Build scalable REST APIs using the open source tools and standards you already know.

baucis v1.0.5

Build scalable REST APIs using the open source tools and standards you and your team already know — Mongoose, Express, and Node.js streams. Baucis takes the boilerplate out of building and maintaining scalable HATEOAS/Level 3 REST APIs.

Baucis has thousands of users and is used in production by startups, departments, and Fortune 500 companies, at hackathons and conferences worldwide. MongoDB, Inc. awarded baucis first place in their 2014 Open Source Hack Contest.

Who's using Baucis?

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Please consider adding bounties to issues in the issue tracker or contributing via GitTip.

The official baucis documentation is being moved to but is still right here for the moment!

  • Awesomely scalable.
  • Fully takes advantage of Node streaming to nimbly process large datasets (both incoming and outgoing)
  • Automatically build controllers from your Mongoose schemata, then easily configure them.
  • Built on Express 4 so adding custom middleware is a snap. 100% compatible with existing middleware such as passport.
  • Supports geolocation and full text search.
  • Highly customizable, simple interface. Can be extended with plugins.
  • Automatically generate interactive Swagger documentation for the API.
  • Perform rich queries of the API using an expressive JSON syntax via query string.
  • Mongoose middleware, inheritence, and validation methods are automatically respected.
  • Version your API using semver.
  • Over 140 Mocha.js tests in addition to express' and mongoose's.

Check the change log for info on all the latest features.

David Rijckaert - Philemon and Baucis Giving Hospitality to Jupiter and Mercury

To install:

npm install --save baucis

An example of creating a REST API from a couple Mongoose schemata.

// Create a mongoose schema. 
var Vegetable = new mongoose.Schema({ name: String });
// Register new models with mongoose. 
mongoose.model('vegetable', Vegetable);
// Create a simple controller.  By default these HTTP methods 
// are activated: HEAD, GET, POST, PUT, DELETE'vegetable');
// Create the app and listen for API requests 
var app = express();
app.use('/api', baucis());

Later, make requests:

HTTP Verb/vegetables/vegetables/:id
GETGet all documents, or documents that match the query conditions.Get the addressed document.
POSTCreates new documents and sends them back. You can POST a single document or an array of documents.n/a
PUTn/aUpdate the addressed document.
DELETEDelete all documents, or documents that match the query conditions.Delete the addressed object.
HEADCheck what headers would be sent for the given GET request.Check what headers would be returned for the instance. creates a new controller associated with a given model.

You can pass in a mongoose model name:

var controller ='robot');

Or, pass in a Mongoose model:

var controller ='robot'));

Calling also adds the newly created controller to the current API. When baucis() is called, the API is finalized and any subsequent controllers will be added to another API instance.

// Creating the first API.'legume');
var api = baucis();
// Creating another API.'tuber');
var api2 = baucis();

Controllers also have the usual Express features. Controllers are Express 4 Routes.

// Add middleware before API routes 
controller.use('/qux', function (requestresponsenext) {
  // Do something cool… 
controller.get('/readme', function (requestresponsenext) {
  // Send a readme document about the resource (for example) 
// Do other stuff... 
controller.set('some option name', 'value');

Customize them with Express middleware, including pre-existing modules like passport.

Baucis also adds controller the request and query methods to interact with the baucis interal Mongoose query. (See the middleware section.)

controller.request(function (requestresponsenext) {
  if (request.isAuthenticated()) return next();
  return response.send(401);


This property sets the controller's mongoose model. You can pass in a string or a directly pass in a mongoose model.


Select or deselect fields for all queries.'field -password');'+groats');

Note that mixing inluding and excluding fields causes an error.

By default the response Link header is set with various useful links based on context. This is especially useful for paging. May be disabled using this method.


The unique path used to identify documents for this controller. Defaults to _id.


Allow sending an index hint for the query from the client. Disabled by default.


Allow sending a query comment from the client. Disabled by default.


