useful
* while rehashing the cluster. */
void slotToKeyAdd(robj *key) {
// hash 定位 slot, 下面我们简单看下该算法
unsigned int hashslot = keyHashSlot(key->ptr,sdslen(key->ptr));
sds sdskey = sdsdup(key->ptr);
// 添加key 到 server.cluster->slots_to_keys 的 跳表中
zslInsert(server.cluster->slots_to_keys,hashslot,sdskey);
}
// cluster.c, slot 定位算法, 其实就是 crc16算法与上 0x3FFF，该算法决定了 slot 最多只能有 16383 个
/* We have 16384 hash slots. The hash slot of a given key is obtained
* as the least significant 14 bits of the crc16 of the key.
*
* However if the key contains the {...} pattern, only the part between
* { and } is hashed. This may be useful in the future to force certain
* keys to be in the same node (assuming no resharding is in progress). */
unsigned int keyHashSlot(char *key, int keylen) {
int s, e; /* start-end indexes of { and } */
for (s = 0; s < keylen; s++)
if (key[s] == '{') break;
/* No '{' ? Hash the whole key. This is the base case. */
if (s == keylen) return crc16(key,keylen) & 0x3FFF;
/* '{' found? Check if we have the corresponding '}'. */
for (e = s+1; e < keylen; e++)
if (key[e] == '}') break;
/* No '}' or nothing betweeen {} ? Hash the whole key. */
if (e == keylen || e == s+1) return crc16(key,keylen) & 0x3FFF;
/* If we are here there is both a { and a } on its right. Hash
* what is in the middle between { and }. */
return crc16(key+s+1,e-s-1) & 0x3FFF;
}
// db.c, 设置key的超时标识
void setExpire(redisDb *db, robj *key, long long when) {
dictEntry *kde, *de;
/* Reuse the sds from the main dict in the expire dict */
kde = dictFind(db->dict,key->ptr);
serverAssertWithInfo(NULL,key,kde != NULL);
// 将需要超时检测的 key 添加到 db->expires 队列中
de = dictReplaceRaw(db->expires,dictGetKey(kde));
// 设置超时时间为 when
dictSetSignedIntegerVal(de,when);
}
// dict.h
#define dictSetSignedIntegerVal(entry, _val_) \
do { entry->v.s64 = _val_; } while(0)总体来说，set操作会分为几步:
- 判断出多重参数，如是否是NX/EX/PX/XX, 是否超时设置;
- 编码转换数据, 如将字符串转换为long型;
- 解析超时字段;
- set kv, 添加或者覆盖数据库值, 同时清理过期队列;
- 设置超时时间;
- 触发事件监听;
- 响应客户端;
最后，我们以set的整个时序图作为结尾，也让我们明白一点，不是每个hello world 都很简单：
