Bounded sorted set

[Adityanarayan Vaidya]

Hi All,

Does redis have a bounded sorted set ?

Regards,

Aditya

[Itamar Haber]

Hi Aditya,

All sets in Redis are bounded - they can grow "only" up to 2^32 members. I suspect, however, that that isn't your intention.

What do you mean by "bounded sorted set"? Is it bounded in terms of cardinality, scores, both? Also, once the bounds are met, what do you expect will happen when trying to add a new member - an error, discard the new member, or remove an existing member according to some logic & then add the new one?

Cheers,

Itamar

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[AlexanderB]

You might want to look into redis lua scripting. While Redis probably doesn't natively support a data type that exactly fits your needs, with scripting you could build a higher level interface to get / set from a normal sorted set that also enforces the extra logic you want.

 This redis green post is a pretty good reference to get a sense of what's possible. https://www.redisgreen.net/blog/intro-to-lua-for-redis-programmers/

[Adityanarayan Vaidya]

Hi Ita,

I was more specifically looking at bound in terms of cardinality. In terms of adding a new member when a bound is met I was looking to remove member by least score if the score of the new member is higher.

The use case is related to TOP N view by score. So I say need at max TOP 100 (N) in the sorted set at any point in time. Currently I am trimming the sorted set every x secs to 100 (N) just to

keep it bounded to 100 (N).

Regards,

Aditya

[Adityanarayan Vaidya]

Sure Alex.

Regards,

Aditya

[Itamar Haber]

Thanks for the clarification. As Alex had suggested, the way to go here is Lua. Specifically, with a script such as the following:

$ cat ~/zaddcapped.lua 

--[[

usage: EVAL "thisscript" 1 <sorted-set-key> <cap> <score> <member> [<score> <member> ...]

reply: integer, the number of members added (not final set cardinality)

performs **no** error/type checking

]]--

local cap = table.remove(ARGV,1)

local added = redis.call('ZADD',KEYS[1],unpack(ARGV))

redis.call('ZREMRANGEBYRANK',KEYS[1],0,-(cap+1))

return added

$ ./redis-cli DEL z

(integer) 1

$ ./redis-cli ZADD z 1 a 2 b 3 c

(integer) 3

./redis-cli --eval ~/zaddcapped.lua z , 5 4 d 5 e 6 f 7 g

(integer) 4

./redis-cli ZRANGE z 0 -1 WITHSCORES

 1) "c"

 2) "3"

 3) "d"

 4) "4"

 5) "e"

 6) "5"

 7) "f"

 8) "6"

 9) "g"

10) "7"

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[Adityanarayan Vaidya]

Also i need to pop the element with the least score from the sorted set.

Is there any other way I might be missing then using ZRANGE & then ZREM?

Regards,

Aditya

[AlexanderB]

Either a transaction or a lua script with those two commands is exactly the way to go. 

Zpop has been one of the classic use cases for scripting, here's a quick one that's been sitting around on github for close to 5 years. https://gist.github.com/droot/1033867

You can also see a way of doing it with a WATCH style transaction in the redis docs. 

http://redis.io/topics/transactions

[Adityanarayan Vaidya]

Thanks Ita.

Regards,

Aditya

[Adityanarayan Vaidya]

Thanks Alex.

Regards,

Aditya