Redis数据类型介绍

 Redis不是一个普通的键值存储,它实际上是一个数据结构服务器,支持不同类型的值。这意味着,在传统的键值存储中将字符串键与字符串值相关联,而在redis中,该值不仅限于一个简单的字符串,而且还可以保存更复杂的数据结构。以下是Redis支持的所有数据结构的列表,本教程将分别介绍这些结构:
  • 二进制安全的字符串。
  • 列表(Lists):按照插入顺序排列的字符串元素的集合。从根本上说它们是 Linked Lists。
  • 集合(Sets):唯一、无序的字符串元素的集合。
  • 有序集合(Sorted sets): 类似于集合,但每个字符串元素都与一个称为score的浮点数值相关联。元素总是按分数排序,因此与集合不同,可以检索一系列元素(例如,您可能会问:给出前10个或后10个)。
  • 哈希(Hashes): 它是由字段和值组成的map。字段和值都是字符串。这和Ruby或Python的hashes非常相似。
  • 位图 Bit arrays (或bitmaps):使用特殊命令,可以像处理一组位那样处理字符串值:可以设置或清除某个位,统计所有被设置为1的位,找到第一个被设置或未设置的位,等等。 
  • 超日志(HyperLogLogs):这是一种概率数据结构,用于估计集合的基数。别害怕,这比看起来简单…请参阅本教程后面的“超级日志”部分。
  • 流(Streams0): 只追加的集合,它由类map提供抽象日志数据类型的元素组成。会在 Introduction to Redis Streams 深入介绍。

掌握这些数据类型是如何工作的,以及从命令参考中选择哪种类型来解决给定的问题并不总是很容易的,因此本文档是Redis数据类型及其最常见模式的一个速成课程。

对于所有示例,我们将使用redis cli实用程序(一个简单但方便的命令行实用程序)来针对redis服务器发出命令。

Redis keys

Redis keys是二进制安全的,这意味着你可以使用任意的二进制序列作为key,从像‘foo’这样的字符串到JPEG 文件的内容。空字符串也是有效的key。

关于key的一些规则:

  • 很长的key不是个好的选择。例如,一个1024字节的key不仅在使用内存方面是一个选择,而且因为在数据集中查找key可能需要一些代价高昂的键比较。即使手头的任务是匹配一个大值的存在,对它进行散列(例如使用sha1)是一个更好的选择,特别是从内存和带宽的角度。
  • 非常短的key通常也不是好的选择。如果你能写出“user:1000:followers”,那么把“u1000flw”作为一个键就没有什么意义了。与键对象本身和值对象使用的空间相比,前者可读性更强,增加的空间较小。尽管短键显然会消耗更少的内存,但你的工作是找到正确的平衡点。
  • 试着坚持使用模式。例如,“object-type:id”是一个好主意,就像“user:1000”一样。点或破折号通常用于多单词字段,如“comment:1234:reply.to”或“comment:1234:reply-to”。
  • key最大允许的长度是512MB。

Redis Strings

Redis 字符串类型是可以用键关联的最简单的值类型。它是Mamcached的唯一数据类型,所以对于新来者来说,在Redis中使用它是非常自然的。

由于Redis的键是字符串,当我们也用字符串作为值的时候,我们从一个字符串映射另一个字符串。字符串类型在大量的用例中都是有用的,像缓存HTML片段和页面。

使用 redis-cli 测试字符串类型:

> set mykey somevalue
OK
> get mykey
"somevalue"

上面使用  SET 和 GET 命令来设置和读取一个字符串值。需要注意的是 SET 将替换存储在key中任何已存在的值,即使key关联的是一个非字符串值。 SET 执行的是赋值。

值可以是各种类型的字符串(包括二进制数据),例如,您可以在值中存储一个jpeg图像。值不能大于512 MB。

SET 命令有很多有趣的选项,这些选项被作为额外的参数提供。例如,可以让 SET 命令失败在key已经存在的情况下,或反过来,只有在key存在的情况下成功。:

> set mykey newval nx
(nil)
> set mykey newval xx
OK

虽然字符串是Redis最基本的值类型,但是也可以在字符串上执行很多有趣的操作。例如,原子自增:

