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Lists

  • A sequence of ordered elements 10,20,1,2,3 is a list

  • Redis lists are implemented via Linked Lists

  • Adding a new element in the head with the LPUSH command or in the tail of the list is performed in constant time

  • Same for a list with 10 or 1M elements

  • Every list value (Element) can be up to 512MB

  • Accessing an element by index is not so fast in lists implemented by linked lists

  • Redis Lists are implemented with linked lists

Basic API of Lists

  • LPUSH command adds a new element 
into a list, on the left (at the head)
  • RPUSH command adds a new element 
into a list ,on the right (at the tail)
  • LRANGE command extracts ranges of 
elements from lists
  • Takes two indexes, the first and the last element 
of the range to return
  • Both the indexes can be negative
  • -1 is the last element 
-2 is the penultimate element of the list, and so forth

Example:

127.0.0.1:6379> rpush mylist A
(integer) 1
127.0.0.1:6379> rpush mylist B
(integer) 2
127.0.0.1:6379> lpush mylist first
(integer) 3
127.0.0.1:6379> lrange mylist 0 -1
1) "first"
2) "A"
3) "B"
127.0.0.1:6379>

Please Note:

  • LPUSH and RPUSH allow for variable number of parameters commands
  • You can push multiple elements into a list in a single call

Example:

127.0.0.1:6379> rpush mylist 2 3 4 5 "foo bar"
(integer) 8
127.0.0.1:6379> lrange mylist 0 -1
1) "first"
2) "A"
3) "B"
4) "2"
5) "3"
6) "4"
7) "5"
8) "foo bar"
127.0.0.1:6379>

RPOP and LPOP

Popping elements is the operation of both

  • retrieving the element from the list, and
  • eliminating it from the list
127.0.0.1:6379> lrange mylist 0 -1
1) "first"
2) "A"
3) "B"
4) "2"
5) "3"
6) "4"
7) "5"
8) "foo bar"
127.0.0.1:6379> rpush mylist a b c
(integer) 11
127.0.0.1:6379> rpop mylist
"c"
127.0.0.1:6379> rpop mylist
"b"
127.0.0.1:6379> rpop mylist
"a"
127.0.0.1:6379> rpop mylist
"foo bar"
127.0.0.1:6379>

Common Usage of Lists

  • 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

        LPUSH                        RPOP

Producer =======> | M3 | M2 | M1 | =========> Consumer

  • Redis has special list commands to make this use case both more reliable and efficient  BRPOP and BLPOP are blocking xPOP operation which will wait for a value to be in the list

      LPUSH                        BRPOP

Producer =======> | M3 | M2 | M1 | =========> Consumer

Capped Lists using LTRIM

  • Use the LTRIM command to use lists as a capped collection,
  • Only remembering the latest N items and discarding all the oldest items

Example:

127.0.0.1:6379> flushall
OK
127.0.0.1:6379> rpush mylist 1 2 3 4 5 
(integer) 5
127.0.0.1:6379> lrange mylist 0 -1
1) "1"
2) "2"
3) "3"
4) "4"
5) "5"
127.0.0.1:6379> ltrim mylist 0 2
OK
127.0.0.1:6379> lrange mylist 0 -1
1) "1"
2) "2"
3) "3"
127.0.0.1:6379>