Question

The think I’m trying to implement is an id table. Basically it has the structure (user_id, lecturer_id) which user_id refers to the primary key in my User table and lecturer_id refers to the primary key of my Lecturer table.

I’m trying to implement this in redis but if I set the key as User’s primary id, when I try to run a query like get all the records with lecturer id=5 since lecturer is not the key, but value I won’t be able to reach it in O(1) time.

How can I form a structure like the id table I mentioned in above, or Redis does not support that?

Answer 1

One of the things you learn fast while working with redis is that you get to design your data structure around your accessing needs, specially when it comes to relations (it’s not a relational database after all)

There is no way to search by “value” with a O(1) time complexity as you already noticed, but there are ways to approach what you describe using redis. Here’s what I would recommend:

• Store your user data by user id (in e.g. a hash) as you are already doing.
• Have an additional set for each lecturer id containing all user ids that correspond to the lecturer id in question.

This might seem like duplicating the data of the relation, since your user data would have to store the lecture id, and your lecture data would store user ids, but that’s the (tiny) price to pay if one is to build relations in a no-relational data store like redis. In practical terms this works well; memory is rarely a bottleneck for small-ish data-sets (think thousands of ids).

To get a better picture at how are people using redis to model applications with relations, I recommend reading Design and implementation of a simple Twitter clone and the source code of Lamernews, both of which are written by redis author Salvatore Sanfilippo.