When to use MongoDB or other document oriented database systems? [closed]

Question

We offer a platform for video- and audio-clips, photos and vector-grafics. We started with MySQL as the database backend and recently included MongoDB for storing all meta-information of the files, because MongoDB better fits the requirements. For example: photos may have Exif information, videos may have audio-tracks where we to want to store the meta-information of, too. Videos and vector-graphics don't share any common meta-information, etc. so I know, that MongoDB is perfect to store this unstructured data and keep it searchable.

However, we continue developing our platform and adding features. Now one of the next steps will be providing a forum for our users. The question that now arises is: use the MySQL database, which would be a good choice for storing forums and forum-posts, etc. or use MongoDB for this, too?

So the question is: when to use MongoDB and when to use a RDBMS. What would you take, mongoDB or MySQL, if you had the choice and why would you take it?

Answer 1

In NoSQL: If Only It Was That Easy, the author writes about MongoDB:

MongoDB is not a key/value store, it’s quite a bit more. It’s definitely not a RDBMS either. I haven’t used MongoDB in production, but I have used it a little building a test app and it is a very cool piece of kit. It seems to be very performant and either has, or will have soon, fault tolerance and auto-sharding (aka it will scale). I think Mongo might be the closest thing to a RDBMS replacement that I’ve seen so far. It won’t work for all data sets and access patterns, but it’s built for your typical CRUD stuff. Storing what is essentially a huge hash, and being able to select on any of those keys, is what most people use a relational database for. If your DB is 3NF and you don’t do any joins (you’re just selecting a bunch of tables and putting all the objects together, AKA what most people do in a web app), MongoDB would probably kick ass for you.

Then, in the conclusion:

The real thing to point out is that if you are being held back from making something super awesome because you can’t choose a database, you are doing it wrong. If you know mysql, just use it. Optimize when you actually need to. Use it like a k/v store, use it like a rdbms, but for god sake, build your killer app! None of this will matter to most apps. Facebook still uses MySQL, a lot. Wikipedia uses MySQL, a lot. FriendFeed uses MySQL, a lot. NoSQL is a great tool, but it’s certainly not going to be your competitive edge, it’s not going to make your app hot, and most of all, your users won’t care about any of this.

What am I going to build my next app on? Probably Postgres. Will I use NoSQL? Maybe. I might also use Hadoop and Hive. I might keep everything in flat files. Maybe I’ll start hacking on Maglev. I’ll use whatever is best for the job. If I need reporting, I won’t be using any NoSQL. If I need caching, I’ll probably use Tokyo Tyrant. If I need ACIDity, I won’t use NoSQL. If I need a ton of counters, I’ll use Redis. If I need transactions, I’ll use Postgres. If I have a ton of a single type of documents, I’ll probably use Mongo. If I need to write 1 billion objects a day, I’d probably use Voldemort. If I need full text search, I’d probably use Solr. If I need full text search of volatile data, I’d probably use Sphinx.

I like this article, I find it very informative, it gives a good overview of the NoSQL landscape and hype. But, and that's the most important part, it really helps to ask yourself the right questions when it comes to choose between RDBMS and NoSQL. Worth the read IMHO.

Alternate link to article

Answer 2

After two years using MongoDb for a social app, I have witnessed what it really means to live without a SQL RDBMS.

1. You end up writing jobs to do things like joining data from different tables/collections, something that an RDBMS would do for you automatically.
2. Your query capabilities with NoSQL are drastically crippled. MongoDb may be the closest thing to SQL but it is still extremely far behind. Trust me. SQL queries are super intuitive, flexible and powerful. MongoDb queries are not.
3. MongoDb queries can retrieve data from only one collection and take advantage of only one index. And MongoDb is probably one of the most flexible NoSQL databases. In many scenarios, this means more round-trips to the server to find related records. And then you start de-normalizing data - which means background jobs.
4. The fact that it is not a relational database means that you won't have (thought by some to be bad performing) foreign key constrains to ensure that your data is consistent. I assure you this is eventually going to create data inconsistencies in your database. Be prepared. Most likely you will start writing processes or checks to keep your database consistent, which will probably not perform better than letting the RDBMS do it for you.
5. Forget about mature frameworks like hibernate.

I believe that 98% of all projects probably are way better with a typical SQL RDBMS than with NoSQL.

