Programming Web Services on the Cloud with Node.js

Workshop 402 @ SIGCSE 2016. March 5, 2016. Memphis, TN. U.S.A.

1. Introduction

In this workshop we’ll learn how to write web services using Node.js on the Cloud9 development environment. But first let’s cover some concepts.

1.1. What is a web service?

A web service is a service provided by a program (the server) that is consumed by other programs (the clients). The server and the clients talk to each other using the HTTP protocol. In a simplified way a web service is like a web page, but meant for a computer instead of a person.

Because the communication between clients and server is carried out through a language agnostic protocol (HTTP), the clients don’t have to be implemented in the same language as the server. So, for example, we can write a web service in JavaScript using Node but clients can be programmed in Python.

1.2. What is Node.js?

Node.js (or just Node) is a software platform that can be used to build scalable network (especially server-side) applications.

Created by Ryan Dahl and debuted in the year 2009.
Built on Google Chrome’s V8 JavaScript Engine. It implements most parts of the 6^th edition (June 2015) of ECMA-262, also known as Harmony.
Achieves high throughput via non-blocking I/O and a single-threaded event loop.
Contains a built-in HTTP server library, making it possible to run a web server without the use of external software, such as Apache, and allowing more control of how the web server works.
Written in C, C++, and JavaScript.

1.3. Preparing the Cloud Development Environment

Go to the Cloud9 (https://c9.io) web site. Create an account if you haven’t done so already.

Create a workspace: give it a name and a brief description, make it public, and select the costum template.

At the moment of this writing, Cloud9 free accounts allow having one private workspace. A private workspace uses an elaborate access scheme that makes testing web services difficult, so it’s important to make your workspaces public.

We now need to install the Node software modules that we’ll be requiring. At the terminal window (the black window labeled bash) type:
```
npm install express mysql body-parser cors request
```

2. Hello World!

Let’s start with a simple “Hello World!” server program.

2.1. The Code

Create a new file in the IDE called hello_app.js. Copy the following content into this file:

File: hello_app.js

'use strict'; (1)

const express = require('express'); (2)
const app = new express(); (3)

app.get('/hello', (req, res) => { (4)
  res.type('text').send('Hello World!\n'); (5)
});

app.listen(process.env.PORT, () => { (6)
  console.log('Server running as: ' + process.env.C9_HOSTNAME); (7)
});

Let’s analyze this program:

1	Declare that the program is in strict mode. Strict mode is a way to introduce better error-checking into your code and also adds support to some new ECMAScript 6 features.
2	Import the express module. Express is a web framework, designed to simplify the construction of web applications.
3	`app` is a newly created web application object.
4	Define an endpoint (or route) for `app`, which is a URI (`'/hello'`) and a specific HTTP request method (`GET` in this case, but it can others like: `POST`, `PUT`, and `DELETE`). Whenever we receive a request from a client using this endpoint, the callback function will be called. It takes two parameters, `req` and `res`, that contain the information related to the HTTP request and response.
5	Set the response content type to `text/plain` and the body to `'Hello World\n'`.
6	Bind the application to a specific port (in this case the value of the environment variable `PORT`, which in practice is 8080) and start listening to client requests. The callback function is called once the binding has taken effect.
7	Print a message with the the name of our host computer (this is contained in the `C9_HOSTNAME` environment variable).

To test the code, do the following:

Run the server program we just wrote. At the terminal type:
```
node hello_app
```
When running the node command, the specified file to execute may include the .js extension, but it’s not required.

The output should be something like this:
```
Server running as: host-name
```
Take note of host-name because you’ll need it when testing your web services from a web browser.
Test the server using the curl command as the client. Open a new terminal window (from the Cloud9 menu select “Window/New Terminal”) and type there:
```
curl -i localhost:8080/hello
```
We use localhost:8080 because curl is running in the same machine as the server. If this is not the case, use the host-name obtained in step 1.

