There are quite a few documents on the architecture of Couchbase Server including the appropriate sections in the manual for 1.8 and the manual for 2.0.  Those obviously reflect how the components come together, but much like an automotive mechanic has a different view of your vehicle than you do, developers of Couchbase see things a little differently.

Couchbase Component Architecture

All of the source for Couchbase Server resides on github.