Redis List Datatype

Introduction#

The List datatype in Redis is an ordered collection of items referenced by a Redis key. Redis allows you to access and modify a list by index or push/pop operations. In Redis, the two ends of a list are referred to as the left and the right. The left corresponds to the first element or head of a list and the right coresponds to the last element or tail of a list.

Syntax#

LPUSH key value [value …]
RPUSH key value [value …]
LPOP key
RPOP key
LLEN key

Remarks#

More detail on the List datatype and all the commands that can be used in conjunction with them can be found in the official Redis documentation at Redis.io.

Adding Items to a List

Redis allows you to add items to either the right or the left of a list.

If I was working with a list, my_list and I wanted to prepend 3 to the list, I could do that using the Redis LPUSH command:

LPUSH my_list 3

If I wanted to append 3 to my_list, I would instead use the RPUSH command:

RPUSH my_list 3

Both the LPUSH and RPUSH command will automatically create a new list for you if the supplied key doesn’t exist. Two alternative commands LPUSHX and RPUSHX can be used to only operate on the list key if it already exists.

Getting Items from a List

Redis provides the LPOP and RPOP commands as a counterpart to the LPUSH and RPUSH commands for fetching data items.

If I was working with a list my_list that had several data items in it already, I can get the first item in the list using the LPOP command:

LPOP my_list

The result of this command will return the value of the first element from the list and remove it from my_list. For example, if I had the list [1, 3, 2, 4] and I applied LPOP to it, I would have the list [3, 2, 4] in memory afterwards.

Similarly, I can remove from the end of the list using RPOP:

RPOP my_list

would return the value fo the last element form the list and then remove it from my_list. Using our example, [1, 2, 3, 4] after calling RPOP on this list, the list in memory would be [1, 2, 3].

Size of a List

The size of a Redis list can be deterimed using the LLEN command. If I have a four element list stored at the key my_list, I can get the size using:

LLEN my_list

which will return 4.

If a user specifies a key that doesn’t exist to LLEN, it will return a zero, but if a key is used that points to an item of a different datatype, an error will be returned.