Used to disable specific HTTP methods for the controller.

controller.methods('post put delete', false);

BYPASSES VALIDATION Use this method to enable non-defualt update operators. The update method can be set using the Update-Operator header field.

controller.operators('$push $set', 'foo some.path some.other.path');
controller.operators('$pull', 'another.path');

This is the fragment to match request URLs agains. Defaults to the plural name of the model.


This can be used to set what status code & body are returned for requests that yield empty query results. The default is status code 200 with a JSON body containing an empty array. Other possible options are 204 No Content and 404 Not Found.


Baucis sets the response status code based on different errors. By default, it also catches and builds responses for certain errors. Set this to false to have the controller only set status codes, and not handle errors further.


Versioning is implemented using semver. Supported releases are specified when calling baucis(). The release(s) that a controller belongs to are specified with the versions controller option.'cat').versions('0.0.1');'cat').versions('>0.0.1 <1.0.0');'cat').versions('~1');'cat').versions('>2.0.0');
var api = baucis();
app.use('/api', api);

Later, make requests and set the API-Version header to a semver range, such as ~1, >2 <3, *, etc. Baucis will use the highest release number that satisfies the range. If no API-Version is specified in a request, the highest release will be used.

Baucis takes full advantage of Node streams internally to offer even more performance, especially when dealing with large datasets. Both outgoing and incoming documents are streamed!

To alter or inspect documents being sent or process, add a through stream that transforms or processes them. As a shortcut, a map function can be passed in. It will be used to create a map stream internally. Here's an example of adding a stream to alter POST'd or PUT'd request bodies:

controller.request(function (requestresponsenext) {
  request.baucis.incoming(function (contextcallback) { = 'Feverfew';
    callback(null, context);

For PUT requests, the document is available to the stream. For POSTs context.doc will be set to null.

controller.request(function (requestresponsenext) {
  request.baucis.incoming(function (contextcallback) {
    if (context.doc.created !== context.incoming.created) {
      callback(baucis.errors.Forbidden('The created date cannot be updated'));
    callback(null, context);

Passing in through streams is also allowed. Here's an example using the through module to create a stream that checks for a forbidden sort of whiskey and alters the name of incoming (POSTed) documents.

controller.request(function (requestresponsenext) {
  request.baucis.incoming(through(function (context) {
    if (context.incoming.whiskey === 'Canadian') {
      // Errors will be passed off to `next` later, and the stream will 
      // be stopped. 
      this.emit('error', baucis.errors.Forbidden('Too smooth.'));
    } = 'SHAZAM';

If request.baucis.incoming or request.baucis.outgoing is called multiple times, the multiple through streams will be piped together.

Here's an example of how a stream that interacts with outgoing documents may be added:

controller.request(function (requestresponsenext) {
  request.baucis.outgoing(through(function (context) {
    if (context.doc.owner !== request.user) {
      // Errors will be passed off to `next` later, and the stream will 
      // be stopped. 
      this.emit('error', baucis.errors.Forbidden());
    context.doc.password = undefined;

  • For POSTs, if request.body is present, the incoming request will be parsed as a whole, negating many of the benefits of streaming. However, especially when POSTing only one new document at a time, this is not an issue. If you want to POST many objects at once, using the default streaming behavior is highly recommened.
  • If you set request.baucis.documents, this will be streamed out instead of the Mongoose query results.
  • The document stage of middleware has been deprecated. Use an outgoing through stream instead.

Baucis adds middleware registration functions for two stages of the request cycle:

This stage of middleware will be called after baucis applies defaults based on the request, but before the Mongoose query is generated.

This stage of middleware will be called after baucis applies defaults to the Mongoose query object, but before the documents are streamed out through the response. The Mongoose query can be accessed and changed in your custom middleware via request.baucis.query. Query middleware cannot be added explicitly for POST and will be ignored when added for POST implicitly.

