You can never have multi-master with MongoDB, which is required for "always writable." However, it can be readable. Our CEO did a series of posts on distributed consistency, see http://blog.mongodb.org/post/475279604/on-distributed-consis....
One love.
- Lil' B
This does sacrifice availability (in the CAP sense), but I haven't heard anyone claim otherwise.
One would hope that reading and writing a single node database was consistent. This is table stakes for something calling itself a persistent store. Claiming partition tolerance in the above is the same as claiming availability. The former claim has been made. Rest left as exercise for the reader.
Namasté.
- Lil' B
-------------------
Choosing Consistency Over Availability
If a system chooses to provide Consistency over Availability in the presence of partitions (again, read: failures), it will preserve the guarantees of its atomic reads and writes by refusing to respond to some requests. It may decide to shut down entirely (like the clients of a single-node data store), refuse writes (like Two-Phase Commit), or only respond to reads and writes for pieces of data whose "master" node is inside the partition component (like Membase).
This is perfectly reasonable. There are plenty of things (atomic counters, for one) which are made much easier (or even possible) by strongly consistent systems. They are a perfectly valid type of tool for satisfying a particular set of business requirements.
-------------------