Redis系列（九）：資料結構Hash原始碼解析和HSET、HGET命令
阿新 • • 發佈：2020-07-01
2.原始碼解析
1.相關命令如下：
{"hset",hsetCommand,,"wmF",,NULL,,,,,},
{"hsetnx",hsetnxCommand,,"wmF",,NULL,,,,,},
{"hget",hgetCommand,,"rF",,NULL,,,,,},
{"hmset",hmsetCommand,-,"wm",,NULL,,,,,},
{"hmget",hmgetCommand,-,"r",,NULL,,,,,},
{"hincrby",hincrbyCommand,,"wmF",,NULL,,,,,},
{"hincrbyfloat",hincrbyfloatCommand,,"wmF",,NULL,,,,,},
{"hlen",hlenCommand,,"rF",,NULL,,,,,},
{"hstrlen",hstrlenCommand,,"rF",,NULL,,,,,},
{"hkeys",hkeysCommand,,"rS",,NULL,,,,,},
{"hvals",hvalsCommand,,"rS",,NULL,,,,,},
{"hgetall",hgetallCommand,,"r",,NULL,,,,,},
{"hexists",hexistsCommand,,"rF",,NULL,,,,,},
2.ziplist資料結構
/* We use this function to receive information about a ziplist entry.
* Note that this is not how the data is actually encoded, is just what we
* get filled by a function in order to operate more easily. */
typedef struct zlentry {
unsigned int prevrawlensize; /* Bytes used to encode the previous entry len*/unsigned int prevrawlen; /* Previous entry len. */
unsigned int lensize; /* Bytes used to encode this entry type/len.
For example strings have a 1, 2 or 5 bytes
header. Integers always use a single byte.*/
unsigned int len; /* Bytes used to represent the actual entry.
For strings this is just the string length
while for integers it is 1, 2, 3, 4, 8 or
0 (for 4 bit immediate) depending on the
number range. */
unsigned int headersize; /* prevrawlensize + lensize. */
unsigned char encoding; /* Set to ZIP_STR_* or ZIP_INT_* depending on
the entry encoding. However for 4 bits
immediate integers this can assume a range
of values and must be range-checked. */
unsigned char *p; /* Pointer to the very start of the entry, that
is, this points to prev-entry-len field. */
} zlentry;
3.hashtable資料結構
typedef struct dictEntry {
void *key;
union {
void *val;
uint64_t u64;
int64_t s64;
double d;
} v;
struct dictEntry *next;
} dictEntry;
typedef struct dictType {
uint64_t (*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;
/* This is our hash table structure. Every dictionary has two of this as we
* implement incremental rehashing, for the old to the new table. */
typedef struct dictht {
dictEntry **table;
unsigned long size;
unsigned long sizemask;
unsigned long used;
} dictht;
typedef struct dict {
dictType *type;
void *privdata;
dictht ht[];
long rehashidx; /* rehashing not in progress if rehashidx == -1 */
unsigned long iterators; /* number of iterators currently running */
} dict;
hset
// t_hash.c, set key field value
void hsetCommand(client *c) {
int update;
robj *o;
// 1. 查詢hash的key是否存在，不存在則新建一個，然後在其上進行資料操作
if ((o = hashTypeLookupWriteOrCreate(c,c->argv[])) == NULL) return;
// 2. 檢查2-3個引數是否需要將簡單版（ziplist）hash錶轉換為複雜的hash表，轉換後的表通過 o->ptr 體現
hashTypeTryConversion(o,c->argv,,);
// 3. 新增kv到 o 的hash表中
update = hashTypeSet(o,c->argv[]->ptr,c->argv[]->ptr,HASH_SET_COPY);
addReply(c, update ? shared.czero : shared.cone);
// 變更命令傳播
signalModifiedKey(c->db,c->argv[]);
notifyKeyspaceEvent(NOTIFY_HASH,"hset",c->argv[],c->db->id);
server.dirty++;
} // 1. 獲取db外部的key, 即整體hash資料例項
// t_hash.c
robj *hashTypeLookupWriteOrCreate(client *c, robj *key) {
