Paul Joseph Davis:
For a bit of background, CouchDB has had an API for managing external OS processes  that are capable of handling HTTP requests for a given URL prefix. These OS processes communicate with CouchDB using JSON over stdio. They’re dead simple to write and provide CouchDB users an easy way to extend CouchDB functionality.
Even though they’re dead simple to write, there are a few issues. The implementation in CouchDB does not provide fancy pooling semantics. The current API is explicitly synchronous which prevents people from writing
event driven code in an external handler. In the end, they may be simple, but their simplicity is also quite limiting.
Original title and link: CouchDB Externals API: Did you Know About it? (NoSQL databases © myNoSQL)