LLEN for list length in Redis

Introduction

LLEN helps you find out how many items are in a list stored in Redis. It tells you the list's length quickly.

You want to check how many messages are waiting in a chat queue.

You need to know how many tasks are in a to-do list stored in Redis.

You want to monitor the size of a list to decide if you should add more items.

You want to confirm if a list is empty before processing it.

You want to limit the number of items processed by checking the list length first.

Syntax

Redis

LLEN key

Replace 'key' with the name of your Redis list.

If the key does not exist, LLEN returns 0.

Examples

Returns the length of the list stored at 'mylist'.

Redis

LLEN mylist

Finds out how many tasks are waiting in the 'tasks_queue' list.

Redis

LLEN tasks_queue

Returns 0 if 'emptylist' does not exist or has no items.

Redis

LLEN emptylist

Sample Program

First, we add three fruits to 'mylist'. Then, we check the length of 'mylist', which is 3. Finally, we check the length of a list that does not exist, 'unknownlist', which returns 0.

Redis

127.0.0.1:6379> RPUSH mylist "apple"
(integer) 1
127.0.0.1:6379> RPUSH mylist "banana"
(integer) 2
127.0.0.1:6379> RPUSH mylist "cherry"
(integer) 3
127.0.0.1:6379> LLEN mylist
(integer) 3
127.0.0.1:6379> LLEN unknownlist
(integer) 0

Important Notes

LLEN runs in constant time O(1), so it is very fast even for long lists.

It uses very little memory since it only returns a number.

A common mistake is to expect LLEN to return the list items; it only returns the count.

Use LLEN when you only need the size of the list, not the items themselves.

Summary

LLEN returns the number of items in a Redis list.

If the list does not exist, LLEN returns 0.

It is a fast and simple way to check list size.