> set counter 100
OK
> incr counter
(integer) 101
> incr counter
(integer) 102
> incrby counter 50
(integer) 152

INCR 命令把字符串解析成整数,并把值加一,并最终把计算的结果作为新值。还有一些类似的命令像 INCRBYDECR 和 DECRBY。本质上它们都是同样的命令,只是表现行为上有一点不同。

“INCR是原子的”是什么意思?即使多个客户端发送INCR命令操作同一个key,也永远不会进入一个竞争状态。例如,客户端1读到10,客户端2同时都到10,它们同时各自自增为11,然后把新值设置为11的情况永远不会发生。最终的值总会是12,读-自增-写操作执行被执行的同时,所有其它客户端都没有在执行命令。 

有许多操作字符串的命令。例如, GETSET 命令设置新值,并返回旧值作为结果。 它的使用实例是:每当有新的访客都把key的值加一,使用这个命令可以准确获取每小时的访问量,通过GETSET 命令,把key的新值设置为0,返回旧值。

在单个命令中同时设置或读取多个key值的能力也是有用的。所有有了 MSET 和 MGET 命令:

> mset a 10 b 20 c 30
OK
> mget a b c
1) "10"
2) "20"
3) "30"

 使用MGET 的时候,Redis返回一个值的数组。

修改和查询key空间

有些命令没有在特定类型上定义,但对于与键的空间交互非常有用,因此可以与任何类型的键一起使用。

例如 EXISTS 命令返回1或0来表明给定的key在数据库中是否存在,而 DEL 命令删除某个key和它关联的值,不管值是什么类型。

> set mykey hello
OK
> exists mykey
(integer) 1
> del mykey
(integer) 1
> exists mykey
(integer) 0

从例子中也可以看出 DEL 命令怎样返回1或0,它依赖于key被删除(key存在)与否(没有这个名字的key)。

有很多key空间相关的命令,但是上面这两个命令和TYPE命令一起是必须的,返回存储在特定key中的值类型。

> set mykey x
OK
> type mykey
string
> del mykey
(integer) 1
> type mykey
none

Redis 过期(Redis expires): 带有有限生存时间的key

开始介绍更复杂的数据结构之前,我们先讨论另一个对任何类型都有作用的特性,叫做Redis 过期(Redis expires)。你可以给key设置超时,这是一个有限的生存时间。当生存期走完之后,这个key会被自动删除,跟直接调用 DEL 命令效果一样。

关于Redis超时的几个简短信息:

  • 它们可以使用秒或毫秒精度进行设置。
  • 但是,过期时间分辨率始终为1毫秒。
  • 有关过期的信息将被复制并保存在磁盘上,当您的redis服务器保持停止状态时,时间实际上会过去(这意味着redis会保存key过期的日期)。

设置超时很简单:

> set key some-value
OK
> expire key 5
(integer) 1
> get key (立刻执行)
"some-value"
> get key (5秒以后执行)
(nil)

因为第二次调用GET延迟时间超过5秒,所以key在两次 GET 之间消失。上面的例子当中使用 EXPIRE 来设置超时(可以给存在超时的key设置一个不同超时,例如PERSIST 可以用来删除超时和设置key为持久的)。也可以使用其它Redis命令来这是超时,例如使用  SET 选项:

> set key 100 ex 10
OK
> ttl key
(integer) 9

上面的例子设置了一个名为key的键,它的值是字符串格式的100,它的超时时间是10秒。接着调用 TTL 命令来检查key的剩余生存时间。

如果像设置或检查超时以毫秒为单位,可以参考 PEXPIRE 和 PTTL 命令,和完整的 SET 选项。

Redis列表(Redis Lists)

为了解释列表数据类型,最好从一点理论入手,因为信息技术人员经常以不正确的方式使用列表这个术语。例如,“python-lists”并不是这个名字所暗示的(链表),而是数组(同一数据类型实际上在ruby中称为array)。

