Redis 数据结构-简单动态字符串
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Redis 之所以快主要得益于它的数据结构、操作内存数据库、单线程和多路 I/O 复用模型,进一步窥探下它常见的五种基本数据的底层数据结构。
Redis 常见数据类型对应的的底层数据结构。
String是Redis 最基本的类型,最大能存储 512MB 的数据,String类型是二进制安全的,它可以存储任何数据包括数字、图片、序列化对象等。虽然Redis 是C 语言写的,但Redis 中并没有使用 C 中 char 来表示字符串,而是自定义了一种新的字符串结构 简单动态字符串(Simple Dynamic Strings,SDS)来存储字符串和整型数据。
C 语言字符串有以下几个问题:
例如执行以下命令
set name "tjt"
set命令执行后,Redis将在底层创建两个SDS,一个是包含name 的SDS,另一个是包含“tjt”的SDS。
从Redis 源码的sds.h 文件中可以看到SDS 的结构体。
从sds.h 源码中截取出sdshdr8 如下。
1 struct __attribute__ ((__packed__)) sdshdr8 {
2 uint8_t len; /* used */
3 uint8_t alloc; /* excluding the header and null terminator */
4 unsigned char flags; /* 3 lsb of type, 5 unused bits */
5 char buf[];
6 };
sdshdr5
、sdshdr8
、sdshdr16
、sdshdr32
、sdshdr64
。如下图对应的几种SDS_TYPE。例如,一个包含字符串“tjt"的SDS 结构如下:
动态字符串SDS 具备动态扩容的能力,例如给SDS 'tjt' 追加一段字符串 ",go”,这里首先会申请新内存空间。
1、SDS可以通过常数级别获取字符串的长度
redis的结构中存储了字符串的长度,所以获取字符串的长度复杂度为O(1),c 中字符串没记录长度,需要遍历整个长度,复杂度为O(N)。
2、杜绝缓冲区溢出
3、减少修改字符串时的内存分配次数
4、二进制安全
5、Int、Raw和 embstr 动态存储
简单动态字符串结构在数据存储过程中,字符串对象会根据保存值的类型、长度不同,动态匹配三种存储结构:Int、Raw和 embstr 。
sds.h 文件完整源码
1 /* SDSLib 2.0 -- A C dynamic strings library
2 *
3 * Copyright (c) 2006-2015, Salvatore Sanfilippo
4 * Copyright (c) 2015, Oran Agra
5 * Copyright (c) 2015, Redis Labs, Inc
6 * All rights reserved.
7 *
8 * Redistribution and use in source and binary forms, with or without
9 * modification, are permitted provided that the following conditions are met:
10 *
11 * * Redistributions of source code must retain the above copyright notice,
12 * this list of conditions and the following disclaimer.
13 * * Redistributions in binary form must reproduce the above copyright
14 * notice, this list of conditions and the following disclaimer in the
15 * documentation and/or other materials provided with the distribution.
16 * * Neither the name of Redis nor the names of its contributors may be used
17 * to endorse or promote products derived from this software without
18 * specific prior written permission.
19 *
20 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
21 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
22 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
23 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
24 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
25 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
26 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
27 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
28 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
29 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
30 * POSSIBILITY OF SUCH DAMAGE.
31 */
32
33 #ifndef __SDS_H
34 #define __SDS_H
35
36 #define SDS_MAX_PREALLOC (1024*1024)
37 const char *SDS_NOINIT;
38
39 #include
40 #include
41 #include
42
43 typedef char *sds;
44
45 /* Note: sdshdr5 is never used, we just access the flags byte directly.
