How to Enable Erlang Views

Since version 0.10.0, CouchDB has a native Erlang view server, allowing you to write your map/reduce functions in Erlang. There is no-longer the need to manually install erlview, unless you are running an old version of CouchDB.

First, you'll need to edit your local.ini to include a native_query_servers section:

[native_query_servers]
erlang = {couch_native_process, start_link, []}

Your local.ini will most likely be at /usr/local/etc/couchdb/local.ini or /etc/couchdb/local.ini. To see these changes you will also need to restart the server:

sudo /etc/init.d/couchdb restart

To test out using Erlang views, visit the Futon admin interface, create a new database and open a temporary view. You should now be able to select erlang from the language drop-down.

Let's try an example of map/reduce functions which count the total documents at each number of revisions (there are x many documents at version "1", and y documents at "2"... etc). Add a few documents to the database, then enter the following functions as a temporary view:

%% Map Function
fun({Doc}) ->
  <<K,_/binary>> = proplists:get_value(<<"_rev">>, Doc, null),
  V = proplists:get_value(<<"_id">>, Doc, null),
  Emit(<<K>>, V)
end.

%% Reduce Function
fun(Keys, Values, ReReduce) -> length(Values) end.

If all has gone well, after running the view you should see a list of the total number of documents at each revision number.