Why I think Mongo is to Databases what Rails was to Frameworks

Spot on, I say. Mongo is usually filed in the K/V bin, while in reality it actually falls into a much more exciting middle ground between RDMBS, document store and K/V store.

Mongo's killer feature for me is the support. The team has been absolutely awesome about helping me track down bugs and sort out my issues.

By contrast I've spent a lot of time on Tokyo, but I'm actively trying to move away from it these days because there have just been too many mysterious problems. I've tracked down some of them and submitted patches, but it's cost me a lot of time I'd rather be spending working on my product. Don't get me wrong, Tokyo is chock-full of great code, but anything as general-purpose as a database is going to have all sorts of interface and configuration issues that require more time than your typical open-source project founder can put in.

Agreed, the comparison of Mongo to databases as Rails is to frameworks is appropriate. I recently started playing with Mongo in a Rails app and the Array type led to an a-ha moment followed by giddy handclapping and a fair amount of code deletion.

Working at a large company that's pretty invested in RDBMS, I'm curious to see where the downside of using something like MongoDB comes in. E.g. James Golick's creation of Friendly because they hit a scaling wall — how big do you have to get for it to become an issue?

MongoDB really is simple to use. Unlike CouchDB, you can make adhoc queries without defining map/reduce functions. In MongoDB, array elements are indexed so searching for words, tags, etc. is easy. The Ruby client support is excellent. Casandra is really easy to set up (installing the cassandra gem, then doing 'cassandra_helper cassandra' installs all cassandra dependencies). But Casandra requires a lot of application specific configuration. I always leave MongoDB running on my MacBook and two of my servers so it is ready at hand.

From my personal experiences with the people who are working full-time on Mongo and Rails, there is little comparison in their skill level - the guys developing Mongo are without doubt better hackers.

They're really responsive to feedback. Documentation is great and usually you get questions in the mailing list answered same-day (within a few hours) by one of the staff.

I've been using MongoMapper with Rails for a while and I've really enjoyed it. It's so freeing to be able to include embedded documents or other modules and not have to deal with migrations.

ActiveRecord is great, but MongoDB and MongoMapper feel so much more natural to me.

This looks good. Care to reveal some plans of this product? Is it going to be commercial or open source? Thanks for sharing.

i think the main issue about mongodb is its license, AGPL 3, and they should think about it. from feature pov, it has lots of small niceties but real big thing about mongodb is auto-sharding and this feature is still alpha.