robj *o = lookupKeyWrite(c->db,key);
if (o == NULL) {
// 此處建立的hashObject是以 ziplist 形式的
o = createHashObject();
dbAdd(c->db,key,o);
} else {
// 不是hash型別的鍵已存在，不可覆蓋，返回錯誤
if (o->type != OBJ_HASH) {
addReply(c,shared.wrongtypeerr);
return NULL;
}
}
return o;
}
// object.c, 建立hashObject, 以 ziplist 形式建立
robj *createHashObject(void) {
unsigned char *zl = ziplistNew();
robj *o = createObject(OBJ_HASH, zl);
o->encoding = OBJ_ENCODING_ZIPLIST;
return o;
}
// ziplist.c
static unsigned char *createList() {
unsigned char *zl = ziplistNew();
zl = ziplistPush(zl, (unsigned char*)"foo", , ZIPLIST_TAIL);
zl = ziplistPush(zl, (unsigned char*)"quux", , ZIPLIST_TAIL);
zl = ziplistPush(zl, (unsigned char*)"hello", , ZIPLIST_HEAD);
zl = ziplistPush(zl, (unsigned char*)"", , ZIPLIST_TAIL);
return zl;
} // 2. 檢查引數，是否需要將 ziplist 形式的hash錶轉換為真正的hash表
/* Check the length of a number of objects to see if we need to convert a
* ziplist to a real hash. Note that we only check string encoded objects
* as their string length can be queried in constant time. */
void hashTypeTryConversion(robj *o, robj **argv, int start, int end) {
int i; if (o->encoding != OBJ_ENCODING_ZIPLIST) return; for (i = start; i <= end; i++) {
// 引數大於設定的 hash_max_ziplist_value (預設: 64)時，會直接將 ziplist 轉換為 ht
// OBJ_ENCODING_RAW, OBJ_ENCODING_EMBSTR
// 迴圈檢查引數，只要發生了一次轉換就結束檢查（沒必要繼續了）
if (sdsEncodedObject(argv[i]) &&
sdslen(argv[i]->ptr) > server.hash_max_ziplist_value)
{
// 這個轉換過程很有意思，我們深入看看
hashTypeConvert(o, OBJ_ENCODING_HT);
break;
}
}
}
// t_hash.c, 轉換編碼方式 (如上, ziplist -> ht)
void hashTypeConvert(robj *o, int enc) {
if (o->encoding == OBJ_ENCODING_ZIPLIST) {
// 此處我們只處理這種情況
hashTypeConvertZiplist(o, enc);
} else if (o->encoding == OBJ_ENCODING_HT) {
serverPanic("Not implemented");
} else {
serverPanic("Unknown hash encoding");
}
}
// t_hash.c, 轉換編碼 ziplist 為目標 enc (實際只能是 OBJ_ENCODING_HT)
void hashTypeConvertZiplist(robj *o, int enc) {
serverAssert(o->encoding == OBJ_ENCODING_ZIPLIST); if (enc == OBJ_ENCODING_ZIPLIST) {
/* Nothing to do... */ } else if (enc == OBJ_ENCODING_HT) {
hashTypeIterator *hi;
dict *dict;
int ret;
// 迭代器建立
hi = hashTypeInitIterator(o);
// 一個hash的資料結構就是一個 dict, 從這個級別來說, hash 與 db 是一個級別的
dict = dictCreate(&hashDictType, NULL);
// 依次迭代 o, 賦值到 hi->fptr, hi->vptr
// 依次新增到 dict 中
while (hashTypeNext(hi) != C_ERR) {
sds key, value;
// 從 hi->fptr 中獲取key
// 從 hi->vptr 中獲取value
key = hashTypeCurrentObjectNewSds(hi,OBJ_HASH_KEY);
value = hashTypeCurrentObjectNewSds(hi,OBJ_HASH_VALUE);
// 新增到 dict 中
ret = dictAdd(dict, key, value);
if (ret != DICT_OK) {
serverLogHexDump(LL_WARNING,"ziplist with dup elements dump",
o->ptr,ziplistBlobLen(o->ptr));
serverPanic("Ziplist corruption detected");
}
}
// 釋放迭代器
hashTypeReleaseIterator(hi);
zfree(o->ptr);
// 將變更反映到o物件上返回
o->encoding = OBJ_ENCODING_HT;
o->ptr = dict;
} else {
serverPanic("Unknown hash encoding");
}
}
// 2.1. 迭代ziplist元素
// t_hash.c, 迭代器
/* Move to the next entry in the hash. Return C_OK when the next entry
* could be found and C_ERR when the iterator reaches the end. */
int hashTypeNext(hashTypeIterator *hi) {
