The relational database may not be dead, and so-called NoSQL solutions may have been slightly overhyped, but that isn’t stopping investors from betting that the market for new types of data stores is going to be very, very big.

The latest example of that: 10gen, which is behind one of the more prominent NoSQL databases, MongoDB, has just raised a new $42m round of funding.

As reported by VentureBeat’s Sean Ludgwig, 10gen’s round of funding was provided by New Enterprise Associates, Sequoia Capital, Union Square Ventures and Flybridge Capital. All told, 10gen has now raised more than $70m.

That’s a lot of money for a company that generates revenue primarily through services like training and support. Although the company does offer commercial licenses, the MongoDB database and drivers are available for free under open-source licenses. That, of course, doesn’t mean that investors are throwing their money away. As we’ve seen from companies like MySQL (which was acquired by Sun, which was acquired by Oracle) and Red Hat (which now has more than $1bn in annual revenue), there is money in open source,

Success, however, isn’t guaranteed. One of the problems facing NoSQL providers like 10gen is that, in a rush to move away from relational database, some companies may have been a little too optimistic about what NoSQL solutions can offer. No solution is perfect, and that includes the latest and greatest data stores. Case in point: ad startup Kiip recently detailed publicly how it ran into problems with MongoDB and moved away from the next-gen database. Kiip’s experience isn’t unique, raising questions about MongoDB’s future prospects.

Of course, you’ll find criticism of some of the most widely used relational databases. So criticism of the latest crop of companies trying to become innovative forces in the database space shouldn’t come as a surprise. Which solutions succeed long-term, of course, remains to be seen, but with valuations in some of the other tech markets, namely the consumer internet, reaching incredibly frothy levels, expect more investors to play the “when there’s a gold rush, sell tools to the miners” strategy, leading to more sizable investments in companies like 10gen.