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restl

restl

Restl - A hypermedia client for nodejs

Introduction

This NPM package is an attempt at creating a 'generic' hypermedia client, it supports an opinionated set of modern features REST services might have.

This means that there's a strong focus on links and link-relationships. Initially we'll build in strong support for Web Linking, a.k.a. the HTTP Link header, and HAL.

Installation

npm install --save restl

Features overview

Restl is a library that sits on top of a Fetch API to provide a RESTful interface and make it easier to follow REST best practices more strictly.

It provides some useful abstractions that make it easier to work with true hypermedia / HATEAOS servers. It currently parses HAL and has a deep understanding of links and embedded resources. There's also support for parsing and following links from HTML documents.

Using this library it becomes very easy to follow links from a single bookmark, and discover resources and features on the server. Embedded resources are completely hidden. Embedded resources just show up as links, but when you're asking for the representation, the response to the GET request will be served from a cache.

This feature allows HAL servers to upgrade links to embedded resources, and allows any client to transparently take advantage of this change and issue less HTTP requests.

Usage

Fetching a resource and following a link:

var restl = require('restl')('http://my-hal-api.example.org/');
 
// Fetch the home resource
var home = restl.getResource()
// Then get the 'author' relationship from _links
home.follow('author')
  .then(function(authorResource)) {
 
    // Follow the 'me' resource.
    return authorResource.follow('me');
 
  }.then(function(meResource) {
 
    // Get the full body
    return meResource.get();
 
  }.then(function(meBody) {
 
    // Output the body
    console.log(meBody);
 
  }).catch(function(err) {
 
    // Error
    console.log(err);
 
  });

Following a chain of links

It's possible to follow a chain of links with follow:

client.follow('rel1')
  .then(function(resource1) {
    return resource1.follow('rel2');
  })
  .then(function(resource2) {
    return resource2.follow('rel3');
  })
  .then(function(resource3) {
    console.log(resource3.getLinks());
  });

As you can see, follow() returns a Promise. However, the returned promise has an additional follow() function itself, which makes it possible to shorten this to:

client
  .follow('rel1')
  .follow('rel2')
  .follow('rel3')
  .then(function(resource3) {
    console.log(resource3.getLinks());
  });

Providing custom options

Options can be passed via the constructor o the client.

Example:

var bookMark = 'https://my-hal-api.example.org';
var options {
  auth: {
    type: 'basic',
    userName: 'foo',
    password: 'bar'
  },
  accept: 'application/json'
}
 
var restl = require('restl')(bookMark, options);

Currently the following options are supported:

• auth, an object with autentication information.
• accept a list of Content-Types which are accepted. Must follow the same format as the HTTP header.
• contentType the default contentType the client sends over. By default this is application/hal+json.

API

Client

var client = new Client(bookMark, options);

• bookMark - The base URL of the web service.
• options optional - A list of options.

Client.getResource()

Returns a 'Resource' object based on the url. If

var resource = client.getResource(url);

• url - URL to fetch. Might be relative. If not provided, the bookMark is fetched instead.

This function returns a Resource.

Resource

Resource.get()

Returns the result of a GET request. This function returns a Promise.

resource.get().then(function(body) {
  console.log(body);
});

If the resource was fetched earlier, it will return a cached copy.

Resource.put()

Updates the resource with a new representation

resource.put({ 'foo' : 'bar' });

This function returns a Promise that resolves to null.

Resource.delete()

Deletes the resource.

resource.delete();

This function returns a Promise that resolves to null.

Resource.post()

This function is meant to be an easy way to create new resources. It's not necessarily for any type of POST request, but it is really meant as a convenience method APIs that follow the typical pattern of using POST for creation.

If the HTTP response from the server was successful and contained a Location header, this method will resolve into a new Resource. For example, this might create a new resource and then get a list of links after creation:

resource.post({ property: 'value'})
  .then(function(newResource) {
    return newResource.links();
  })
  .then(function(links) {
    console.log(links);
  });

Resource.refresh()

Refreshes the internal cache for a resource and does a GET request again. This function returns a Promise that resolves when the operation is complete, but the Promise does not have a value.

resource.refresh().then(function() {
  return resource.get()
}).then(function(body) {
  // A fresh body!
});

Resource.links()

Returns a list of Link objects for the resource.

resource.links().then(function(links) {
  console.log(links);
});

You can also request only the links for a relation-type you are interested in:

resource.links('item').then(function(links) {
 
});

Resource.follow()

Follows a link, by it's relation-type and returns a new resource for the target.

resource.follow('author').then(function(author) {
  return author.get();
}).then(function(body) {
  console.log(body);
});

The follow function returns a special kind of Promise that has a follow() function itself.

This makes it possible to chain follows:

resource
  .follow('author')
  .follow('homepage')
  .follow('icon');

Lastly, it's possible to follow RFC6570 templated links, using the second argument.

For example, a link specified as:

{ href: "/foo{?a}", templated: true}

May be followed using

resource
  .follow('some-templated-link', { a: 'bar'})

This would result following a link to the /foo?a=bar uri.

Resource.followAll()

This method works like follow() but resolves into a list of resources. Multiple links with the same relation type can appear in resources; for example in collections.

resource.followAll('item')
  .then(function(items) {
    console.log(items);
  });

Resource.representation()

This function is similar to GET, but instead of just returning a response body, it returns a Representation object.

Representation

The Representation is typically the 'body' of a resource in REST terminology. It's the R in REST.

The Representation is what gets sent by a HTTP server in response to a GET request, and it's what gets sent by a HTTP client in a POST request.

The Representation provides access to the body, a list of links and HTTP headers that represent real meta-data of the resource. Currently this is only Content-Type but this might be extended to include encoding, language and cache-related information.

Representation.body

The body property has the body contents of a PUT request or a GET response.

Representation.links

The links property has the list of links for a resource.

Representation.contentType

The contentType property has the value of the Content-Type header for both requests and responses.