Redis sorted sets
Introduction to Redis sorted sets
A Redis sorted set is a collection of unique strings (members) ordered by an associated score. When more than one string has the same score, the strings are ordered lexicographically. Some use cases for sorted sets include:
 Leaderboards. For example, you can use sorted sets to easily maintain ordered lists of the highest scores in a massive online game.
 Rate limiters. In particular, you can use a sorted set to build a slidingwindow rate limiter to prevent excessive API requests.
You can think of sorted sets as a mix between a Set and a Hash. Like sets, sorted sets are composed of unique, nonrepeating 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 set 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 B and A are two elements with a different score, then A > B if A.score is > B.score.
 If B and A have exactly the same score, then A > B if the A string is lexicographically greater than the B string. B and A strings can't be equal since sorted sets only have unique elements.
Let's start with a simple example, we'll add all our racers and the score they got in the first race:
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 scorevalue
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
dualported 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 sorted. Note that the ZRANGE
order is low to high, while the ZREVRANGE
order is high to low:
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).
It is possible to return scores as well, using the WITHSCORES
argument:
Operating on ranges
Sorted sets are more powerful than this. They can operate on ranges.
Let's get all the racers with 10 or fewer points. We
use the ZRANGEBYSCORE
command to do it:
We asked Redis to return all the elements with a score between negative infinity and 10 (both extremes are included).
To remove an element we'd simply call ZREM
with the racer's name.
It's also possible to remove ranges of elements. Let's remove racer Castilla along with all
the racers with strictly fewer than 10 points:
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 getrank operation. It is possible to ask what is the
position of an element in the set of ordered elements.
The ZREVRANK
command is also available in order to get the rank, considering
the elements sorted a descending way.
Lexicographical scores
In version 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
memcmp
function, 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 ZRANGEBYLEX
,
ZREVRANGEBYLEX
, ZREMRANGEBYLEX
and ZLEXCOUNT
.
For example, let's add again our list of famous hackers, but this time
using a score of zero for all the elements. We'll see that because of the sorted sets ordering rules, they are already sorted lexicographically. Using ZRANGEBYLEX
we can ask for lexicographical ranges:
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 128bit 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 a 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: leaderboards
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 leaderboards. The typical application is a Facebook game where you combine the ability to take users sorted by their high score, plus the getrank operation, in order to show the topN users, and the user rank in the leader board (e.g., "you are the #4932 best score here").
Examples
 There are two ways we can use a sorted set to represent a leaderbaord. If we know a racer's new score, we can update it directly via the
ZADD
command. However, if we want to add points to an existing score, we can use theZINCRBY
command.
You'll see that ZADD
returns 0 when the member already exists (the score is updated), while ZINCRBY
returns the new score. The score for racer Henshaw went from 100, was changed to 150 with no regard for what score was there before, and then was incremented by 50 to 200.
Basic commands
ZADD
adds a new member and associated score to a sorted set. If the member already exists, the score is updated.ZRANGE
returns members of a sorted set, sorted within a given range.ZRANK
returns the rank of the provided member, assuming the sorted is in ascending order.ZREVRANK
returns the rank of the provided member, assuming the sorted set is in descending order.
See the complete list of sorted set commands.
Performance
Most sorted set operations are O(log(n)), where n is the number of members.
Exercise some caution when running the ZRANGE
command with large returns values (e.g., in the tens of thousands or more).
This command's time complexity is O(log(n) + m), where m is the number of results returned.
Alternatives
Redis sorted sets are sometimes used for indexing other Redis data structures. If you need to index and query your data, consider the JSON data type and the Search and query features.
Learn more

Redis Sorted Sets Explained is an entertaining introduction to sorted sets in Redis.

Redis University's RU101 explores Redis sorted sets in detail.

Redis Sorted Sets Explained is an entertaining introduction to sorted sets in Redis.

Redis University's RU101 explores Redis sorted sets in detail.