Document Databases are an important part of the NoSql movement taking place. They are appealing to developers because of their intuitive Apis and also to stakeholders as they lower the boundaries to feature development that would have once been hugely difficult a Sql-only environment. Some key benefits of document databases include:
- They permit fast development of complex objects graphs, making it easy to employ DDD. Though hand-scripting stored procedures are now largely a thing of the past thanks to modern ORMs, they are difficult to retro-fit to a legacy system and can introduce new bugs of their own. I would argue that there is more confidence in storing and accessing aggregates roots in a document database as their is 'truer' representation of the model being stored on disk, while with an ORM there will always be a layer of translation.
- Are schema-less and therefore 'backwards compatible', meaning no application downtime while the database is administered. Again, ORMs have advanced enough to support deployment tasks, see Microsoft Web Deploy and RoundhousE of the Chuck Norris Dev Ops suite, but the risk still exists of losing or corrupting data with a tool or not.
- When you need to scale, document databases have been designed from the ground up to be able to distribute the data across large numbers of machines with aim of spreading system load, increasing availability and preventing failures. There is a physical limit that Sql will reach when scaling simply by throwing more resources at it (vertical scaling). Document databases deal with sharding more effectively as they don't have relationships to maintain as in a relational SQL database. There are ways in which you can scale a sql database that are not so simple.
- When you need to perform calculations over a large dataset very quickly. Document databases lend themselves to map-reduce type queries especially well because of their distributed nature. Map reduce queries are designed to be performed in parallel, which is a great fit for a shared document database. Queries are also optimized for 'eventual consistency', meaning that result accuracy is traded for low response times.
In modern applications, its hard to argue that any of these requirements can be overlooked. But if you don't have a bursty traffic model and do not need a globally accessible service then you are not yet excluded from using a Sql database...
Sql databases still have a very good use. Leaving aside the fact that they remain largely a more stable platform due to there maturity compared with the relative youthful NoSql technologies, Sql databases make a great tool for data-mining. That is, to make use of the Sql JOIN operator across multiple tables to 'slice' the data. If you are working with a normalized schema, Sql queries provide the ability to draw conclusions from a body of data with more freedom than a document database. Document databases demand that you specify the kind of report you will want up front at design-time. With a Sql database, or at least a tool that provides a Sql type language interface, you can build up reports based on the relationships built into the tables.
By reporting in this context, I primarily mean the types of back end office reports that might be needed on a daily, weekly or monthly basis. These type of reports are largely back office type reports, that tend to grow as the business rules change. The type of reports I mean reveal insight and are built up further on top of existing tables and views. This type of gradual evolution is brought about by in-house report writers, who know the schema well. These reports take time to write, as well as time to run. But that time is acceptable because of the advantages that they lend the business.
In contrast, when lots of users need to see top level summaries of data quickly, a document database would be a much better tool for the job. Dashboards headline figures can be useful debugging as well as marketing tools, but they are not the type of reports that are suited to a Sql database. When website response time is important, do not put a report generated from a Sql database on the home page (hopefully we all learned this by now, right?!)
It is easy to get caught up in the momentum and success of the NoSql movement. But it can be difficult to tell what benefits it will bring to your application, and at what cost. Hopefully this article offers you a balanced view. Its fun to be an early adopter, but I would exercise caution before binning the old Sql reporting database in favour of the latest and greatest.