Get off the Table
10gen is hardly alone in splitting from MySQL's path – so-called NoSQL databases are the hype du jour – but Merriman argues that MongoDB is better suited than many to running modern day "cloud" applications because it uses a document-oriented data model. Mapping object-oriented code to a relational database is "painful work," Merriman says, and he argues that you have much the same problem with tabular NoSQL databases such as Google's proprietary BigTable, which underpins App Engine, and BigTable's open source doppelgänger, HBase
"Today, 95 per cent of the code we're writing is in an object-oriented language," he says. "We're to the point where object-oriented programming is ubiquitous enough, having a database that works well with that sort of thing is important." There are certain advantages to a tabular model – it's closer to what the average database programmer is used to – but you still have to map from objects in programs to tables.
"JSON gives us a standards-based, language-independent way to store object-style data. So we're not storing methods. We're not storing class hierarchies. We're not storing the object 'graph' of your app. But we are storing the documents that are data of objects, including a good amount of embedding and nesting and lists. So the documents are pretty rich."
Speaking with The Register, Merriman acknowledged that MongoDB isn't the only NoSQL that uses a document-oriented model. CouchDB is the other obvious example. And he made it clear that MongoDB and other NoSQL database can't completely replace relational databases. They're not suited to complex transactions, and ad hoc queries aren't as easy. But MongoDB can scale, he says, and it can still handle ad hoc queries, and it offers strong consistency.
"We want Mongo to be a full-featured product, but it's never going to be quite as full-featured as a traditional RDMS. We're always going to leave a few things out so we can scale," Merriman says. Mongo may be suited to some tasks behind the firewall, but it is particularly suited for services in the cloud. Like the one it was originally built for. Today, MongoDB can be used in tandem with the new platform clouds from VMware (Cloud Foundry) and Red Hat (OpenShift), and there's a plug-in for the Ruby on Rails cloud from Heroku. "We want Mongo to be used in many, many platform-as-a-service stacks," Merriman says.
You can't run it with App Engine. Google's platform is necessarily tied to the company's own BigTable-based back-end infrastructure. Apparently, MongoDB owes a debt to BigTable. Merriman indicates that 10gen's distributed data–sharing setup is based on Google's famous BigTable research paper. But data sharing aside, MongoDB is very much a different animal.
With the introduction of Megastore – the high-availability layer built atop BigTable – Google's setup is closer to Mongo. "We read [Google's Megastore research paper] and we were almost laughing at the similarities," Merriman says. But mapping object-oriented code to a tabular database like BigTable, he argues, still lacks a certain elegance. And we would add that MongoDB is open source. ®