if (hi->encoding == OBJ_ENCODING_ZIPLIST) {
unsigned char *zl;
unsigned char *fptr, *vptr;
// 每次都是基於原始字元器進行計算偏移
// 迭代的是 fptr,vptr
zl = hi->subject->ptr;
fptr = hi->fptr;
vptr = hi->vptr;
// 第一次查詢時使用index查詢，後續則使用 fptr,vptr 進行迭代
if (fptr == NULL) {
/* Initialize cursor */
serverAssert(vptr == NULL);
fptr = ziplistIndex(zl, );
} else {
/* Advance cursor */
serverAssert(vptr != NULL);
fptr = ziplistNext(zl, vptr);
}
if (fptr == NULL) return C_ERR; /* Grab pointer to the value (fptr points to the field) */
vptr = ziplistNext(zl, fptr);
serverAssert(vptr != NULL); /* fptr, vptr now point to the first or next pair */
hi->fptr = fptr;
hi->vptr = vptr;
} else if (hi->encoding == OBJ_ENCODING_HT) {
if ((hi->de = dictNext(hi->di)) == NULL) return C_ERR;
} else {
serverPanic("Unknown hash encoding");
}
return C_OK;
}
// ziplist.c, 查詢 index 的元素
/* Returns an offset to use for iterating with ziplistNext. When the given
* index is negative, the list is traversed back to front. When the list
* doesn't contain an element at the provided index, NULL is returned. */
unsigned char *ziplistIndex(unsigned char *zl, int index) {
unsigned char *p;
unsigned int prevlensize, prevlen = ;
if (index < ) {
// 小於0時，反向查詢
index = (-index)-;
p = ZIPLIST_ENTRY_TAIL(zl);
if (p[] != ZIP_END) {
ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
while (prevlen > && index--) {
p -= prevlen;
ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
}
}
} else {
p = ZIPLIST_ENTRY_HEAD(zl);
while (p[] != ZIP_END && index--) {
p += zipRawEntryLength(p);
}
}
// 迭代完成還沒找到元素 p[0]=ZIP_END
// index 超出整體ziplist大小則遍歷完成後 index>0
return (p[] == ZIP_END || index > ) ? NULL : p;
}
// ziplist.c, 由 fptr,vptr 進行迭代元素
/* Return pointer to next entry in ziplist.
*
* zl is the pointer to the ziplist
* p is the pointer to the current element
*
* The element after 'p' is returned, otherwise NULL if we are at the end. */
unsigned char *ziplistNext(unsigned char *zl, unsigned char *p) {
((void) zl); /* "p" could be equal to ZIP_END, caused by ziplistDelete,
* and we should return NULL. Otherwise, we should return NULL
* when the *next* element is ZIP_END (there is no next entry). */
if (p[] == ZIP_END) {
return NULL;
}
// 當前指標偏移當前元素長度（根據ziplist協議），即到下一元素指標位置
p += zipRawEntryLength(p);
if (p[] == ZIP_END) {
return NULL;
} return p;
}
/* Return the total number of bytes used by the entry pointed to by 'p'. */
static unsigned int zipRawEntryLength(unsigned char *p) {
unsigned int prevlensize, encoding, lensize, len;
ZIP_DECODE_PREVLENSIZE(p, prevlensize);
ZIP_DECODE_LENGTH(p + prevlensize, encoding, lensize, len);
return prevlensize + lensize + len;
} // 2.2. t_hash.c, 獲取 hashTypeIterator 的具體值，寫入 vstr, vlen 中
/* Return the key or value at the current iterator position as a new
* SDS string. */
sds hashTypeCurrentObjectNewSds(hashTypeIterator *hi, int what) {
unsigned char *vstr;
unsigned int vlen;
long long vll; hashTypeCurrentObject(hi,what,&vstr,&vlen,&vll);
if (vstr) return sdsnewlen(vstr,vlen);
return sdsfromlonglong(vll);
}
/* Higher level function of hashTypeCurrent*() that returns the hash value
* at current iterator position.