46 * However is here to document the layout of type 5 SDS strings. */
47 struct __attribute__ ((__packed__)) sdshdr5 {
48 unsigned char flags; /* 3 lsb of type, and 5 msb of string length */
49 char buf[];
50 };
51 struct __attribute__ ((__packed__)) sdshdr8 {
52 uint8_t len; /* used */
53 uint8_t alloc; /* excluding the header and null terminator */
54 unsigned char flags; /* 3 lsb of type, 5 unused bits */
55 char buf[];
56 };
57 struct __attribute__ ((__packed__)) sdshdr16 {
58 uint16_t len; /* used */
59 uint16_t alloc; /* excluding the header and null terminator */
60 unsigned char flags; /* 3 lsb of type, 5 unused bits */
61 char buf[];
62 };
63 struct __attribute__ ((__packed__)) sdshdr32 {
64 uint32_t len; /* used */
65 uint32_t alloc; /* excluding the header and null terminator */
66 unsigned char flags; /* 3 lsb of type, 5 unused bits */
67 char buf[];
68 };
69 struct __attribute__ ((__packed__)) sdshdr64 {
70 uint64_t len; /* used */
71 uint64_t alloc; /* excluding the header and null terminator */
72 unsigned char flags; /* 3 lsb of type, 5 unused bits */
73 char buf[];
74 };
75
76 #define SDS_TYPE_5 0
77 #define SDS_TYPE_8 1
78 #define SDS_TYPE_16 2
79 #define SDS_TYPE_32 3
80 #define SDS_TYPE_64 4
81 #define SDS_TYPE_MASK 7
82 #define SDS_TYPE_BITS 3
83 #define SDS_HDR_VAR(T,s) struct sdshdr##T *sh = (void*)((s)-(sizeof(struct sdshdr##T)));
84 #define SDS_HDR(T,s) ((struct sdshdr##T *)((s)-(sizeof(struct sdshdr##T))))
85 #define SDS_TYPE_5_LEN(f) ((f)>>SDS_TYPE_BITS)
86
87 static inline size_t sdslen(const sds s) {
88 unsigned char flags = s[-1];
89 switch(flags&SDS_TYPE_MASK) {
90 case SDS_TYPE_5:
91 return SDS_TYPE_5_LEN(flags);
92 case SDS_TYPE_8:
93 return SDS_HDR(8,s)->len;
94 case SDS_TYPE_16:
95 return SDS_HDR(16,s)->len;
96 case SDS_TYPE_32:
97 return SDS_HDR(32,s)->len;
98 case SDS_TYPE_64:
99 return SDS_HDR(64,s)->len;
100 }
101 return 0;
102 }
103
104 static inline size_t sdsavail(const sds s) {
105 unsigned char flags = s[-1];
106 switch(flags&SDS_TYPE_MASK) {
107 case SDS_TYPE_5: {
108 return 0;
109 }
110 case SDS_TYPE_8: {
111 SDS_HDR_VAR(8,s);
112 return sh->alloc - sh->len;
113 }
114 case SDS_TYPE_16: {
115 SDS_HDR_VAR(16,s);
116 return sh->alloc - sh->len;
117 }
118 case SDS_TYPE_32: {
119 SDS_HDR_VAR(32,s);
120 return sh->alloc - sh->len;
121 }
122 case SDS_TYPE_64: {
123 SDS_HDR_VAR(64,s);
124 return sh->alloc - sh->len;
125 }
126 }
127 return 0;
128 }
129
130 static inline void sdssetlen(sds s, size_t newlen) {
131 unsigned char flags = s[-1];
132 switch(flags&SDS_TYPE_MASK) {
133 case SDS_TYPE_5:
134 {
135 unsigned char *fp = ((unsigned char*)s)-1;
136 *fp = SDS_TYPE_5 | (newlen << SDS_TYPE_BITS);
137 }
138 break;
139 case SDS_TYPE_8:
140 SDS_HDR(8,s)->len = newlen;
141 break;
142 case SDS_TYPE_16:
143 SDS_HDR(16,s)->len = newlen;
144 break;
145 case SDS_TYPE_32:
146 SDS_HDR(32,s)->len = newlen;
147 break;
148 case SDS_TYPE_64:
149 SDS_HDR(64,s)->len = newlen;
150 break;
151 }
152 }
153
154 static inline void sdsinclen(sds s, size_t inc) {
155 unsigned char flags = s[-1];
156 switch(flags&SDS_TYPE_MASK) {
157 case SDS_TYPE_5:
158 {
159 unsigned char *fp = ((unsigned char*)s)-1;
160 unsigned char newlen = SDS_TYPE_5_LEN(flags)+inc;
161 *fp = SDS_TYPE_5 | (newlen << SDS_TYPE_BITS);
162 }
163 break;
164 case SDS_TYPE_8:
165 SDS_HDR(8,s)->len += inc;
166 break;
167 case SDS_TYPE_16:
