.. Licensed under the Apache License, Version 2.0 (the "License"); you may not .. use this file except in compliance with the License. You may obtain a copy of .. the License at .. .. http://www.apache.org/licenses/LICENSE-2.0 .. .. Unless required by applicable law or agreed to in writing, software .. distributed under the License is distributed on an "AS IS" BASIS, WITHOUT .. WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the .. License for the specific language governing permissions and limitations under .. the License. .. highlight:: ini ============= Query Servers ============= .. _config/query_servers: Query Servers Definition ======================== .. config:section:: query_servers :: Query Servers Definition .. versionchanged:: 1.2: Added CoffeeScript query server CouchDB delegates computation of :ref:`design documents ` functions to external query servers. The external query server is a special OS process which communicates with CouchDB over standard input/output using a very simple line-based protocol with JSON messages. The external query server may be defined in configuration file following next pattern:: [query_servers] LANGUAGE = PATH ARGS Where: - ``LANGUAGE``: is a programming language which code this query server may execute. For instance, there are `python`, `ruby`, `clojure` and other query servers in wild. This value is also used for `ddoc` field ``language`` to determine which query server processes the functions. Note, that you may set up multiple query servers for the same programming language, but you have to name them different (like `python-dev` etc.). - ``PATH``: is a system path to the executable binary program that runs the query server. - ``ARGS``: optionally, you may specify additional command line arguments for the executable ``PATH``. The default query server is written in :ref:`JavaScript `, running via `Mozilla SpiderMonkey`_:: [query_servers] javascript = /usr/bin/couchjs /usr/share/couchdb/server/main.js coffeescript = /usr/bin/couchjs /usr/share/couchdb/server/main-coffee.js .. _Mozilla SpiderMonkey: https://developer.mozilla.org/en/docs/SpiderMonkey .. seealso:: :ref:`Native Erlang Query Server ` that allows to process Erlang `ddocs` and runs within CouchDB bypassing stdio communication and JSON serialization/deserialization round trip overhead. .. _config/query_server_config: Query Servers Configuration =========================== .. config:section:: query_server_config :: Query Servers Configuration .. config:option:: commit_freq :: View index commit delay Specifies the delay in seconds before view index changes are committed to disk. The default value is ``5``:: [query_server_config] commit_freq = 5 .. config:option:: os_process_limit :: Query Server process hard limit Hard limit on the number of OS processes usable by Query Servers. The default value is ``100``:: [query_server_config] os_process_limit = 100 Setting `os_process_limit` too low can result in starvation of Query Servers, and manifest in `os_process_timeout` errors, while setting it too high can potentially use too many system resources. Production settings are typically 10-20 times the default value. .. config:option:: os_process_soft_limit :: Query Server process soft limit Soft limit on the number of OS processes usable by Query Servers. The default value is ``100``:: [query_server_config] os_process_soft_limit = 100 Idle OS processes are closed until the total reaches the soft limit. For example, if the hard limit is 200 and the soft limit is 100, the total number of OS processes will never exceed 200, and CouchDB will close all idle OS processes until it reaches 100, at which point it will leave the rest intact, even if some are idle. .. config:option:: reduce_limit :: Reduce limit control Controls `Reduce overflow` error that raises when output of :ref:`reduce functions ` is too big:: [query_server_config] reduce_limit = true Normally, you don't have to disable (by setting ``false`` value) this option since main propose of `reduce` functions is to *reduce* the input. .. _config/native_query_servers: Native Erlang Query Server ========================== .. config:section:: native_query_servers :: Native Erlang Query Server .. warning:: Due to security restrictions, the Erlang query server is disabled by default. Unlike the JavaScript query server, the Erlang one does not runs in a sandbox mode. This means that Erlang code has full access to your OS, file system and network, which may lead to security issues. While Erlang functions are faster than JavaScript ones, you need to be careful about running them, especially if they were written by someone else. CouchDB has a native Erlang query server, allowing you to write your map/reduce functions in Erlang. 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, []} To see these changes you will also need to restart the server. 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 view: .. code-block:: erlang %% Map Function fun({Doc}) -> <> = proplists:get_value(<<"_rev">>, Doc, null), V = proplists:get_value(<<"_id">>, Doc, null), Emit(<>, 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.