Checking if a value exists in a list already Redis

I'm wondering if there's a way to check if a key already exists in a redis list?

I can't use a set because I don't want to enforce uniqueness, but I do want to be able to check if the string is actually there.

Solution 1:

Your options are as follows:

1. Using LREM and replacing it if it was found.
2. Maintaining a separate SET in conjunction with your LIST
3. Looping through the LIST until you find the item or reach the end.

Redis lists are implemented as a http://en.wikipedia.org/wiki/Linked_list, hence the limitations.

I think your best option is maintaining a duplicate SET. This is what I tend to do. Just think of it as an extra index. Regardless, make sure your actions are atomic with MULTI-EXEC or Lua scripts.

Solution 2:

Lists allow duplicates but do not provide a simple way to check for existence and as @Fritzy advised, you either need to:

• Make multiple operations (remove then add again if found during removal) for simple checks = Cost in time
• Maintain a separate set = Cost in memory

I am surprised no one advised you to use either a Hash Table or a Sorted Set which combine advantages of allowing duplicity (by storing the number of elements as value - Hash Table, or score - Sorted Set) and indexing members by nature of a hash table/set.

Hash Table

To check for a key existence, use HEXISTS for a specific field which returns 0 if the specified member does not exist. You could also use the HGETcommand. It returns a nil answer if the specified member does not exist.

To add a new member, simply use HINCRBY which will either update the value (ie the number of elements with the member name) or create a new member if it does not exist.

Sorted Set

To check for a key existence, use either one of the three following commands:

• ZSCORE
• ZRANK
• ZREVRANK

They return a nil answer if the specified member does not exist.

To add a new member, simply use ZINCRBY which will either update the score (ie the number of elements with the member name) or create a new member if it does not exist.

To sum up: Sorted Sets or Hash Tables allow you to make all the operations with your requirements with a single command.