168 SDS_HDR(16,s)->len += inc;
169 break;
170 case SDS_TYPE_32:
171 SDS_HDR(32,s)->len += inc;
172 break;
173 case SDS_TYPE_64:
174 SDS_HDR(64,s)->len += inc;
175 break;
176 }
177 }
178
179 /* sdsalloc() = sdsavail() + sdslen() */
180 static inline size_t sdsalloc(const sds s) {
181 unsigned char flags = s[-1];
182 switch(flags&SDS_TYPE_MASK) {
183 case SDS_TYPE_5:
184 return SDS_TYPE_5_LEN(flags);
185 case SDS_TYPE_8:
186 return SDS_HDR(8,s)->alloc;
187 case SDS_TYPE_16:
188 return SDS_HDR(16,s)->alloc;
189 case SDS_TYPE_32:
190 return SDS_HDR(32,s)->alloc;
191 case SDS_TYPE_64:
192 return SDS_HDR(64,s)->alloc;
193 }
194 return 0;
195 }
196
197 static inline void sdssetalloc(sds s, size_t newlen) {
198 unsigned char flags = s[-1];
199 switch(flags&SDS_TYPE_MASK) {
200 case SDS_TYPE_5:
201 /* Nothing to do, this type has no total allocation info. */
202 break;
203 case SDS_TYPE_8:
204 SDS_HDR(8,s)->alloc = newlen;
205 break;
206 case SDS_TYPE_16:
207 SDS_HDR(16,s)->alloc = newlen;
208 break;
209 case SDS_TYPE_32:
210 SDS_HDR(32,s)->alloc = newlen;
211 break;
212 case SDS_TYPE_64:
213 SDS_HDR(64,s)->alloc = newlen;
214 break;
215 }
216 }
217
218 sds sdsnewlen(const void *init, size_t initlen);
219 sds sdsnew(const char *init);
220 sds sdsempty(void);
221 sds sdsdup(const sds s);
222 void sdsfree(sds s);
223 sds sdsgrowzero(sds s, size_t len);
224 sds sdscatlen(sds s, const void *t, size_t len);
225 sds sdscat(sds s, const char *t);
226 sds sdscatsds(sds s, const sds t);
227 sds sdscpylen(sds s, const char *t, size_t len);
228 sds sdscpy(sds s, const char *t);
229
230 sds sdscatvprintf(sds s, const char *fmt, va_list ap);
231 #ifdef __GNUC__
232 sds sdscatprintf(sds s, const char *fmt, …)
233 __attribute__((format(printf, 2, 3)));
234 #else
235 sds sdscatprintf(sds s, const char *fmt, …);
236 #endif
237
238 sds sdscatfmt(sds s, char const *fmt, …);
239 sds sdstrim(sds s, const char *cset);
240 void sdsrange(sds s, ssize_t start, ssize_t end);
241 void sdsupdatelen(sds s);
242 void sdsclear(sds s);
243 int sdscmp(const sds s1, const sds s2);
244 sds *sdssplitlen(const char *s, ssize_t len, const char *sep, int seplen, int *count);
245 void sdsfreesplitres(sds *tokens, int count);
246 void sdstolower(sds s);
247 void sdstoupper(sds s);
248 sds sdsfromlonglong(long long value);
249 sds sdscatrepr(sds s, const char *p, size_t len);
250 sds *sdssplitargs(const char *line, int *argc);
251 sds sdsmapchars(sds s, const char *from, const char *to, size_t setlen);
252 sds sdsjoin(char **argv, int argc, char *sep);
253 sds sdsjoinsds(sds *argv, int argc, const char *sep, size_t seplen);
254
255 /* Low level functions exposed to the user API */
256 sds sdsMakeRoomFor(sds s, size_t addlen);
257 void sdsIncrLen(sds s, ssize_t incr);
258 sds sdsRemoveFreeSpace(sds s);
259 size_t sdsAllocSize(sds s);
260 void *sdsAllocPtr(sds s);
261
262 /* Export the allocator used by SDS to the program using SDS.
263 * Sometimes the program SDS is linked to, may use a different set of
264 * allocators, but may want to allocate or free things that SDS will
265 * respectively free or allocate. */
266 void *sds_malloc(size_t size);
267 void *sds_realloc(void *ptr, size_t size);
268 void sds_free(void *ptr);
269
270 #ifdef REDIS_TEST
271 int sdsTest(int argc, char *argv[]);
272 #endif
273
274 #endif
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