数据库键空间

Redis 是一个键值对(key-value pair)数据库服务器, 服务器中的每个数据库都由一个 redis.h/redisDb 结构表示, 其中, redisDb 结构的 dict 字典保存了数据库中的所有键值对, 我们将这个字典称为键空间(key space):

typedef struct redisDb {

    // ...

    // 数据库键空间,保存着数据库中的所有键值对
    dict *dict;

    // ...

} redisDb;

键空间和用户所见的数据库是直接对应的:

  • 键空间的键也就是数据库的键, 每个键都是一个字符串对象。
  • 键空间的值也就是数据库的值, 每个值可以是字符串对象、列表对象、哈希表对象、集合对象和有序集合对象在内的任意一种 Redis 对象。

举个例子, 如果我们在空白的数据库中执行以下命令:

redis> SET message "hello world"
OK

redis> RPUSH alphabet "a" "b" "c"
(integer) 3

redis> HSET book name "Redis in Action"
(integer) 1

redis> HSET book author "Josiah L. Carlson"
(integer) 1

redis> HSET book publisher "Manning"
(integer) 1

那么在这些命令执行之后, 数据库的键空间将会是图 IMAGE_DB_EXAMPLE 所展示的样子:

  • alphabet 是一个列表键, 键的名字是一个包含字符串 "alphabet" 的字符串对象, 键的值则是一个包含三个元素的列表对象。
  • book 是一个哈希表键, 键的名字是一个包含字符串 "book" 的字符串对象, 键的值则是一个包含三个键值对的哈希表对象。
  • message 是一个字符串键, 键的名字是一个包含字符串 "message" 的字符串对象, 键的值则是一个包含字符串 "hello world" 的字符串对象。

digraph {

    label = "\n图 IMAGE_DB_EXAMPLE    数据库键空间例子";

    rankdir = LR;

    node [shape = record];

    //

    redisDb [label = "redisDb | ... | <dict> dict | ..."];

    dict [label = "<dict> dict | <alphabet> StringObject \n \"alphabet\" | <book> StringObject \n \"book\" | <message> StringObject \n \"message\""];

    subgraph cluster_alphabet {

        a [label = " StringObject \n \"a\" "];
        b [label = " StringObject \n \"b\" "];
        c [label = " StringObject \n \"c\" "];

        a -> b -> c;

        label = "ListObject";

    }

    //alphabet [label = "<head> ListObject | { StringObject \n \"a\" | \"b\" | \"c\" }"];

    book [label = "<head> HashObject | <name> StringObject \n \"name\" | <author> StringObject \n \"author\" | <publisher> StringObject \n \"publisher\""];

    //name [label = " StringObject \n \"Redis in Action\""];
    name [label = " StringObject \n \"Redis in Action\""];

    author [label = " StringObject \n \"Josiah L. Carlson\""];

    publisher [label = " StringObject \n \"Manning\""];

    message [label = " StringObject \n \"hello world\""];

    //

    redisDb:dict -> dict:dict;

    dict:alphabet -> a;
    dict:book -> book:head;
    dict:message -> message;

    book:name -> name;
    book:publisher -> publisher;
    book:author -> author;

}

因为数据库的键空间是一个字典, 所以所有针对数据库的操作 —— 比如添加一个键值对到数据库, 或者从数据库中删除一个键值对, 又或者在数据库中获取某个键值对, 等等, 实际上都是通过对键空间字典进行操作来实现的, 以下几个小节将分别介绍数据库的添加、删除、更新、取值等操作的实现原理。

添加新键

添加一个新键值对到数据库, 实际上就是将一个新键值对添加到键空间字典里面, 其中键为字符串对象, 而值则为任意一种类型的 Redis 对象。

举个例子, 如果键空间当前的状态如图 IMAGE_DB_EXAMPLE 所示, 那么在执行以下命令之后:

redis> SET date "2013.12.1"
OK

键空间将添加一个新的键值对, 这个新键值对的键是一个包含字符串 "date" 的字符串对象, 而键值对的值则是一个包含字符串 "2013.12.1" 的字符串对象, 如图 IMAGE_DB_AFTER_ADD_NEW_KEY 所示。

digraph {

    label = "\n图 IMAGE_DB_AFTER_ADD_NEW_KEY    添加 date 键之后的键空间";

    rankdir = LR;

    node [shape = record];

