What exactly is NoSQL?

I'm not agree with the answers I'm seeing, although it's true that NoSQL solutions tends to break the ACID rules, not all are created from that approach.

I think first you should define what is a SQL Solution and then you can put the "Not Only" in front of it, that will be more accurate definition of what is a NoSQL solution.

With this approach in mind:

SQL databases are a way to group all the data stores that are accessible using Structured Query Language as the main (and most of the time only) way to communicate with them, this means it requires that the database support the structures that are common to those systems like "tables", "columns", "rows", "relationships", etc.

Now, put the "Not Only" in front of the last sentence and you will get a definition of what means "NoSQL". NoSQL groups all the stores created as an attempt to solve problems which cannot fit into the table/column/rows structures or even in SQL Statements, in most of the cases these databases will not support relationships, they're abandoning the well known structures just because the problems have changed since their conception.

If you have a text file, and you create an API to store/retrieve/organize this information, then you have a NoSQL database in your hands.

All of these means that there are several solutions to store the information in a way that traditional SQL systems will not allow to achieve better performance, flexibility, etc etc. Every NoSQL provider tries to solve a different problem and that's why you wont be able to compare two different solutions, for example:

• djondb is a document store created to be used asNoSQL enterprise solution supporting transactions, consistency, etc.but sacrifice performance of its counterparts.
• MongoDB is a document store (similar todjondb) which accomplish great performance but trades some of theACID properties to achieve this.
• CouchDB is another document store whichsolves the queries slightly different providing views to retrieve theinformation without doing a full query every time.
• ...

As you may have noticed I only talked about the document stores, that's because I wanted to show you that 3 different document stores implementations have different approach, therefore you should keep in mind the golden rule of NoSQL stores "Use the right tool for the right job".

I'm the creator of djondb and I've been doing a lot of research even before trying to start my own NoSQL implementation, but this is a field where the concepts will keep changing the way we see the information storage.

From wikipedia:

NoSQL is an umbrella term for a loosely defined class of non-relational data stores that break with a long history of relational databases and ACID guarantees. Data stores that fall under this term may not require fixed table schemas, and usually avoid join operations. The term was first popularised in early 2009.

The motivation for such an architecture was high scalability, to support sites such as Facebook, advertising.com, etc...

To quickly get a handle on NoSQL systems, see this blog post I wrote: Visual Guide to NoSQL Systems. Essentially, NoSQL systems sacrifice either consistency or availability in favor of tolerance to network partitions.