 从一个非常一般的角度来看,列表只是一系列有序元素:10、20、1、2、3是一个列表。但是,使用数组实现的列表的属性与使用链接列表实现的列表的属性非常不同。

Redis列表通过链接列表实现。这意味着,即使列表中有数百万个元素,在列表的头部或尾部添加新元素的操作也将在恒定时间内执行。使用 LPUSH 命令将一个新元素添加到包含10个元素的列表头的速度与向包含1000万个元素的列表头添加元素的速度相同。

 

缺点是什么?通过索引访问元素在使用数组实现的列表中速度非常快(常量时间索引访问),而在由链接列表实现的列表中速度却不太快(在该操作中需要与被访问元素的索引成比例的工作量)。

Redis列表是通过链表实现的,因为对于数据库系统来说,能够以非常快的方式将元素添加到一个非常长的列表中是至关重要的。另一个优势,如您稍后将看到的,是Redis列表可以在恒定的时间内处理恒定的长度。

当快速访问大量元素集合的中间部分很重要时,可以使用不同的数据结构,叫做有序集。本教程稍后将介绍有序集。

First steps with Redis Lists

The LPUSH command adds a new element into a list, on the left (at the head), while the RPUSH command adds a new element into a list ,on the right (at the tail). Finally the LRANGE command extracts ranges of elements from lists:

> rpush mylist A
(integer) 1
> rpush mylist B
(integer) 2
> lpush mylist first
(integer) 3
> lrange mylist 0 -1
1) "first"
2) "A"
3) "B"

Note that LRANGE takes two indexes, the first and the last element of the range to return. Both the indexes can be negative, telling Redis to start counting from the end: so -1 is the last element, -2 is the penultimate element of the list, and so forth.

As you can see RPUSH appended the elements on the right of the list, while the final LPUSH appended the element on the left.

Both commands are variadic commands, meaning that you are free to push multiple elements into a list in a single call:

> rpush mylist 1 2 3 4 5 "foo bar"
(integer) 9
> lrange mylist 0 -1
1) "first"
2) "A"
3) "B"
4) "1"
5) "2"
6) "3"
7) "4"
8) "5"
9) "foo bar"

An important operation defined on Redis lists is the ability to pop elements. Popping elements is the operation of both retrieving the element from the list, and eliminating it from the list, at the same time. You can pop elements from left and right, similarly to how you can push elements in both sides of the list:

> rpush mylist a b c
(integer) 3
> rpop mylist
"c"
> rpop mylist
"b"
> rpop mylist
"a"

We added three elements and popped three elements, so at the end of this sequence of commands the list is empty and there are no more elements to pop. If we try to pop yet another element, this is the result we get:

> rpop mylist
(nil)

Redis returned a NULL value to signal that there are no elements in the list.

Common use cases for lists

Lists are useful for a number of tasks, two very representative use cases are the following:

  • Remember the latest updates posted by users into a social network.
  • Communication between processes, using a consumer-producer pattern where the producer pushes items into a list, and a consumer (usually a worker) consumes those items and executed actions. Redis has special list commands to make this use case both more reliable and efficient.

For example both the popular Ruby libraries resque and sidekiq use Redis lists under the hood in order to implement background jobs.

The popular Twitter social network takes the latest tweets posted by users into Redis lists.

To describe a common use case step by step, imagine your home page shows the latest photos published in a photo sharing social network and you want to speedup access.

  • Every time a user posts a new photo, we add its ID into a list with LPUSH.
  • When users visit the home page, we use LRANGE 0 9 in order to get the latest 10 posted items.

Capped lists

In many use cases we just want to use lists to store the latest items, whatever they are: social network updates, logs, or anything else.

Redis allows us to use lists as a capped collection, only remembering the latest N items and discarding all the oldest items using the LTRIM command.

The LTRIM command is similar to LRANGE, but instead of displaying the specified range of elements it sets this range as the new list value. All the elements outside the given range are removed.

An example will make it more clear:

> rpush mylist 1 2 3 4 5
(integer) 5
> ltrim mylist 0 2
OK
> lrange mylist 0 -1
1) "1"
2) "2"
3) "3"

The above LTRIM command tells Redis to take just list elements from index 0 to 2, everything else will be discarded. This allows for a very simple but useful pattern: doing a List push operation + a List trim operation together in order to add a new element and discard elements exceeding a limit:

LPUSH mylist <some element>
LTRIM mylist 0 999

The above combination adds a new element and takes only the 1000 newest elements into the list. With LRANGE you can access the top items without any need to remember very old data.

