I've spent a few days researching the pros and cons of mysql against nosql solutions (specifically mongodb) for my project.
The project needs to be able to eventually scale to handle tens of thousands of simultaneous users - millions of users in total. The site is heavily user focussed and will interact with the database as much if not more than a site like facebook - it is very relational, all functionality is dependant on the relation to the user and their relationship with other users. It's also data heavy - lots of files, images, audio, messaging, personal news feed etc.
I like the look on mongodb a lot, I like the way it works, and I like how it scales - but can't get my head around how this would work for a site such as I describe. Would all interactions for a specific user have to be stored in a single document?
I am however very comfortable using mysql and like the relational aspect of it. I am just worried without a lot of work there will be scalability issues with this project - although perhaps with memcached and sharding this won't be an issue?
I'd like to know from those with experience with the two databases on large projects, out of mysql and mongodb which is the right tool for this particular job?
If the data is highly relational, use a relational database. If it's not, don't. NoSQL is great, don't get me wrong, but it's not suited to all tasks. It may be suited to your task, but the only way to find out is for you to build some tests for your specific usecase. Add a bunch of dummy data (millions if not hundreds of millions of rows). And then load test it.
As far as scaling, that's more of a component of how you build your application than the backend you choose. Do you have a solid schema? Do you have a strong cache layer with write-through caching? Do you access the backend as efficiently as possible (queries and such)? Can you shard based upon your application?
Those are the kind of questions which are appropriate here. Not "which will scale for me better". And not "which is the right tool". Both can do the job fine. Which is best is up to you...