viane/loopback-connector-rest

Connect Loopback to a REST API

loopback-connector-rest

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 data source generator to add a REST data source to your application.

For LoopBack 2.x:

$ apic create --type datasource

For LoopBack 2.x or 3.0:

$ lb 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):

/server/datasources.json

...
"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"]
    }
  }]
}
...

The operations property is an array of objects, each of which can have these properties:

• template: An object that defines a custom method using a template; see Defining a custom method using a template.
• functions: An object that maps a JavaScript function to a list of parameter names.

The example above creates a function geocode(street, city, zipcode) whose first argument is street, second is city, and third is zipcode.  LoopBack application code can call the function anywhere; for example, in a boot script, via middleware, or within a model's JavaScript file if 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)

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

/server/datasources.json

{
  "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, 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:

/server/boot/script.js

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

The template object specifies the REST API invocation as a JSON template, with the following properties:

Property	Description	Type
method	HTTP method	String (one of "GET", "POST", "PUT", and so on).
url	The URL of the request	String; template values allowed.
headers	HTTP headers	Object
query	Query strings	Object; template values allowed.
responsePath	Optional JSONPath applied to the HTTP body. See https://github.com/s3u/JSONPath for syntax of JSON paths.	String

The template variable syntax is:

{name=defaultValue:type}

To specify that the variable value is required, add the prefix ! or ^.

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 following table provides several examples:

Variable definition	Description
'{x=100:number}'	Define a variable x of number type and default value 100.
'{x:number}'	Define a variable x of number type
'{x}'	Define a variable x
'{x=100}ABC{y}123'	Define two variables x and y. The default value of x is 100. The resolved value will be a concatenation of x, 'ABC', y, and '123'. For example, x=50, y=YYY will produce '50ABCYYY123'
'{!x}'	Define a required variable x
'{x=100}ABC{^y}123'	Define two variables, x and y. The default value of x is 100, and y is required.

To use custom methods, configure the REST connector with the operations property, which is an array of objects, each of which can have these properties:

• template defines the API structure.
• functions defines JavaScript methods that accept the specified 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 in Node.js as follows:

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

By default, the 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)

NOTE: This feature is available with loopback-connector-rest version 2.0.0 and later.

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

Root property	HTTP source
url	path
query	query
body	body
headers	header

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.