Today, someone came to the #couchdb channel asking about
_externals. For a long while it's been on my mind that perhaps
we should deprecate the entire _externals feature for a number
of reasons:
1. Couch is not a great reverse proxy. Making it into one is
as hard as rewriting nginx or haproxy in erlang. It's a
distraction to our development team and far outside our
core competency.
2. In a clustered CouchDB (the default in 2.0), the
assumptions around externals change drastically. For an
_external to work, it must be stateless and not rely upon
multiple sequential requests to hit the same node (assuming
the standard n-node cluster + a load balancer/reverse proxy
at the front.)
People who wrote a CouchDB 1.x external could reasonably
expect to write an old-school singleton app (i.e., the only
copy of that external process running, on a single machine).
If they engaged in any of a number of bad behaviours for
distributed systems - storing content on local disk, locking
or blocking connections to other services/databases in a
"single-threaded" pattern, or even expecting CouchDB not to
possibly introduce a conflict or "read your writes" - they
will probably fail outright at best, or at worst introduce
subtle and confusing behaviour.
TL;DR: We're changing the contract we give to _externals in a
reverse-compatibility-breaking way. We either need to document it
straight up, along with all of the admonishments required for
people who expect it to operate the same as in 1.x, or we need to
remove it.
My opinion is that now that the default CouchDB rollout will be
a cluster with a reverse proxy, that _externals should be exposed
through the load balancer, which can then reference 1 or more
processes distributed either on the same CouchDB nodes, or on
different hosts should compute needs demand it.
The exception here would be a single-node CouchDB, which could
still use the same approach. However I don't see the issue with
deploying an haproxy on that same node and using the same approach
I describe above.
Thoughts, comments, suggestions?
-Joan