Note: while LRANGE is technically an O(N) command, accessing small ranges towards the head or the tail of the list is a constant time operation.

Blocking operations on lists

Lists have a special feature that make them suitable to implement queues, and in general as a building block for inter process communication systems: blocking operations.

Imagine you want to push items into a list with one process, and use a different process in order to actually do some kind of work with those items. This is the usual producer / consumer setup, and can be implemented in the following simple way:

  • To push items into the list, producers call LPUSH.
  • To extract / process items from the list, consumers call RPOP.

However it is possible that sometimes the list is empty and there is nothing to process, so RPOP just returns NULL. In this case a consumer is forced to wait some time and retry again with RPOP. This is called polling, and is not a good idea in this context because it has several drawbacks:

  1. Forces Redis and clients to process useless commands (all the requests when the list is empty will get no actual work done, they’ll just return NULL).
  2. Adds a delay to the processing of items, since after a worker receives a NULL, it waits some time. To make the delay smaller, we could wait less between calls to RPOP, with the effect of amplifying problem number 1, i.e. more useless calls to Redis.

So Redis implements commands called BRPOP and BLPOP which are versions of RPOP and LPOP able to block if the list is empty: they’ll return to the caller only when a new element is added to the list, or when a user-specified timeout is reached.

This is an example of a BRPOP call we could use in the worker:

> brpop tasks 5
1) "tasks"
2) "do_something"

It means: “wait for elements in the list tasks, but return if after 5 seconds no element is available”.

Note that you can use 0 as timeout to wait for elements forever, and you can also specify multiple lists and not just one, in order to wait on multiple lists at the same time, and get notified when the first list receives an element.

A few things to note about BRPOP:

  1. Clients are served in an ordered way: the first client that blocked waiting for a list, is served first when an element is pushed by some other client, and so forth.
  2. The return value is different compared to RPOP: it is a two-element array since it also includes the name of the key, because BRPOP and BLPOP are able to block waiting for elements from multiple lists.
  3. If the timeout is reached, NULL is returned.

There are more things you should know about lists and blocking ops. We suggest that you read more on the following:

  • It is possible to build safer queues or rotating queues using RPOPLPUSH.
  • There is also a blocking variant of the command, called BRPOPLPUSH.

Automatic creation and removal of keys

So far in our examples we never had to create empty lists before pushing elements, or removing empty lists when they no longer have elements inside. It is Redis’ responsibility to delete keys when lists are left empty, or to create an empty list if the key does not exist and we are trying to add elements to it, for example, with LPUSH.

This is not specific to lists, it applies to all the Redis data types composed of multiple elements — Streams, Sets, Sorted Sets and Hashes.

Basically we can summarize the behavior with three rules:

  1. When we add an element to an aggregate data type, if the target key does not exist, an empty aggregate data type is created before adding the element.
  2. When we remove elements from an aggregate data type, if the value remains empty, the key is automatically destroyed. The Stream data type is the only exception to this rule.
  3. Calling a read-only command such as LLEN (which returns the length of the list), or a write command removing elements, with an empty key, always produces the same result as if the key is holding an empty aggregate type of the type the command expects to find.

例子 of rule 1:

> del mylist
(integer) 1
> lpush mylist 1 2 3
(integer) 3

However we can’t perform operations against the wrong type if the key exists:

> set foo bar
OK
> lpush foo 1 2 3
(error) WRONGTYPE Operation against a key holding the wrong kind of value
> type foo
string

Example of rule 2:

> lpush mylist 1 2 3
(integer) 3
> exists mylist
(integer) 1
> lpop mylist
"3"
> lpop mylist
"2"
> lpop mylist
"1"
> exists mylist
(integer) 0

The key no longer exists after all the elements are popped.

