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6. Query Servers

6.1. JavaScript

Note

While every design function has access to all JavaScript objects, the table below describes appropriate usage cases. For example, you may use emit() in List functions, but getRow() is not permitted during Map functions.

JS Function Reasonable to use in design doc functions
emit() Map functions
getRow() List functions
JSON any
isArray() any
log() any
provides() Show functions, List functions
registerType() Show functions, List functions
require() any, except Reduce and rereduce functions
send() List functions
start() List functions
sum() any
toJSON() any

6.1.1. Design functions context

Each design function executes in a special context of predefined objects, modules and functions:

emit(key, value)

Emits a key-value pair for further processing by CouchDB after the map function is done.

Arguments:
  • key – The view key
  • value – The key‘s associated value
 
function(doc){
  emit(doc._id, doc._rev);
}
getRow()

Extracts the next row from a related view result.

Returns:View result row
Return type:object 
function(head, req){
  send('[');
  row = getRow();
  if (row){
    send(toJSON(row));
    while(row = getRow()){
      send(',');
      send(toJSON(row));
    }
  }
  return ']';
}
JSON

JSON2 object.

isArray(obj)

A helper function to check if the provided value is an Array.

Arguments:
  • obj – Any Javascript value
Returns:

true if obj is Array-typed, false otherwise
Return type:

boolean
log(message)

Log a message to the CouchDB log (at the INFO level).

Arguments:
  • message – Message to be logged
 
function(doc){
  log('Procesing doc ' + doc['_id']);
  emit(doc['_id'], null);
}

After the map function has run, the following line can be found in CouchDB logs (e.g. at /var/log/couchdb/couch.log):

[Sat, 03 Nov 2012 17:38:02 GMT] [info] [<0.7543.0>] OS Process #Port<0.3289> Log :: Processing doc 8d300b86622d67953d102165dbe99467
provides(key, func)

Registers callable handler for specified MIME key.

Arguments:
  • key – MIME key previously defined by registerType()
  • func – MIME type handler
registerType(key, *mimes)

Registers list of MIME types by associated key.

Arguments:
  • key – MIME types
  • mimes – MIME types enumeration

Predefined mappings (key-array):

  • all: */*
  • text: text/plain; charset=utf-8, txt
  • html: text/html; charset=utf-8
  • xhtml: application/xhtml+xml, xhtml
  • xml: application/xml, text/xml, application/x-xml
  • js: text/javascript, application/javascript, application/x-javascript
  • css: text/css
  • ics: text/calendar
  • csv: text/csv
  • rss: application/rss+xml
  • atom: application/atom+xml
  • yaml: application/x-yaml, text/yaml
  • multipart_form: multipart/form-data
  • url_encoded_form: application/x-www-form-urlencoded
  • json: application/json, text/x-json
require(path)

Loads CommonJS module by a specified path. The path should not start with a slash.

Arguments:
  • path – A CommonJS module path started from design document root
Returns:

Exported statements
send(chunk)

Sends a single string chunk in response.

Arguments:
  • chunk – Text chunk
 
function(head, req){
  send('Hello,');
  send(' ');
  send('Couch');
  return !
}
start(init_resp)

Initiates chunked response. As an option, a custom response object may be sent at this point. For list-functions only!

Note

list functions may set the HTTP response code and headers by calling this function. This function must be called before send(), getRow() or a return statement; otherwise, the query server will implicitly call this function with the empty object ({}).

function(head, req){
  start({
    "code": 302,
    "headers": {
      "Location": "http://couchdb.apache.org"
    }
  });
  return "Relax!";
}
sum(arr)

Sum arr‘s items.

Arguments:
  • arr – Array of numbers
Return type:

number
toJSON(obj)

Encodes obj to JSON string. This is an alias for the JSON.stringify method.

Arguments:
  • obj – JSON encodable object
Returns:

JSON string

6.1.2. CommonJS Modules

Support for CommonJS Modules (introduced in CouchDB 0.11.0) allows you to create modular design functions without the need for duplication of functionality.