To apply middleware to all API routes, just pass the function or array to the method for the appropriate stage:

controller.request(function (requestresponsenext) {
  if (request.isAuthenticated()) return next();
  return response.send(401);
controller.query(function (requestresponsenext) {

To add middleware that applies only to specific HTTP methods, use the second form. It adds a paramater that must contain a space delimted list of HTTP methods that the middleware should be applied to.

controller.query('head get', function (requestresponsenext) {

The final form is the most specific. The first argument lets you specify whether the middleware applies to document instances (paths like /cheeses/:id) or to collection requests (paths like /cheeses).

controller.request('instance', 'head get delete', middleware);
controller.request('collection', 'post', middleware);

Here's how to use swagger. First, install the plugin:

npm install --save baucis-swagger

Next, download the swagger-ui client.

git clone
open swagger-ui/dist/index.html

Then, create your API as normal, but be sure to require baucis-swagger.

var baucis = require('baucis');
var swagger = require('baucis-swagger');
app.use('/api', baucis());

Point the swagger client at your API:


Now you have documentation and a test client!

To customize the swagger definition, simply alter the controler's swagger data directly:

var controller ='sauce');
  'path': '/sauces/awesome',
  'description': 'Awesome sauce.',
  'operations': [
      'httpMethod': 'GET',
      'nickname': 'getAwesomeSauce',
      'responseClass': 'Sauce',
      'summary': 'Carolina BBQ Sauce.'
Header FieldNotes
API-VersionSet this to a valid semver range to request a specific version or range of versions for the requested controller.
Update-OperatorUse this to perform updates (that BYPASS VALIDATION) using a special update operator such as $set, $push, or $pull. The fields that may be updated must be whitelisted per controller.
Header FieldNotes
ETagUsed for HTTP caching based on response body. Supported automatically for single object endpoints.
Last-ModifiedUsed for HTTP caching. Can be set automatically by Baucis for single object endpoints.
AcceptSet to application/json for all responses.
AllowSet automatically, correctly removing HTTP methods when those methods have been disabled.
LocationSet to the URL of the created entity for POST responses.
LinkBaucis adds related links to the header for you. Especially useful for paging through a query. first, last, next, and previous links are added when paging through a collection when using the limit & skip query options.

Use query options from the client to make dynamic requests. Query options can be mixed as you see fit.

Set the Mongoose query's find or remove arguments. This can take full advtange of the MongoDB query syntax, using geolocation, regular expressions, or full text search. Special query operators are fine, and in fact geolocation, regular expression, and full text search capabilities are available to your API clients by default!

GET /api/people?conditions={ "location": { "$near": [44, -97] } }
GET /api/people?conditions={ "$text": { "$search": "dog bites man" } }
GET /api/cats?sort=-name&limit=1&conditions={ "features": "stripes" }
DELETE /api/people?conditions={ "name": { "$regex": "^Bob W", "$flags": "i" } }

Skip sending the first n matched documents in the response. Useful for paging.

GET /api/horses?skip=3

Limit the response document count to n at maximum.

GET /api/horses?limit=3

If both limit and skip are used on a request, the response Link header will be set with extra relations that give URLs for paging.

Sort response documents by the given criteria. Here's how you'd sort the collection by name in ascending order, then by age in descending order.

GET /api/cheeses?sort=name -age

Set which fields should be selected for response documents.

GET /api/phones?select=-_id -year

It is not permitted to use the select query option to select deselected paths. This is to allow a mechanism for hiding fields from client software.

You can deselect paths in the Mongoose schema definition using select: false or in the controller by calling e.g.'-foo'). Your server middleware will be able to select these fields as usual using, while preventing the client from selecting the field.

Note that mixing inluding and excluding fields causes an error.

Set which fields should be populated for response documents. See the Mongoose population documentation for more information. The string or object syntax can be used:

GET /api/boats?populate=captain
GET /api/cities?populate={ "path": "captain", "match": { "age": "44" } }

The select option of populate is disallowed. Only paths deselected at the model level will be deselected in populate queries.

May be set to true for GET requests to specify that a count should be returned instead of documents

GET /api/stereos?count=true

Set to a path name to retrieve an array of distinct values.