Example of rule 3:

> del mylist
(integer) 0
> llen mylist
(integer) 0
> lpop mylist
(nil)

Redis Hashes

Redis hashes look exactly how one might expect a “hash” to look, with field-value pairs:

> hmset user:1000 username antirez birthyear 1977 verified 1
OK
> hget user:1000 username
"antirez"
> hget user:1000 birthyear
"1977"
> hgetall user:1000
1) "username"
2) "antirez"
3) "birthyear"
4) "1977"
5) "verified"
6) "1"

While hashes are handy to represent objects, actually the number of fields you can put inside a hash has no practical limits (other than available memory), so you can use hashes in many different ways inside your application.

The command HMSET sets multiple fields of the hash, while HGET retrieves a single field. HMGET is similar to HGETbut returns an array of values:

> hmget user:1000 username birthyear no-such-field
1) "antirez"
2) "1977"
3) (nil)

There are commands that are able to perform operations on individual fields as well, like HINCRBY:

> hincrby user:1000 birthyear 10
(integer) 1987
> hincrby user:1000 birthyear 10
(integer) 1997

You can find the full list of hash commands in the documentation.

It is worth noting that small hashes (i.e., a few elements with small values) are encoded in special way in memory that make them very memory efficient.

Redis Sets

Redis Sets are unordered collections of strings. The SADD command adds new elements to a set. It’s also possible to do a number of other operations against sets like testing if a given element already exists, performing the intersection, union or difference between multiple sets, and so forth.

> sadd myset 1 2 3
(integer) 3
> smembers myset
1. 3
2. 1
3. 2

Here I’ve added three elements to my set and told Redis to return all the elements. As you can see they are not sorted — Redis is free to return the elements in any order at every call, since there is no contract with the user about element ordering.

Redis has commands to test for membership. For example, checking if an element exists:

> sismember myset 3
(integer) 1
> sismember myset 30
(integer) 0

“3” is a member of the set, while “30” is not.

Sets are good for expressing relations between objects. For instance we can easily use sets in order to implement tags.

A simple way to model this problem is to have a set for every object we want to tag. The set contains the IDs of the tags associated with the object.

One illustration is tagging news articles. If article ID 1000 is tagged with tags 1, 2, 5 and 77, a set can associate these tag IDs with the news item:

> sadd news:1000:tags 1 2 5 77
(integer) 4

We may also want to have the inverse relation as well: the list of all the news tagged with a given tag:

> sadd tag:1:news 1000
(integer) 1
> sadd tag:2:news 1000
(integer) 1
> sadd tag:5:news 1000
(integer) 1
> sadd tag:77:news 1000
(integer) 1

To get all the tags for a given object is trivial:

> smembers news:1000:tags
1. 5
2. 1
3. 77
4. 2

Note: in the example we assume you have another data structure, for example a Redis hash, which maps tag IDs to tag names.

There are other non trivial operations that are still easy to implement using the right Redis commands. For instance we may want a list of all the objects with the tags 1, 2, 10, and 27 together. We can do this using the SINTERcommand, which performs the intersection between different sets. We can use:

> sinter tag:1:news tag:2:news tag:10:news tag:27:news
... results here ...

In addition to intersection you can also perform unions, difference, extract a random element, and so forth.

The command to extract an element is called SPOP, and is handy to model certain problems. For example in order to implement a web-based poker game, you may want to represent your deck with a set. Imagine we use a one-char prefix for (C)lubs, (D)iamonds, (H)earts, (S)pades:

>  sadd deck C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 CJ CQ CK
   D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 DJ DQ DK H1 H2 H3
   H4 H5 H6 H7 H8 H9 H10 HJ HQ HK S1 S2 S3 S4 S5 S6
   S7 S8 S9 S10 SJ SQ SK
   (integer) 52

Now we want to provide each player with 5 cards. The SPOP command removes a random element, returning it to the client, so it is the perfect operation in this case.

However if we call it against our deck directly, in the next play of the game we’ll need to populate the deck of cards again, which may not be ideal. So to start, we can make a copy of the set stored in the deck key into the game:1:deck key.

