libcouchbase is a callback oriented client which makes it very easy to
write high performance, thread safe programs. If you're interested in
the early history of libcouchbase you might want to check out the blog
post

http://trondn.blogspot.com/2011/10/libcouchbase-explore-full-features-of.html

The key component of libcouchbase is that its asynchronous, giving you
full freedom in adding it to your application logic. From using the
asynchronous interface you may schedule a lot of operations to be
performed, and then you'll get the callbacks whenever they are
performed. I do know that there are a lot of people who don't care
about an async interface, so you may also enable synchronous
mode. When synchronous mode is enabled you can't use this batching.