*
* The returned element is returned by reference in either *vstr and *vlen if
* it's returned in string form, or stored in *vll if it's returned as
* a number.
*
* If *vll is populated *vstr is set to NULL, so the caller
* can always check the function return by checking the return value
* type checking if vstr == NULL. */
void hashTypeCurrentObject(hashTypeIterator *hi, int what, unsigned char **vstr, unsigned int *vlen, long long *vll) {
if (hi->encoding == OBJ_ENCODING_ZIPLIST) {
*vstr = NULL;
hashTypeCurrentFromZiplist(hi, what, vstr, vlen, vll);
} else if (hi->encoding == OBJ_ENCODING_HT) {
sds ele = hashTypeCurrentFromHashTable(hi, what);
*vstr = (unsigned char*) ele;
*vlen = sdslen(ele);
} else {
serverPanic("Unknown hash encoding");
}
} // t_hash.c, 從ziplist中獲取某個 hashTypeIterator 的具體值，結果定稿 vstr, vlen
/* Get the field or value at iterator cursor, for an iterator on a hash value
* encoded as a ziplist. Prototype is similar to `hashTypeGetFromZiplist`. */
void hashTypeCurrentFromZiplist(hashTypeIterator *hi, int what,
unsigned char **vstr,
unsigned int *vlen,
long long *vll)
{
int ret; serverAssert(hi->encoding == OBJ_ENCODING_ZIPLIST);
// OBJ_HASH_KEY 從 fptr 中獲取, 否則從 vptr 中獲取
if (what & OBJ_HASH_KEY) {
ret = ziplistGet(hi->fptr, vstr, vlen, vll);
serverAssert(ret);
} else {
ret = ziplistGet(hi->vptr, vstr, vlen, vll);
serverAssert(ret);
}
}
// ziplist.c,
/* Get entry pointed to by 'p' and store in either '*sstr' or 'sval' depending
* on the encoding of the entry. '*sstr' is always set to NULL to be able
* to find out whether the string pointer or the integer value was set.
