LLEN

Introduction and Use Case(s)

The LLEN command in Redis is used to get the length of a list stored at a specified key. This command is particularly useful in scenarios where you need to monitor the size of a list for purposes such as rate limiting, queue length monitoring, or validating that operations are performed correctly on lists.

Syntax

  1. LLEN key

Parameter Explanations

  • key: The name of the list whose length you want to retrieve. It must be an existing Redis list key.

Return Values

The length of the list at the specified key. If the key does not exist, it returns 0 because an absent list is considered to have zero elements.

Examples:

  • For a non-empty list: (integer) 3
  • For an empty list: (integer) 0
  • For a non-existent key: (integer) 0
  • For a key that holds a different type (like a string or set): Error message indicating wrong data type.

Code Examples

  1. dragonfly> RPUSH mylist "first"
  2. (integer) 1
  3. dragonfly> RPUSH mylist "second"
  4. (integer) 2
  5. dragonfly> RPUSH mylist "third"
  6. (integer) 3
  7. dragonfly> LLEN mylist
  8. (integer) 3
  9. dragonfly> DEL mylist
  10. (integer) 1
  11. dragonfly> LLEN mylist
  12. (integer) 0
  13. dragonfly> SET mystring "value"
  14. OK
  15. dragonfly> LLEN mystring
  16. (error) WRONGTYPE Operation against a key holding the wrong kind of value

Best Practices

  • Always ensure the key you’re using with LLEN refers to a list to avoid type errors.
  • Monitor your list lengths regularly if you’re using lists to handle tasks like job queues to prevent overflows and performance issues.

Common Mistakes

  • Using LLEN on keys that do not hold lists can lead to errors. Make sure to check the type of the key before performing operations.

FAQs

What happens if I use LLEN on a key that doesn’t exist?

LLEN will return 0 if the specified key does not exist in the database.

Can LLEN be used with other data types?

No, LLEN is specifically designed for lists. Using it with other data types will result in a WRONGTYPE error.