This is accomplished using SUNIONSTORE, which normally performs the union between multiple sets, and stores the result into another set. However, since the union of a single set is itself, I can copy my deck with:

> sunionstore game:1:deck deck
(integer) 52

Now I’m ready to provide the first player with five cards:

> spop game:1:deck
"C6"
> spop game:1:deck
"CQ"
> spop game:1:deck
"D1"
> spop game:1:deck
"CJ"
> spop game:1:deck
"SJ"

One pair of jacks, not great…

This is a good time to introduce the set command that provides the number of elements inside a set. This is often called the cardinality of a set in the context of set theory, so the Redis command is called SCARD.

> scard game:1:deck
(integer) 47

The math works: 52 – 5 = 47.

When you need to just get random elements without removing them from the set, there is the SRANDMEMBERcommand suitable for the task. It also features the ability to return both repeating and non-repeating elements.

Redis Sorted sets

Sorted sets are a data type which is similar to a mix between a Set and a Hash. Like sets, sorted sets are composed of unique, non-repeating string elements, so in some sense a sorted set is a set as well.

However while elements inside sets are not ordered, every element in a sorted set is associated with a floating point value, called the score (this is why the type is also similar to a hash, since every element is mapped to a value).

Moreover, elements in a sorted sets are taken in order (so they are not ordered on request, order is a peculiarity of the data structure used to represent sorted sets). They are ordered according to the following rule:

  • If A and B are two elements with a different score, then A > B if A.score is > B.score.
  • If A and B have exactly the same score, then A > B if the A string is lexicographically greater than the B string. A and B strings can’t be equal since sorted sets only have unique elements.

Let’s start with a simple example, adding a few selected hackers names as sorted set elements, with their year of birth as “score”.

> zadd hackers 1940 "Alan Kay"
(integer) 1
> zadd hackers 1957 "Sophie Wilson"
(integer) 1
> zadd hackers 1953 "Richard Stallman"
(integer) 1
> zadd hackers 1949 "Anita Borg"
(integer) 1
> zadd hackers 1965 "Yukihiro Matsumoto"
(integer) 1
> zadd hackers 1914 "Hedy Lamarr"
(integer) 1
> zadd hackers 1916 "Claude Shannon"
(integer) 1
> zadd hackers 1969 "Linus Torvalds"
(integer) 1
> zadd hackers 1912 "Alan Turing"
(integer) 1

As you can see ZADD is similar to SADD, but takes one additional argument (placed before the element to be added) which is the score. ZADD is also variadic, so you are free to specify multiple score-value pairs, even if this is not used in the example above.

With sorted sets it is trivial to return a list of hackers sorted by their birth year because actually they are already sorted.

Implementation note: Sorted sets are implemented via a dual-ported data structure containing both a skip list and a hash table, so every time we add an element Redis performs an O(log(N)) operation. That’s good, but when we ask for sorted elements Redis does not have to do any work at all, it’s already all sorted:

> zrange hackers 0 -1
1) "Alan Turing"
2) "Hedy Lamarr"
3) "Claude Shannon"
4) "Alan Kay"
5) "Anita Borg"
6) "Richard Stallman"
7) "Sophie Wilson"
8) "Yukihiro Matsumoto"
9) "Linus Torvalds"

Note: 0 and -1 means from element index 0 to the last element (-1 works here just as it does in the case of the LRANGE command).

What if I want to order them the opposite way, youngest to oldest? Use ZREVRANGE instead of ZRANGE:

> zrevrange hackers 0 -1
1) "Linus Torvalds"
2) "Yukihiro Matsumoto"
3) "Sophie Wilson"
4) "Richard Stallman"
5) "Anita Borg"
6) "Alan Kay"
7) "Claude Shannon"
8) "Hedy Lamarr"
9) "Alan Turing"

It is possible to return scores as well, using the WITHSCORES argument:

> zrange hackers 0 -1 withscores
1) "Alan Turing"
2) "1912"
3) "Hedy Lamarr"
4) "1914"
5) "Claude Shannon"
6) "1916"
7) "Alan Kay"
8) "1940"
9) "Anita Borg"
10) "1949"
11) "Richard Stallman"
12) "1953"
13) "Sophie Wilson"
14) "1957"
15) "Yukihiro Matsumoto"
16) "1965"
17) "Linus Torvalds"
18) "1969"

Operating on ranges

Sorted sets are more powerful than this. They can operate on ranges. Let’s get all the individuals that were born up to 1950 inclusive. We use the ZRANGEBYSCORE command to do it:

> zrangebyscore hackers -inf 1950
1) "Alan Turing"
2) "Hedy Lamarr"
3) "Claude Shannon"
4) "Alan Kay"
5) "Anita Borg"

We asked Redis to return all the elements with a score between negative infinity and 1950 (both extremes are included).