* Return 0 if 'p' points to the end of the ziplist, 1 otherwise. */
unsigned int ziplistGet(unsigned char *p, unsigned char **sstr, unsigned int *slen, long long *sval) {
zlentry entry;
if (p == NULL || p[] == ZIP_END) return ;
if (sstr) *sstr = NULL;
// 按照ziplist的編碼協議, 獲取頭部資訊
zipEntry(p, &entry);
if (ZIP_IS_STR(entry.encoding)) {
if (sstr) {
*slen = entry.len;
*sstr = p+entry.headersize;
}
} else {
if (sval) {
*sval = zipLoadInteger(p+entry.headersize,entry.encoding);
}
}
return ;
}
// ziplist.c, 解析原始字串為 zlentry
/* Return a struct with all information about an entry. */
static void zipEntry(unsigned char *p, zlentry *e) {
// 按照ziplist的編碼協議,依次讀取 prevrawlensize, prevrawlen
ZIP_DECODE_PREVLEN(p, e->prevrawlensize, e->prevrawlen);
// 指向下一位置偏移，按照ziplist的編碼協議,依次讀取 encoding, lensize, len
ZIP_DECODE_LENGTH(p + e->prevrawlensize, e->encoding, e->lensize, e->len);
// 除去header得到 body偏移
e->headersize = e->prevrawlensize + e->lensize;
e->p = p;
}
header
// ziplist.c
/* Decode the length of the previous element, from the perspective of the entry
* pointed to by 'ptr'. */
#define ZIP_DECODE_PREVLEN(ptr, prevlensize, prevlen) do { \
// 解析第1個字元為 prevlensize
ZIP_DECODE_PREVLENSIZE(ptr, prevlensize); \
if ((prevlensize) == ) { \
(prevlen) = (ptr)[]; \
} else if ((prevlensize) == ) { \
assert(sizeof((prevlensize)) == ); \
// 當ptr[0]>254時，代表內容有點大，需要使用 5個字元儲存上一字元長度
memcpy(&(prevlen), ((char*)(ptr)) + , ); \
memrev32ifbe(&prevlen); \
} \
} while();
/* Decode the number of bytes required to store the length of the previous
* element, from the perspective of the entry pointed to by 'ptr'. */
#define ZIP_DECODE_PREVLENSIZE(ptr, prevlensize) do { \
if ((ptr)[] < ZIP_BIGLEN) { \
(prevlensize) = ; \
} else { \
(prevlensize) = ; \
} \
} while();
/* Decode the length encoded in 'ptr'. The 'encoding' variable will hold the
* entries encoding, the 'lensize' variable will hold the number of bytes
* required to encode the entries length, and the 'len' variable will hold the
* entries length. */
#define ZIP_DECODE_LENGTH(ptr, encoding, lensize, len) do { \
// 解析第1個字元為 編碼格式 &ZIP_STR_MASK=0xc0
ZIP_ENTRY_ENCODING((ptr), (encoding)); \
if ((encoding) < ZIP_STR_MASK) { \
// 0 << 6 =0
// 具體解析如下程式碼,
if ((encoding) == ZIP_STR_06B) { \
(lensize) = ; \
(len) = (ptr)[] & 0x3f; \
}
// 1 << 6 =64
else if ((encoding) == ZIP_STR_14B) { \
(lensize) = ; \
(len) = (((ptr)[] & 0x3f) << ) | (ptr)[]; \
}
// 2 << 6 =128
else if (encoding == ZIP_STR_32B) { \
(lensize) = ; \
(len) = ((ptr)[] << ) | \
((ptr)[] << ) | \
((ptr)[] << ) | \
((ptr)[]); \
} else { \
assert(NULL); \
} \
} else { \
// 超過 0xc0 的長度了，直接使用 1,2,3,4 表示len
(lensize) = ; \
(len) = zipIntSize(encoding); \
} \
} while();
/* Extract the encoding from the byte pointed by 'ptr' and set it into
* 'encoding'. */
#define ZIP_ENTRY_ENCODING(ptr, encoding) do { \
(encoding) = (ptr[]); \
if ((encoding) < ZIP_STR_MASK) (encoding) &= ZIP_STR_MASK; \
} while() /* Different encoding/length possibilities */
#define ZIP_STR_MASK 0xc0