    //

    redisDb [label = "redisDb | ... | <dict> dict | ..."];

    dict [label = "<dict> dict | <alphabet> StringObject \n \"alphabet\" | <book> StringObject \n \"book\" | <message> StringObject \n \"message\" | <date> StringObject \n \"date\""];

    subgraph cluster_alphabet {

        a [label = " StringObject \n \"a\" "];
        b [label = " StringObject \n \"b\" "];
        c [label = " StringObject \n \"c\" "];

        a -> b -> c;

        label = "ListObject";

    }

    book [label = "<head> HashObject | <name> StringObject \n \"name\" | <author> StringObject \n \"author\" | <publisher> StringObject \n \"publisher\""];

    name [label = " StringObject \n \"Redis in Action\""];

    author [label = " StringObject \n \"Josiah L. Carlson\""];

    publisher [label = " StringObject \n \"Manning\""];

    message [label = " StringObject \n \"hello world\""];

    date [label = " StringObject \n \"2013.12.1\""];

    //

    redisDb:dict -> dict:dict;

    dict:alphabet -> a;
    dict:book -> book:head;
    dict:message -> message;

    book:name -> name;
    book:publisher -> publisher;
    book:author -> author;

    dict:date -> date;

    //

    node [shape = plaintext]

    newadd [label = "新添加"]

    newadd -> dict:date [style = dashed]

}

删除键

删除数据库中的一个键, 实际上就是在键空间里面删除键所对应的键值对对象。

举个例子, 如果键空间当前的状态如图 IMAGE_DB_EXAMPLE 所示, 那么在执行以下命令之后:

redis> DEL book
(integer) 1

book 以及它的值将从键空间中被删除, 如图 IMAGE_DB_AFTER_DEL 所示。

digraph {

    label = "\n图 IMAGE_DB_AFTER_DEL    删除 book 键之后的键空间";

    rankdir = LR;

    node [shape = record];

    //

    redisDb [label = "redisDb | ... | <dict> dict | ..."];

    dict [label = "<dict> dict | <alphabet> StringObject \n \"alphabet\" |  <message> StringObject \n \"message\""];

    subgraph cluster_alphabet {

        a [label = " StringObject \n \"a\" "];
        b [label = " StringObject \n \"b\" "];
        c [label = " StringObject \n \"c\" "];

        a -> b -> c;

        label = "ListObject";

    }

    message [label = " StringObject \n \"hello world\""];

    //

    redisDb:dict -> dict:dict;

    dict:alphabet -> a;
    dict:message -> message;

}

更新键

对一个数据库键进行更新, 实际上就是对键空间里面键所对应的值对象进行更新, 根据值对象的类型不同, 更新的具体方法也会有所不同。

举个例子, 如果键空间当前的状态如图 IMAGE_DB_EXAMPLE 所示, 那么在执行以下命令之后:

redis> SET message "blah blah"
OK

message 的值对象将从之前包含 "hello world" 字符串更新为包含 "blah blah" 字符串, 如图 IMAGE_DB_UPDATE_CAUSE_SET 所示。

digraph {

    label = "\n图 IMAGE_DB_UPDATE_CAUSE_SET    使用 SET 命令更新 message 键";

    rankdir = LR;

    node [shape = record];

    //

    redisDb [label = "redisDb | ... | <dict> dict | ..."];

    dict [label = "<dict> dict | <alphabet> StringObject \n \"alphabet\" | <book> StringObject \n \"book\" | <message> StringObject \n \"message\""];

    subgraph cluster_alphabet {

        a [label = " StringObject \n \"a\" "];
        b [label = " StringObject \n \"b\" "];
        c [label = " StringObject \n \"c\" "];

        a -> b -> c;

        label = "ListObject";

    }

    book [label = "<head> HashObject | <name> StringObject \n \"name\" | <author> StringObject \n \"author\" | <publisher> StringObject \n \"publisher\""];

    name [label = " StringObject \n \"Redis in Action\""];

    author [label = " StringObject \n \"Josiah L. Carlson\""];

    publisher [label = " StringObject \n \"Manning\""];

    message [label = " StringObject \n \"blah blah\""];