Answer 3

to store this unstructured data

As you said, MongoDB is best suitable to store unstructured data. And this can organize your data into document format. These RDBMS altenatives called NoSQL data stores (MongoDB, CouchDB, Voldemort) are very useful for applications that scales massively and require faster data access from these big data stores.

And the implementation of these databases are simpler than the regular RDBMS. Since these are simple key-valued or document style binary objects directly serialized into disk. These data stores don't enforce the ACID properties, and any schemas. This doesn't provide any transaction abilities. So this can scale big and we can achieve faster access (both read and write).

But in contrast, RDBM enforces ACID and schemas on datas. If you wanted to work with structured data you can go ahead with RDBM.

I would choose MySQL for creating forums for this kind of stuff. Because this is not going to scale big. And this is a very simple (common) application which has structured relations among the data.

Answer 4

Note that Mongo essentially stores JSON. If your app is dealing with a lot of JS Objects (with nesting) and you want to persist these objects then there is a very strong argument for using Mongo. It makes your DAL and MVC layers ultra thin, because they are not un-packaging all the JS object properties and trying to force-fit them into a structure (schema) that they don't naturally fit into.

We have a system that has several complex JS Objects at its heart, and we love Mongo because we can persist everything really, really easily. Our objects are also rather amorphous and unstructured, and Mongo soaks up that complication without blinking. We have a custom reporting layer that deciphers the amorphous data for human consumption, and that wasn't that difficult to develop.

Answer 5

I would say use an RDBMS if you need complex transactions. Otherwise I would go with MongoDB - more flexible to work with and you know it can scale when you need to. (I'm biased though - I work on the MongoDB project)

Answer 6

Who needs distributed, sharded forums? Maybe Facebook, but unless you're creating a Facebook-competitor, just use Mysql, Postgres or whatever you are most comfortable with. If you want to try MongoDB, ok, but don't expect it to do magic for you. It'll have its quirks and general nastiness, just as everything else, as I'm sure you've already discovered if you really have been working on it already.

Sure, MongoDB may be hyped and seem easy on the surface, but you'll run into problems which more mature products have already overcome. Don't be lured so easily, but rather wait until "nosql" matures, or dies.

Personally, I think "nosql" will wither and die from fragmentation, as there are no set standards (almost by definition). So I will not personally bet on it for any long-term projects.

Only thing that can save "nosql" in my book, is if it can integrate into Ruby or similar languages seamlessly, and make the language "persistent", almost without any overhead in coding and design. That may come to pass, but I'll wait until then, not now, AND it needs to be more mature of course.

Btw, why are you creating a forum from scratch? There are tons of open source forums which can be tweaked to fit most requirements, unless you really are creating The Next Generation of Forums (which I doubt).

Answer 7

I've seen at lot of companies are using MongoDB for realtime analytics from application logs. Its schema-freeness really fits for application logs, where record schema tends to change time-to-time. Also, its Capped Collection feature is useful because it automatically purges old data to keep the data fit into the memory.

That is one area I really think MongoDB fits for, but MySQL/PostgreSQL is more recommended in general. There're a lot of documentations and developer resources on the web, as well as their functionality and robustness.

Answer 8

The 2 main reason why you might want to prefer Mongo are

• Flexibility in schema design (JSON type document store).
• Scalability - Just add up nodes and it can scale horizontally quite well.

It is suitable for big data applications. RDBMS is not good for big data.

Answer 9

You know, all this stuff about the joins and the 'complex transactions' -- but it was Monty himself who, many years ago, explained away the "need" for COMMIT / ROLLBACK, saying that 'all that is done in the logic classes (and not the database) anyway' -- so it's the same thing all over again. What is needed is a dumb yet incredibly tidy and fast data storage/retrieval engine, for 99% of what the web apps do.

Answer 10

Like said previously, you can choose between a lot of choices, take a look at all those choices: http://kkovacs.eu/cassandra-vs-mongodb-vs-couchdb-vs-redis

What I suggest is to find your best combination: MySQL + Memcache is really great if you need ACID and you want to join some tables MongoDB + Redis is perfect for document store Neo4J is perfect for graph database

What i do: I start with MySQl + Memcache because I'm use to, then I start using others database framework. In a single project, you can combine MySQL and MongoDB for instance !