#define ZIP_INT_MASK 0x30
#define ZIP_STR_06B (0 << 6) // 0x00
#define ZIP_STR_14B (1 << 6) // 0x40
#define ZIP_STR_32B (2 << 6) // 0x80
#define ZIP_INT_16B (0xc0 | 0<<4) // 0xc0
#define ZIP_INT_32B (0xc0 | 1<<4) // 0xd0
#define ZIP_INT_64B (0xc0 | 2<<4) // 0xe0
#define ZIP_INT_24B (0xc0 | 3<<4) // 0xf0
#define ZIP_INT_8B 0xfe // 0xfe
新增kv到對應的key例項中：
// 3. 新增kv到 hash表中, 稍微複雜
// t_hash.c, 做變更到hash表中
int hashTypeSet(robj *o, sds field, sds value, int flags) {
int update = ;
// 針對ziplist 的新增, 與 ht 編碼的新增, 自然是分別處理
if (o->encoding == OBJ_ENCODING_ZIPLIST) {
unsigned char *zl, *fptr, *vptr; zl = o->ptr;
// 找到ziplist 的頭節點指標
fptr = ziplistIndex(zl, ZIPLIST_HEAD);
if (fptr != NULL) {
// 嘗試查詢該 field 對應的元素(從1開始)，如果找到則先刪除原值，然後統一新增
fptr = ziplistFind(fptr, (unsigned char*)field, sdslen(field), );
if (fptr != NULL) {
/* Grab pointer to the value (fptr points to the field) */
// value 不可以為null, 否則 ziplist 將無法工作
vptr = ziplistNext(zl, fptr);
serverAssert(vptr != NULL);
update = ; /* Delete value */
// 先刪除舊的 value, 再以插入的形式更新, 後續講刪除時再詳解
zl = ziplistDelete(zl, &vptr); /* Insert new value */
// 重點，將value新增到 ziplist 中
zl = ziplistInsert(zl, vptr, (unsigned char*)value,
sdslen(value));
}
}
// 沒有找到對應元素，則直接將元素新增到尾部即可
if (!update) {
/* Push new field/value pair onto the tail of the ziplist */
zl = ziplistPush(zl, (unsigned char*)field, sdslen(field),
ZIPLIST_TAIL);
zl = ziplistPush(zl, (unsigned char*)value, sdslen(value),
ZIPLIST_TAIL);
}
o->ptr = zl; /* Check if the ziplist needs to be converted to a hash table */
// 大於設定的閥值後，轉換ziplist為ht(預設: 512)
if (hashTypeLength(o) > server.hash_max_ziplist_entries)
hashTypeConvert(o, OBJ_ENCODING_HT);
} else if (o->encoding == OBJ_ENCODING_HT) {
dictEntry *de = dictFind(o->ptr,field);
if (de) {
sdsfree(dictGetVal(de));
if (flags & HASH_SET_TAKE_VALUE) {
dictGetVal(de) = value;
value = NULL;
} else {
dictGetVal(de) = sdsdup(value);
}
update = ;
} else {
sds f,v;
if (flags & HASH_SET_TAKE_FIELD) {
f = field;
field = NULL;
} else {
f = sdsdup(field);
}
if (flags & HASH_SET_TAKE_VALUE) {
v = value;
value = NULL;
} else {
v = sdsdup(value);
}
dictAdd(o->ptr,f,v);
}
} else {
serverPanic("Unknown hash encoding");
} /* Free SDS strings we did not referenced elsewhere if the flags
* want this function to be responsible. */
if (flags & HASH_SET_TAKE_FIELD && field) sdsfree(field);
if (flags & HASH_SET_TAKE_VALUE && value) sdsfree(value);
return update;
}
// 3.1. 使用ziplist進行儲存 field -> value
// ziplist.c, 查詢某個 field 是否存在於ziplist中
/* Find pointer to the entry equal to the specified entry. Skip 'skip' entries
* between every comparison. Returns NULL when the field could not be found. */
unsigned char *ziplistFind(unsigned char *p, unsigned char *vstr, unsigned int vlen, unsigned int skip) {
int skipcnt = ;
unsigned char vencoding = ;
long long vll = ; while (p[] != ZIP_END) {
unsigned int prevlensize, encoding, lensize, len;
unsigned char *q;
// 解析整個字串p的 prevlensize,encoding,lensize,len
ZIP_DECODE_PREVLENSIZE(p, prevlensize);