    //

    redisDb:dict -> dict:dict;

    dict:alphabet -> a;
    dict:book -> book:head;
    dict:message -> message;

    book:name -> name;
    book:publisher -> publisher;
    book:author -> author;

    //

    node [shape = plaintext]

    update [label = "更新值对象"]

    update -> message [style = dashed]

}

再举个例子, 如果我们继续执行以下命令:

redis> HSET book page 320
(integer) 1

那么键空间中 book 键的值对象(一个哈希对象)将被更新, 新的键值对 page320 会被添加到值对象里面, 如图 IMAGE_UPDATE_BY_HSET 所示。

digraph {

    label = "\n图 IMAGE_UPDATE_BY_HSET    使用 HSET 更新 book 键";

    rankdir = LR;

    node [shape = record];

    //

    redisDb [label = "redisDb | ... | <dict> dict | ..."];

    dict [label = "<dict> dict | <alphabet> StringObject \n \"alphabet\" | <book> StringObject \n \"book\" | <message> StringObject \n \"message\" "];

    subgraph cluster_alphabet {

        a [label = " StringObject \n \"a\" "];
        b [label = " StringObject \n \"b\" "];
        c [label = " StringObject \n \"c\" "];

        a -> b -> c;

        label = "ListObject";

    }

    book [label = "<head> HashObject | <name> StringObject \n \"name\" | <author> StringObject \n \"author\" | <publisher> StringObject \n \"publisher\" | <page> StringObject \n \"page\" "];

    name [label = " StringObject \n \"Redis in Action\""];

    author [label = " StringObject \n \"Josiah L. Carlson\""];

    publisher [label = " StringObject \n \"Manning\""];

    page [label = " StringObject \n 320"];

    message [label = " StringObject \n \"blah blah\""];

    //

    redisDb:dict -> dict:dict;

    dict:alphabet -> a;
    dict:book -> book:head;
    dict:message -> message;

    book:name -> name;
    book:publisher -> publisher;
    book:author -> author;
    book:page -> page;

    //

    node [shape = plaintext]

    update [label = "新添加"]

    update -> book:page [style = dashed]

}

对键取值

对一个数据库键进行取值, 实际上就是在键空间中取出键所对应的值对象, 根据值对象的类型不同, 具体的取值方法也会有所不同。

举个例子, 如果键空间当前的状态如图 IMAGE_DB_EXAMPLE 所示, 那么当执行以下命令时:

redis> GET message
"hello world"

GET 命令将首先在键空间中查找键 message , 找到键之后接着取得该键所对应的字符串对象值, 之后再返回值对象所包含的字符串 "hello world" , 取值过程如图 IMAGE_FETCH_VALUE_VIA_GET 所示。

digraph {

    label = "\n图 IMAGE_FETCH_VALUE_VIA_GET    使用 GET 命令取值的过程";

    rankdir = LR;

    node [shape = record];

    //

    redisDb [label = "redisDb | ... | <dict> dict | ..."];

    dict [label = "<dict> dict | <alphabet> StringObject \n \"alphabet\" | <book> StringObject \n \"book\" | <message> StringObject \n \"message\""];

    subgraph cluster_alphabet {

        a [label = " StringObject \n \"a\" "];
        b [label = " StringObject \n \"b\" "];
        c [label = " StringObject \n \"c\" "];

        a -> b -> c;

        label = "ListObject";

    }

    book [label = "<head> HashObject | <name> StringObject \n \"name\" | <author> StringObject \n \"author\" | <publisher> StringObject \n \"publisher\""];

    name [label = " StringObject \n \"Redis in Action\""];

    author [label = " StringObject \n \"Josiah L. Carlson\""];

    publisher [label = " StringObject \n \"Manning\""];

    message [label = " StringObject \n \"hello world\""];

    get [label = "GET", shape = plaintext];

    //

    redisDb:dict -> dict:dict;

    dict:alphabet -> a;
    dict:book -> book:head;
    dict:message -> message:head [label = "2)取值", style = dashed];

    book:name -> name;
    book:publisher -> publisher;
    book:author -> author;

    get -> dict:message [label = "1)查找键", style = dashed];