It’s also possible to remove ranges of elements. Let’s remove all the hackers born between 1940 and 1960 from the sorted set:

> zremrangebyscore hackers 1940 1960
(integer) 4

ZREMRANGEBYSCORE is perhaps not the best command name, but it can be very useful, and returns the number of removed elements.

Another extremely useful operation defined for sorted set elements is the get-rank operation. It is possible to ask what is the position of an element in the set of the ordered elements.

> zrank hackers "Anita Borg"
(integer) 4

The ZREVRANK command is also available in order to get the rank, considering the elements sorted a descending way.

Lexicographical scores

With recent versions of Redis 2.8, a new feature was introduced that allows getting ranges lexicographically, assuming elements in a sorted set are all inserted with the same identical score (elements are compared with the C memcmpfunction, so it is guaranteed that there is no collation, and every Redis instance will reply with the same output).

The main commands to operate with lexicographical ranges are ZRANGEBYLEXZREVRANGEBYLEXZREMRANGEBYLEX and ZLEXCOUNT.

For example, let’s add again our list of famous hackers, but this time use a score of zero for all the elements:

> zadd hackers 0 "Alan Kay" 0 "Sophie Wilson" 0 "Richard Stallman" 0
  "Anita Borg" 0 "Yukihiro Matsumoto" 0 "Hedy Lamarr" 0 "Claude Shannon"
  0 "Linus Torvalds" 0 "Alan Turing"

Because of the sorted sets ordering rules, they are already sorted lexicographically:

> zrange hackers 0 -1
1) "Alan Kay"
2) "Alan Turing"
3) "Anita Borg"
4) "Claude Shannon"
5) "Hedy Lamarr"
6) "Linus Torvalds"
7) "Richard Stallman"
8) "Sophie Wilson"
9) "Yukihiro Matsumoto"

Using ZRANGEBYLEX we can ask for lexicographical ranges:

> zrangebylex hackers [B [P
1) "Claude Shannon"
2) "Hedy Lamarr"
3) "Linus Torvalds"

Ranges can be inclusive or exclusive (depending on the first character), also string infinite and minus infinite are specified respectively with the + and - strings. See the documentation for more information.

This feature is important because it allows us to use sorted sets as a generic index. For example, if you want to index elements by a 128-bit unsigned integer argument, all you need to do is to add elements into a sorted set with the same score (for example 0) but with an 16 byte prefix consisting of the 128 bit number in big endian. Since numbers in big endian, when ordered lexicographically (in raw bytes order) are actually ordered numerically as well, you can ask for ranges in the 128 bit space, and get the element’s value discarding the prefix.

If you want to see the feature in the context of a more serious demo, check the Redis autocomplete demo.

Updating the score: leader boards

Just a final note about sorted sets before switching to the next topic. Sorted sets’ scores can be updated at any time. Just calling ZADD against an element already included in the sorted set will update its score (and position) with O(log(N)) time complexity. As such, sorted sets are suitable when there are tons of updates.

Because of this characteristic a common use case is leader boards. The typical application is a Facebook game where you combine the ability to take users sorted by their high score, plus the get-rank operation, in order to show the top-N users, and the user rank in the leader board (e.g., “you are the #4932 best score here”).

Bitmaps

Bitmaps are not an actual data type, but a set of bit-oriented operations defined on the String type. Since strings are binary safe blobs and their maximum length is 512 MB, they are suitable to set up to 232 different bits.

Bit operations are divided into two groups: constant-time single bit operations, like setting a bit to 1 or 0, or getting its value, and operations on groups of bits, for example counting the number of set bits in a given range of bits (e.g., population counting).

