which database suits my application mysql or mongodb ? using Node.js , Backbone , Now.js
With MongoDB, you can just store JSON objects and retrieve them fully-formed, so you don't really need an ORM layer and you spend less CPU time translating your data back-and-forth. The developers behind MongoDB have also made horizontally scaling the database a higher priority and let you run arbitrary Javascript code to pre-process data on the DB side (allowing map-reduce style filtering of data).
But you lose some for these gains: You can't join records. Actually, the JSON structure you store could only be done via joins in SQL, but in MongoDB you only have that one structure to your data, while in SQL you can query differently and get your data represented in alternate ways much easier, so if you need to do a lot of analytics on your database, MongoDB will make that harder.
The query language in MongoDB is "rougher", in my opinion, than SQL's, partly because it's less familiar, and partly because the querying features "feel" haphazardly put together, partially to make it valid JSON, and partially because there are literally a couple of ways of doing the same thing, and some are older ways that aren't as useful or regularly-formatted as the others. And there's the added complexity of the array and sub-object types over SQL's simple row-based design, so the syntax has to be able to handle querying for arrays that contain some of the values you defined, contain all of the values you defined, contain only the values you defined, and contain none of the values you defined. The same distinctions apply to object keys and their values, and this makes the query syntax harder to grasp. (And while I can see the need for edge-cases, the $where query parameter, which takes a javascript function that is run on every record of the data and returns a boolean, is a Siren song because you can easily define what objects you want to return or not, but it has to run on every record in the database, no indexes can be used.)
So, it depends on what you want to do, but since you say it's for a Google Docs clone, you probably don't care about any representation but the document representation, itself, and you're probably only going to query based on document ID, document name, or the owner's ID/name, nothing too complex in the querying.
Then, I'd say being able to take the JSON representation of the document your user is editing, and just throw it into the database and have it automatically index these important fields, is worth the price of learning a new database.
I was also struggling with this choice looking at the hype created by using MongoDB for tasks it was not built for. So my 2 cents are:
Storing and retrieving hierarchical objects, that your documents probably are, is easier in MongoDB, as David says. It becomes more complicated if you want to store documents that are bigger than 16Mb though - MongoDB's answer is GridFS.
Organising documents in folders, groups, keeping track of which user owns which documents and who he/she provided access to them is definitely easier with MySQL - you have the advantage of powerful SQL queries with joins etc., built in EXPLAIN optimization, triggers, functions, stored procedures, etc. MongoDB is nowhere near.
So what prevents you from using both MySQL to organize the documents and MongoDB to store one collection of documents identified by id (or several collections - one for each document type)? It seems to me the best choice and using two databases in one application is not a problem, really.
MySQL will store users, groups, folders, permissions - whatever you fancy - and for each document it will store a reference to the collection and the document id (MongoDB has a special format for it - DBRefs). MongoDB will store documents themselves in collections, if they are all less than 16MB, or the previews and metadata of documents in collections and the whole documents in GridFS.
David provided a good answer. A few things to add to it.
- MongoDB's flexible nature permits for easy agile / iterative development.
- MongoDB like node.js is asyncronous in nature and works very well within asyncronous environments.
- Mongoose is a good ODM (object document mapper) that makes working with MongoDB with Node.js feel very natural. Unlike ORMs this is a very thin layer.
For Google Doc like functionality, the flexibility & very rich data structure provided by MongoDB feels like a much better fit.
You can find some good example posts by searching for mongoose, node and MongoDB.
Here's one that also uses backbone.js and looks good http://mattkopala.com/blog/2012/02/12/getting-started-with-nodejs/