}

再举一个例子, 当执行以下命令时:

redis> LRANGE alphabet 0 -1
1) "a"
2) "b"
3) "c"

LRANGE 命令将首先在键空间中查找键 alphabet , 找到键之后接着取得该键所对应的列表对象值, 之后再返回列表对象中包含的三个字符串对象的值, 取值过程如图 IMAGE_FETCH_VALUE_VIA_LRANGE 所示。

digraph {

    label = "\n图 IMAGE_FETCH_VALUE_VIA_LRANGE    使用 LRANGE 命令取值的过程";

    rankdir = LR;

    node [shape = record];

    //

    redisDb [label = "redisDb | ... | <dict> dict | ..."];

    dict [label = "<dict> dict | <alphabet> StringObject \n \"alphabet\" | <book> StringObject \n \"book\" | <message> StringObject \n \"message\""];

    subgraph cluster_alphabet {

        a [label = " StringObject \n \"a\" "];
        b [label = " StringObject \n \"b\" "];
        c [label = " StringObject \n \"c\" "];

        a -> b -> c [style = dashed];

        label = "ListObject";

    }

    book [label = "<head> HashObject | <name> StringObject \n \"name\" | <author> StringObject \n \"author\" | <publisher> StringObject \n \"publisher\""];

    name [label = " StringObject \n \"Redis in Action\""];

    author [label = " StringObject \n \"Josiah L. Carlson\""];

    publisher [label = " StringObject \n \"Manning\""];

    message [label = " StringObject \n \"hello world\""];

    lrange [label = "LRANGE", shape = plaintext];

    //

    redisDb:dict -> dict:dict;

    dict:alphabet -> a [label = "2)取值", style = dashed];
    dict:book -> book:head;
    dict:message -> message:head;

    book:name -> name:head;
    book:publisher -> publisher:head;
    book:author -> author:head;

    lrange -> dict:alphabet [label = "1)查找键", style = dashed];

}

其他键空间操作

除了上面列出的添加、删除、更新、取值操作之外, 还有很多针对数据库本身的 Redis 命令, 也是通过对键空间进行处理来完成的。

比如说, 用于清空整个数据库的 FLUSHDB 命令, 就是通过删除键空间中的所有键值对来实现的。

又比如说, 用于随机返回数据库中某个键的 RANDOMKEY 命令, 就是通过在键空间中随机返回一个键来实现的。

另外, 用于返回数据库键数量的 DBSIZE 命令, 就是通过返回键空间中包含键值对的数量来实现的。

类似的命令还有 EXISTSRENAMEKEYS , 等等, 这些命令都是通过对键空间进行操作来实现的。

读写键空间时的维护操作

当使用 Redis 命令对数据库进行读写时, 服务器不仅会对键空间执行指定的读写操作, 还会执行一些额外的维护操作, 其中包括:

  • 在读取一个键之后(读操作和写操作都要对键进行读取), 服务器会根据键是否存在, 以此来更新服务器的键空间命中(hit)次数或键空间不命中(miss)次数, 这两个值可以在 INFO stats 命令的 keyspace_hits 属性和 keyspace_misses 属性中查看。
  • 在读取一个键之后, 服务器会更新键的 LRU (最后一次使用)时间, 这个值可以用于计算键的闲置时间, 使用命令 OBJECT idletime <key> 命令可以查看键 key 的闲置时间。
  • 如果服务器在读取一个键时, 发现该键已经过期, 那么服务器会先删除这个过期键, 然后才执行余下的其他操作, 本章稍后对过期键的讨论会详细说明这一点。
  • 如果有客户端使用 WATCH 命令监视了某个键, 那么服务器在对被监视的键进行修改之后, 会将这个键标记为脏(dirty), 从而让事务程序注意到这个键已经被修改过, 《事务》一章会详细说明这一点。
  • 服务器每次修改一个键之后, 都会对脏(dirty)键计数器的值增一, 这个计数器会触发服务器的持久化以及复制操作执行, 《RDB 持久化》、《AOF 持久化》和《复制》这三章都会说到这一点。
  • 如果服务器开启了数据库通知功能, 那么在对键进行修改之后, 服务器将按配置发送相应的数据库通知, 本章稍后讨论数据库通知功能的实现时会详细说明这一点。

讨论

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