MongoDB as a cache for frequent joins and queries

Question

Im thinking about doing the following and need suggestions if it makes sense to approach it this way. Basically since I am able to do queries in MongoDB and MongoDb is wicked fast at these since the hotspots of the data are cached in memory. I was thinking of storing data I would normally do a join from in mysql in mongoDB. While I am using memcached to store simple query results (for example a movie description page), for bigger stuff that requires more realtime/ondemand queries I was thinking about storing this in MongoDB. For example the view count for movies and who saw it, and doing analysis on it.

Hopefully I explained it clearly.

more info:

We dont want to keep writing to our mysql server on every rating like etc, MongoDB seemed like a good option to store the ratings,views of movies etc and then later on be able to do processing on that data. Whereas with Memcached data is not persisted and were unable to do queries

Thanks, Faisal

Answer 1

What your talking about is the idea of having normalized and de-normalized data. Using MongoDB as a denormalized data store for your normalized sql data is fine. Using Mongodb as your only data store for certain kinds of data is also fine. Just make sure that its clear in the system design as to where the true data is, and where the denormalized data is.

Normalized data is true fact. Denormalized data is gossip - you aren't sure if its up to date or not.

Answer 2

There's nothing wrong with using both solutions in your application. As a matter of fact, I'm using mysql to store user sessions as suppose to cookies. In addition, I have another project that utilizes mysql but for certain parts of my application, I will be using MongoDB. Why? It's wicked fast and I hate writing join queries. It's so much easier to pop data in/out of mongo as suppose to having to do join queries in mysql.

i.e.

When saving tags for a particular user, it's so gosh darn eary to save/modify/delete the tag that's stored in mongodb. With MySQL, I would've had to write a query that JOINs multiple tables. For data such as user account, password, city, state - I saved everything into MySQL.

Answer 3

Memory caching alone is not a good reason to go with MongoDB. Any properly configured RDBMS will cache frequently used data in memory.

What aspect of MySQL is currently limiting your performance? Do you have enough RAM in your server? Are your disks fast enough? Do you have a low latency cache device like an SSD configured appropriately?