ZIP_DECODE_LENGTH(p + prevlensize, encoding, lensize, len);
q = p + prevlensize + lensize;
// 傳入1, 代表要跳過一個元素, 比如: 查詢key時,跳過1個v,然後繼續迭代
// 跳過了n個元素後，再從此開始key的比對過程
if (skipcnt == ) {
/* Compare current entry with specified entry */
// 針對不同的編碼使用不同的比較方式
if (ZIP_IS_STR(encoding)) {
// 找到相應的元素，直接返回 p 指標
if (len == vlen && memcmp(q, vstr, vlen) == ) {
return p;
}
} else {
/* Find out if the searched field can be encoded. Note that
* we do it only the first time, once done vencoding is set
* to non-zero and vll is set to the integer value. */
if (vencoding == ) {
if (!zipTryEncoding(vstr, vlen, &vll, &vencoding)) {
/* If the entry can't be encoded we set it to
* UCHAR_MAX so that we don't retry again the next
* time. */
vencoding = UCHAR_MAX;
}
/* Must be non-zero by now */
assert(vencoding);
} /* Compare current entry with specified entry, do it only
* if vencoding != UCHAR_MAX because if there is no encoding
* possible for the field it can't be a valid integer. */
if (vencoding != UCHAR_MAX) {
long long ll = zipLoadInteger(q, encoding);
if (ll == vll) {
return p;
}
}
} /* Reset skip count */
// 查詢一次，跳過skip次
skipcnt = skip;
} else {
/* Skip entry */
skipcnt--;
} /* Move to next entry */
p = q + len;
} return NULL;
}
// ziplist.c, 新增value到ziplist中
// zl:ziplist例項, p:要插入的key字串, s:要插入的value字串, len:要插入的value的長度
/* Insert an entry at "p". */
unsigned char *ziplistInsert(unsigned char *zl, unsigned char *p, unsigned char *s, unsigned int slen) {
return __ziplistInsert(zl,p,s,slen);
}
/* Insert item at "p". */
static unsigned char *__ziplistInsert(unsigned char *zl, unsigned char *p, unsigned char *s, unsigned int slen) {
size_t curlen = intrev32ifbe(ZIPLIST_BYTES(zl)), reqlen;
unsigned int prevlensize, prevlen = ;
size_t offset;
int nextdiff = ;
unsigned char encoding = ;
long long value = ; /* initialized to avoid warning. Using a value
that is easy to see if for some reason
we use it uninitialized. */
zlentry tail; /* Find out prevlen for the entry that is inserted. */
if (p[] != ZIP_END) {
ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
} else {
unsigned char *ptail = ZIPLIST_ENTRY_TAIL(zl);
if (ptail[] != ZIP_END) {
prevlen = zipRawEntryLength(ptail);
}
} /* See if the entry can be encoded */
if (zipTryEncoding(s,slen,&value,&encoding)) {
/* 'encoding' is set to the appropriate integer encoding */
reqlen = zipIntSize(encoding);
} else {
/* 'encoding' is untouched, however zipEncodeLength will use the
* string length to figure out how to encode it. */
reqlen = slen;
}
/* We need space for both the length of the previous entry and
* the length of the payload. */
// 加上prevlen,encoding,slen 的長度，以計算value的存放位置
reqlen += zipPrevEncodeLength(NULL,prevlen);
reqlen += zipEncodeLength(NULL,encoding,slen); /* When the insert position is not equal to the tail, we need to
* make sure that the next entry can hold this entry's length in
* its prevlen field. */
nextdiff = (p[] != ZIP_END) ? zipPrevLenByteDiff(p,reqlen) : ; /* Store offset because a realloc may change the address of zl. */
