how to pop multiple items from list

[Tony Wang]

Hi all

Is it possible to pop multiple items from a Redis list? I know that spop could do it with set. 

In my case, I have a producer server and a Redis server, a local machine as a consumer. Because of poor network between local consumer and Redis server, the consuming speed with rpop seems much slower than producing. 15-30 minutes slower in one hour. So I curious if I could get multiple items within one connection from Redis list, or is there any other better solution to speed up？

Thanks !

[Joshua Scott]

You can use LRANGE and LREM inside a MULTI/EXEC to atomically return and remove a certain number of elements from a list.
https://redis.io/commands/lrange
https://redis.io/commands/lrem
https://redis.io/commands/multi

127.0.0.1:6379> RPUSH mylist a b c d

(integer) 4

127.0.0.1:6379> LRANGE mylist 0 -1

1) "a"

2) "b"

3) "c"

4) "d"

127.0.0.1:6379> MULTI

OK

127.0.0.1:6379> LRANGE mylist 0 1

QUEUED

127.0.0.1:6379> LTRIM mylist 2 -1

QUEUED

127.0.0.1:6379> EXEC

1) 1) "a"

   2) "b"

2) OK

127.0.0.1:6379> LRANGE mylist 0 -1

1) "c"

2) "d"

[Itamar Haber]

...or use Lua :)

$ cat lpopn.lua

local key = KEYS[1]

local n = tonumber(ARGV[1])

local rep = {}

local ele = 1

while n > 0 and ele do

  ele = redis.call('LPOP', key)

  n = n - 1

  if ele then

    rep[#rep+1] = ele

  end

end

return rep

$ redis-cli lpush l a b c d e

(integer) 5

$ redis-cli --eval lpopn.lua l , 2

1) "e"

2) "d"

$ redis-cli --eval lpopn.lua l , 4

1) "c"

2) "b"

3) "a"

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Itamar Haber | Chief OSS Education Officer
Redis Labs ~/redis

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