REST connector

Overview

The LoopBack REST connector enables applications to interact with other (third party) REST APIs using a template-driven approach. It supports two different styles of API invocations:

• Resource operations
• Defining a custom method using a template

Installation

In your application root directory, enter:

$ npm install loopback-connector-rest --save

This will install the module from npm and add it as a dependency to the application’s package.json file.

Creating a REST data source

Use the DataSource generator to add a REST data source to your application.

$ apic create --type datasource

$ slc loopback:datasource

When prompted, scroll down in the list of connectors and choose REST services (supported by StrongLoop). This adds an entry to datasources.json (for example):

...
  "myRESTdatasource": {
    "name": "myRESTdatasource",
    "connector": "rest"
  }
...

Configuring a REST data source

Configure the REST connector by editing datasources.json manually (for example using the Google Maps API):

...
"geoRest": {
  "connector": "rest",
  "debug": "false",
  "operations": [{
    "template": {
      "method": "GET",
      "url": "http://maps.googleapis.com/maps/api/geocode/{format=json}",
      "headers": {
        "accepts": "application/json",
        "content-type": "application/json"
      },
      "query": {
        "address": "{street},{city},{zipcode}",
        "sensor": "{sensor=false}"
      },
      "responsePath": "$.results[0].geometry.location"
    },
    "functions": {
      "geocode": ["street", "city", "zipcode"]
    }
  }]
}
...

For a REST data source, you can define an array of  operation objects to specify the REST API mapping. Each operation object can have the following two properties:

• template: See how to define a custom method template below.
• functions: An object that maps a JavaScript function to a list of parameter names. For example, a function geocode(street, city, zipcode) will be created so that the first argument will be the value of street variable in the template, second for city, and third for zipcode.  The function can be executed anywhere by the server (in a boot script, through middleware, or within a model’s .js file if it is attached to the REST datasource). 

Configure options for request

The REST connector uses the request module as the HTTP client. You can configure the same options as for the request() function. See request(options, callback).

You can configure options options property at two levels:

• Data source level (common to all operations)
• Operation level (specific to the declaring operation)

For example, the following example sets Accept and Content-Type to "application/json" for all requests. It also sets strictSSL to false so that the connector allows self-signed SSL certificates.

{
  "connector": "rest",
  "debug": false,
  "options": {
    "headers": {
      "accept": "application/json",
      "content-type": "application/json"
    },
    "strictSSL": false
  },
  "operations": [
    {
      "template": {
        "method": "GET",
        "url": "http://maps.googleapis.com/maps/api/geocode/{format=json}",
        "query": {
          "address": "{street},{city},{zipcode}",
          "sensor": "{sensor=false}"
        },
        "options": {
          "strictSSL": true,
          "useQuerystring": true
        },
        "responsePath": "$.results[0].geometry.location"
      },
      "functions": {
        "geocode": ["street", "city", "zipcode"]
      }
    }
  ]
}

Resource operations

If the REST API supports create, read, update, and delete (CRUD) operations for resources, such as users or orders, you can simply bind the model to a REST endpoint that follows REST conventions.

For example, the following methods would be mixed into your model class:

• create: POST /users
• findById: GET /users/:id
• delete: DELETE /users/:id
• update: PUT /users/:id
• find: GET /users?limit=5&username=ray&order=email

For example:

module.exports = function(app) {
  var ds = app.loopback.createDataSource({
    connector: require("loopback-connector-rest"),
    debug: false,
    baseURL: 'http://localhost:3000'
  });

  var User = ds.createModel('user', {
    name: String,
    bio: String,
    approved: Boolean,
    joinedAt: Date,
    age: Number
  });

  User.create(new User({
    name: 'Mary'
  }), function(err, user) {
    console.log(user);
  });

  User.find(function(err, user) {
    console.log(user);
  });

  User.findById(1, function(err, user) {
    console.log(err, user);
  });

  User.update(new User({
    id: 1,
    name: 'Raymond'
  }), function(err, user) {
    console.log(err, user);
  });
}

Setting the resource URL

You can set the remote URL when using create, read, update, or delete functionality by setting the resourceName property on a model definition. This allows for a local model name that is different from the remote resource name.

For example:

var config = {
  "name": "ServiceTransaction",
  "base": "PersistedModel",
  "resourceName": "transactions"
}

var ServiceTransaction = ds.createModel('ServiceTransaction', {}, config);

Now there will be a resource model named ServiceTransaction, but whose URLs call out to baseUrl - '/transactions'

Without setting resourceName the calls would have been made to baseUrl - '/ServiceTransaction'.

Defining a custom method using a template

Imagine that you use a web browser or REST client to test drive a REST API; you will specify the following HTTP request properties:

• method: HTTP method
• url: The URL of the request
• headers: HTTP headers
• query: Query strings
• responsePath: an optional JSONPath applied to the HTTP body. See https://github.com/s3u/JSONPath for syntax of JSON paths.

Then you define the API invocation as a JSON template. For example:

template: {
    "method": "GET",
    "url": "http://maps.googleapis.com/maps/api/geocode/{format=json}",
    "headers": {
      "accepts": "application/json",
      "content-type": "application/json"
    },
    "query": {
      "address": "{street},{city},{zipcode}",
      "sensor": "{sensor=false}"
    },
    "responsePath": "$.results[0].geometry.location"
  }

The template variable syntax is:

{name=defaultValue:type}

The variable is required if the name has a prefix of ! or ^

For example:

'{x=100:number}'
        

'{x:number}'
        

'{x}'
        

'{x=100}ABC{y}123'
        

'{!x}'
        

'{x=100}ABC{^y}123'
        

To use custom methods, you can configure the REST connector with the operations property, which is an array of objects that contain template and functions. The template property defines the API structure while the functions property defines JavaScript methods that takes the list of parameter names.

var loopback = require("loopback");

var ds = loopback.createDataSource({
  connector: require("loopback-connector-rest"),
  debug: false,
  operations: [{
    template: {
      "method": "GET",
      "url": "http://maps.googleapis.com/maps/api/geocode/{format=json}",
      "headers": {
        "accepts": "application/json",
        "content-type": "application/json"
      },
      "query": {
        "address": "{street},{city},{zipcode}",
        "sensor": "{sensor=false}"
      },
      "responsePath": "$.results[0].geometry.location"
    },
    functions: {
      "geocode": ["street", "city", "zipcode"]
    }
  }]
});

Now you can invoke the geocode API as follows:

Model.geocode('107 S B St', 'San Mateo', '94401', processResponse);

By default, LoopBack REST connector also provides an ‘invoke’ method to call the REST API with an object of parameters, for example:

Model.invoke({street: '107 S B St', city: 'San Mateo', zipcode: '94401'}, processResponse);

Parameter/variable mapping to HTTP (since 2.0.0)

By default, variables in the template are mapped to HTTP sources based on their root property.

You can further customize the source in the parameter array of the function mapping, for example:

{
  "template": {
    "method": "POST",
    "url": "http://localhost:3000/{p}",
    "headers": {
      "accept": "application/{format}"
    },
    "query": {
      "x": "{x}",
      "y": 2
    },
    "body": {
      "a": "{a:number}",
      "b": "{b=true}"
    }
  },
  "functions": {
    "myOp": [
      "p",
      "x",
      "a",
      {
        "name": "b",
        "source": "header"
      }
    ]
  }
}

For the template above, the variables will be mapped as follows:

• p - path
• x - query
• a - body
• b - header

Please note that path variables are appended to the path, for example, /myOp/:p.