// 儲存當前偏移位置，以便在擴容之後，還能找到相應位置
// p = p -zl + zl
offset = p-zl;
zl = ziplistResize(zl,curlen+reqlen+nextdiff);
p = zl+offset; /* Apply memory move when necessary and update tail offset. */
if (p[] != ZIP_END) {
/* Subtract one because of the ZIP_END bytes */
// 字元拷貝
memmove(p+reqlen,p-nextdiff,curlen-offset-+nextdiff); /* Encode this entry's raw length in the next entry. */
zipPrevEncodeLength(p+reqlen,reqlen); /* Update offset for tail */
ZIPLIST_TAIL_OFFSET(zl) =
intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))+reqlen); /* When the tail contains more than one entry, we need to take
* "nextdiff" in account as well. Otherwise, a change in the
* size of prevlen doesn't have an effect on the *tail* offset. */
zipEntry(p+reqlen, &tail);
if (p[reqlen+tail.headersize+tail.len] != ZIP_END) {
ZIPLIST_TAIL_OFFSET(zl) =
intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))+nextdiff);
}
} else {
/* This element will be the new tail. */
ZIPLIST_TAIL_OFFSET(zl) = intrev32ifbe(p-zl);
} /* When nextdiff != 0, the raw length of the next entry has changed, so
* we need to cascade the update throughout the ziplist */
if (nextdiff != ) {
// 如果本次更新後資料位置變化，則需要更新後續的元素位置
offset = p-zl;
zl = __ziplistCascadeUpdate(zl,p+reqlen);
p = zl+offset;
} /* Write the entry */
// 將 value 寫入 p 中, 即寫入了 ziplist 中
p += zipPrevEncodeLength(p,prevlen);
p += zipEncodeLength(p,encoding,slen);
if (ZIP_IS_STR(encoding)) {
memcpy(p,s,slen);
} else {
zipSaveInteger(p,value,encoding);
}
ZIPLIST_INCR_LENGTH(zl,);
return zl;
}
// 另外，如果沒有舊的元素值時，直接在hash表的末尾新增對應的field->value 即可
// ziplist.c, 在尾部進行新增元素，沒有許多的情況要考慮，但是程式碼完全複用 __ziplistInsert()
unsigned char *ziplistPush(unsigned char *zl, unsigned char *s, unsigned int slen, int where) {
unsigned char *p;
p = (where == ZIPLIST_HEAD) ? ZIPLIST_ENTRY_HEAD(zl) : ZIPLIST_ENTRY_END(zl);
return __ziplistInsert(zl,p,s,slen);
}
深入理解ziplist
看起來沒ziplist好像沒那麼簡單呢，為啥還要搞這麼複雜呢？其實以上程式碼，僅是在人看來複雜，對機器來說就是更多的移位計算操作，多消耗點cpu就換來了空間上的節省，是可以的。軟體本身的複雜性帶來了效益，是軟體的價值體現，所以，並非所有的東西都是簡單即美。
接下來，我們來看一下使用 HT 的編碼又如何儲存field->value呢？
// 3.2. OBJ_ENCODING_HT 的 field -> value 的新增
if (o->encoding == OBJ_ENCODING_HT) {
// hash 表中查詢對應的 field
dictEntry *de = dictFind(o->ptr,field);
if (de) {
sdsfree(dictGetVal(de));
// hset 時使用 HASH_SET_COPY, 所以直接使用 sdsdup() 即可
if (flags & HASH_SET_TAKE_VALUE) {
dictGetVal(de) = value;
value = NULL;
} else {
dictGetVal(de) = sdsdup(value);
}
update = ;
} else {
// 新增 field -> value
sds f,v;
if (flags & HASH_SET_TAKE_FIELD) {
f = field;
field = NULL;
} else {
f = sdsdup(field);
}
if (flags & HASH_SET_TAKE_VALUE) {
v = value;
value = NULL;
} else {
v = sdsdup(value);
}
// 新增到 hash 表中，前些篇章講解過，大概就是計算hash，放入v的過程
dictAdd(o->ptr,f,v);
}
}
如此看來，OBJ_ENCODING_HT 的實現反而簡單了哦。
總結下 hash的插入過程，hash 初始建立時都是使用ziplist 進行容納元素的，在特定情況下會觸發 ziplist 為 ht 的編碼方式, 比如:
1. hset時自身的引數大於設定值(預設: 64)時直接轉換 ziplist -> ht;
2. hash表的元素數量大於設定值(預設: 512)時轉換 ziplist -> ht;
這麼設計的原因是，元素較少且佔用空間較小時，使用ziplist會節省空間，且時間消耗與hash表相關並不大，所以 ziplist 是優先的選擇了。但是大量資料還是必須要使用hash表儲存的。