MongoDB Cache or Not Cache using Redis

Question

In my project, main database is mongodb and for caching, i have redis. Now for long and more complex queries, it is obviously better that i use redis to cache them.

But i'm wondering if i should cache simple queries like lookup by id, or lookup by some other mongodb indexed field? Does it make sense to use redis for this kind of indexed lookup ?

or should i just not cache this kind of query because mongodb already has good caching mechanism internally?

Is it faster looking up to mongodb indexed field or is it faster to lookup to redis?

Answer 1

1. Lookup in Redis is definitely faster (because of the key-value nature of Redis).

2. MongoDB Can't cache queries' results:

MongoDB is a Database and can't cache the result of queries for you because data may change anytime. So managing the cache is the responsibility of the Developer.

But also the MongoDB has some good internal mechanisms to use the RAM for better performance. (check this Question for more info)

3. DataBase query is expensive:

When you are executing a query in MongoDB, there will be many processes to find data, even on simple queries. But Redis can find a key very, very fast. So it's clear that you must use Redis for keeping things and use MongoDB only for permanent storage and queries.

My recommendation:

It's recommended to cache any high-usage or heavy query's results in the Redis, Memcached, or other key-value in-memory storage. (It doesn't make sense to look up a simple post in Database/MongoDB a thousand times per day. It's just wasting of resources. The first duty of Cache systems is to keep high-usage data closer)

Also attention you must have a good "cache invalidation" mechanism to update cached data in Redis.

I recommend use the write-through technique to keep models and data in Redis.

I hope this helps.