Here’s a CommonJS module that checks user permissions:

function user_context(userctx, secobj) {
  var is_admin = function() {
    return userctx.indexOf('_admin') != -1;
  }
  return {'is_admin': is_admin}
}

exports['user'] = user_context

Each module has access to additional global variables:

  • module (object): Contains information about the stored module
    • id (string): The module id; a JSON path in ddoc context
    • current (code): Compiled module code object
    • parent (object): Parent frame
    • exports (object): Export statements
  • exports (object): Shortcut to the module.exports object

The CommonJS module can be added to a design document, like so:

{
   "views": {
      "lib": {
         "security": "function user_context(userctx, secobj) { ... }"
      },
      "validate_doc_update": "function(newdoc, olddoc, userctx, secobj) {
        user = require('lib/security').user(userctx, secobj);
        return user.is_admin();
      }"
   },
   "_id": "_design/test"
}

Modules paths are relative to the design document’s views object, but modules can only be loaded from the object referenced via lib. The lib structure can still be used for view functions as well, by simply storing view functions at e.g. views.lib.map, views.lib.reduce, etc.

6.2. Erlang

Warning

Unlike the JavaScript query server, the Erlang query server does not run in a sandbox. This means Erlang code has full access to your OS, filesystem 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.

Note

Due to security restrictions, the Erlang query server is disabled by default. To enable it, you need to edit your local.ini to include a native_query_servers section:

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

Don’t forget to restart CouchDB after updating the configuration, and use the language: "erlang" property in your Erlang design documents.

Emit(Id, Value)

Emits key-value pairs to view indexer process.

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

Helper to iterate over all rows in a list function.

Arguments:
  • Fun – Function object.
  • Acc – The value previously returned by Fun.
 
fun(Head, {Req}) ->
  Fun = fun({Row}, Acc) ->
    Id = couch_util:get_value(<<"id">>, Row),
    Send(list_to_binary(io_lib:format("Previous doc id: ~p~n", [Acc]))),
    Send(list_to_binary(io_lib:format("Current  doc id: ~p~n", [Id]))),
    {ok, Id}
  end,
  FoldRows(Fun, nil),
  ""
end.
GetRow()

Retrieves the next row from a related view result.

%% FoldRows background implementation.
%% https://git-wip-us.apache.org/repos/asf?p=couchdb.git;a=blob;f=src/couchdb/couch_native_process.erl;hb=HEAD#l368
%%
foldrows(GetRow, ProcRow, Acc) ->
  case GetRow() of
    nil ->
      {ok, Acc};
    Row ->
      case (catch ProcRow(Row, Acc)) of
        {ok, Acc2} ->
          foldrows(GetRow, ProcRow, Acc2);
        {stop, Acc2} ->
          {ok, Acc2}
      end
end.
Log(Msg)
Arguments:
  • Msg – Log a message at the INFO level.
 
fun({Doc}) ->
  <<K,_/binary>> = proplists:get_value(<<"_rev">>, Doc, null),
  V = proplists:get_value(<<"_id">>, Doc, null),
  Log(lists:flatten(io_lib:format("Hello from ~s doc!", [V]))),
  Emit(<<K>>, V)
end.

After the map function has run, the following line can be found in CouchDB logs (e.g. at /var/log/couchdb/couch.log):

[Sun, 04 Nov 2012 11:33:58 GMT] [info] [<0.9144.2>] Hello from 8d300b86622d67953d102165dbe99467 doc!
Send(Chunk)

Sends a single string Chunk in response.

fun(Head, {Req}) ->
  Send("Hello,"),
  Send(" "),
  Send("Couch"),
  "!"
end.

The function above produces the following response:

Hello, Couch!
Start(Headers)
Arguments:

Initialize List functions response. At this point, response code and headers may be defined. For example, this function redirects to the CouchDB web site:

fun(Head, {Req}) ->
  Start({[{<<"code">>, 302},
          {<<"headers">>, {[
            {<<"Location">>, <<"http://couchdb.apache.org">>}]
          }}
        ]}),
  "Relax!"
end.

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