One of the biggest advantages of bitmaps is that they often provide extreme space savings when storing information. For example in a system where different users are represented by incremental user IDs, it is possible to remember a single bit information (for example, knowing whether a user wants to receive a newsletter) of 4 billion of users using just 512 MB of memory.

Bits are set and retrieved using the SETBIT and GETBIT commands:

> setbit key 10 1
(integer) 1
> getbit key 10
(integer) 1
> getbit key 11
(integer) 0

The SETBIT command takes as its first argument the bit number, and as its second argument the value to set the bit to, which is 1 or 0. The command automatically enlarges the string if the addressed bit is outside the current string length.

GETBIT just returns the value of the bit at the specified index. Out of range bits (addressing a bit that is outside the length of the string stored into the target key) are always considered to be zero.

There are three commands operating on group of bits:

  1. BITOP performs bit-wise operations between different strings. The provided operations are AND, OR, XOR and NOT.
  2. BITCOUNT performs population counting, reporting the number of bits set to 1.
  3. BITPOS finds the first bit having the specified value of 0 or 1.

Both BITPOS and BITCOUNT are able to operate with byte ranges of the string, instead of running for the whole length of the string. The following is a trivial example of BITCOUNT call:

> setbit key 0 1
(integer) 0
> setbit key 100 1
(integer) 0
> bitcount key
(integer) 2

Common use cases for bitmaps are:

  • Real time analytics of all kinds.
  • Storing space efficient but high performance boolean information associated with object IDs.

For example imagine you want to know the longest streak of daily visits of your web site users. You start counting days starting from zero, that is the day you made your web site public, and set a bit with SETBIT every time the user visits the web site. As a bit index you simply take the current unix time, subtract the initial offset, and divide by 3600*24.

This way for each user you have a small string containing the visit information for each day. With BITCOUNT it is possible to easily get the number of days a given user visited the web site, while with a few BITPOS calls, or simply fetching and analyzing the bitmap client-side, it is possible to easily compute the longest streak.

Bitmaps are trivial to split into multiple keys, for example for the sake of sharding the data set and because in general it is better to avoid working with huge keys. To split a bitmap across different keys instead of setting all the bits into a key, a trivial strategy is just to store M bits per key and obtain the key name with bit-number/M and the Nth bit to address inside the key with bit-number MOD M.

HyperLogLogs

A HyperLogLog is a probabilistic data structure used in order to count unique things (technically this is referred to estimating the cardinality of a set). Usually counting unique items requires using an amount of memory proportional to the number of items you want to count, because you need to remember the elements you have already seen in the past in order to avoid counting them multiple times. However there is a set of algorithms that trade memory for precision: you end with an estimated measure with a standard error, which in the case of the Redis implementation is less than 1%. The magic of this algorithm is that you no longer need to use an amount of memory proportional to the number of items counted, and instead can use a constant amount of memory! 12k bytes in the worst case, or a lot less if your HyperLogLog (We’ll just call them HLL from now) has seen very few elements.

HLLs in Redis, while technically a different data structure, are encoded as a Redis string, so you can call GET to serialize a HLL, and SET to deserialize it back to the server.

Conceptually the HLL API is like using Sets to do the same task. You would SADD every observed element into a set, and would use SCARD to check the number of elements inside the set, which are unique since SADD will not re-add an existing element.

While you don’t really add items into an HLL, because the data structure only contains a state that does not include actual elements, the API is the same:

  • Every time you see a new element, you add it to the count with PFADD.
  • Every time you want to retrieve the current approximation of the unique elements added with PFADD so far, you use the PFCOUNT.

    > pfadd hll a b c d
    (integer) 1
    > pfcount hll
    (integer) 4
    

An example of use case for this data structure is counting unique queries performed by users in a search form every day.

Redis is also able to perform the union of HLLs, please check the full documentation for more information.

其它值得注意的特性

Redis API 有很多重要的东西不能在本文深入探索,但却值得了解:

了解更多

本教程没办法完全覆盖所有内容,只介绍了API的基本概念。可以阅读 command reference 来了解更多。