GET /api/restaurants?distinct=category

Add an index hint to the query (must be enabled per controller).

GET /api/pools?hint={ name: 1 }

Add a comment to a query (must be enabled per controller).

GET /api/wrenches?comment=Something informative

Baucis decorates Mongoose models with a few additional methods to add richer textual and other semantics. The Model API is ubstable. It will be stablized for v1.0.0.

Typically, these methods would be called when the schema is registered with Mongoose:

mongoose.model('cactus', Cactus).plural('cacti');
mongoose.model('hen', Hen).locking(true);

Customize the name used for singular instances of documents associated with this model.


Customize the name used for groups of documents associated with this model. Defaults to the plural of the model's singular name. Uses Mongoose's pluralizer utility method.


Set the Last-Modified HTTP header using the given Date field. Disabled by default.


Enable optimistic locking. (Disabled by default.) Requires that all PUTs must send the document version (__v by default) and will send a 409 response if there would be a version conflict, instead of performing the update.


Baucis supports a rich array of error responses and status codes. For more information on status codes see RFC2616 the HTTP sepcification.

4xx status codes mean that the client, for example a web browser made a mistake and needs to fix the response.

The client made a bad request and should fix the request before trying again. This is also sent when a deprecated command is used.


Sent when the requested action is disallowed for a controller.


Sent when the query does not match any document, or when the requested resource does not exist.


The request HTTP method (i.e one of HEAD, GET, POST, PUT, DELETE) is disabled for this resource. This can be done by e.g. controller.methods('post put delete', false).


The Accept header specified in the request could not be fulfilled. By default JSON is supported. Baucis is pluggable to allow adding formatters for additional content types.


If a controller has optimistic locking enabled, baucis will automatically check each updated document's version and give a 409 error if another API client modified the document before the requester was able to send their update.

In this case, a client could reload the document, present the user with a description of the conflict, and ask them how to procede.


The request body content type was not able to be parsed. By default JSON is supported. Baucis is pluggable to allow adding parsers for additional content types.


This status indicates the request body was syntactically correct and could be parsed, but that it is not semantically correct, and so it could not be processed. Most often countered when a document's validation step fails.


Baucis will send a response body with error 422 that indicates what validation failed for which fields.

  "name": {
  "message": "Path `name` is required.",
  "name": "ValidatorError",
  "path": "name",
      "type": "required"
  "score": {
  "message": "Path `score` (-1) is less than minimum allowed value (1).",
  "name": "ValidatorError",
  "path": "score",
  "type": "min",
      "value": -1

Technically, this error is from RFC4918, the WebDAV specification.

Where as 4xx errors mean the requester messed up, 5xx errors mean the server messed up :)

This means that baucis is misconfigured, or that the server tried to perform the requested action but failed.


The requested functionality is not implemented now, but may be implented in the future.


See what's in store in the GitHub milestones page for this repo.

Baucis can be augmented via Express middleware, incoming and outgoing streams, as well as with decorators.

Add decorators to Controllers and other baucis constructors by using the decorators method. Adding a decorator will affect all subsequently created controllers. Here's how you could add a tiny plugin that makes all subsequently added controllers check authentication for all PUTs and POSTs.

baucis.Controller.decorators(function (optionsprotect) {
  var controller = this;
  controller.request('put post', function (requestresponsenext) {
    if (!request.isAuthenticated()) return next(new Error());
Module Name
baucis-accessConfigure read/write access on a per-attribute basis (created by Pavel Karoukin)
baucis-swaggerautomatically generate interactive API documentation (created by wprl)
bswaggeralternative swagger package (created by j.sedlan)
baucis-gformRapidly develop a administration UI for your baucis API (created by stemey)
baucis-vivifyUsed for embedding controllers under a parent path (created by wprl)
baucis-patchcreated by wprl
baucis-jsoncreated by wprl

The baucis-json and baucis-error plugins are bundled with baucis by default. baucis-json is a good example for writing your own plugins for parsing or formatting custom content types.


© 2012-2014 William P. Riley-Land