You should see an output like this:
```
HTTP/1.1 200 OK
x-powered-by: Express
content-type: text/plain; charset=utf-8
content-length: 13
etag: W/"d-jd2L5LF5pSmvpfL/rkuYWA"
date: Thu, 25 Feb 2016 19:59:33 GMT
X-BACKEND: apps-proxy

Hello World!
```
Curl’s -i option allows us to see the complete HTTP response: status line, headers and body (and an empty line separating the headers and the body).
Let’s now use a web browser as the client for our server. Open a new tab in your browser and type in the address bar:

host-name/hello

Don’t forget to replace host-name with the correct value displayed by the server in step 1. You should see now a page containing only the text: Hello World!.

2.2. Exercise A ★

Modify the hello_app.js program so that it can greet anyone. The name of the greetee will be specified as a query string parameter using URL encoding. This means that the URL to test your program should be something like this:

host-name/hello?who=Random+User

In this case the name of the parameter is who and its value is the string "Random User". Note that in the URL spaces should be replaced with the plus (+) character.

If the parameter who was not provided, your program should respond: Hello Anonymous!.

To read a query string parameter named some_parameter use the following expression inside the callback function for app.get(): req.query.some_parameter

This expression evaluates to undefined if some_parameter is not present.

Test your changes with the following examples. Type at the terminal:

curl localhost:8080/hello?who=Random+User

The output should be:

Hello Random User!

Now type:

curl localhost:8080/hello

Expected output:

Hello Anonymous!

3. Quotations

This example is a much larger program. It consists of a Quotations database in MySQL. We’ll provide a way of accessing this database using a RESTful API.

3.1. RESTful Web Services

REST stands for Representational State Transfer. It relies on a stateless, client-server, cacheable communications protocol. REST is an architectural style for designing networked applications. RESTful applications use HTTP requests to post and put data (create and/or update), read data (make queries), and delete data. Thus, REST uses HTTP for all four CRUD (Create/Read/Update/Delete) operations (see table table 1). When building web services the use of REST is often preferred over the more heavyweight SOAP (Simple Object Access Protocol) style because REST is less complex and does not leverage as much bandwidth, which makes it a better fit for use over the Internet.

Table 1. CRUD/HTTP/SQL Mapping
CRUD Operation	HTTP Method	SQL Statement	Idempotent?	Safe?
Create	POST	INSERT	No	No
Read	GET	SELECT	Yes	Yes
Update	PUT	UPDATE	Yes	No
Delete	DELETE	DELETE	Yes	No

We say an operation is idempotent if it can be applied multiple times without changing the result beyond the initial application. For example, in mathematics the absolute value is an idempotent operation: applying it once or multiple times gives us the same answer.

An operation is safe if it’s only used for data retrieval (it doesn’t have any side-effects on the server).

REST was defined by Roy Thomas Fielding in his 2000 PhD dissertation “Architectural Styles and the Design of Network-based Software Architectures”.

3.2. API Guidelines

REST is more a collection of principles than it is a set of standards. There are “best practices” and de-facto standards but those are constantly evolving. Fortunately for us, there are a couple of documents, both written by Todd Fredrich, that provide some useful guidelines on things to consider when writing RESTful APIs:

Our example will incorporate several recommendations from these documents. Specifically:

We’ll use a noun to name our resource, not a verb, and it will be in plural form ('quotations' instead of 'quotation').
We’ll use the Collection Metaphor. This means that our resource is going to have two endpoints:
- '/quotations' — for the complete resource collection.
- '/quotations/{ID}' — for each individual resource within the collection.
We’ll supply links in the response body for retrieval of the resource object itself or related objects. This is a constraint of the REST application architecture known as HATEOAS (Hypermedia as the Engine of Application State).
We’ll use HTTP methods to mean something useful:
- GET — Read a resource or collection.
- POST — Create.
- PUT — Update.
- DELETE — Remove a resource or collection.
We’ll make sure that the GET, PUT and DELETE operations are idempotent.
We’ll use the JSON format for the request and response bodies.
We’ll use meaningful HTTP status codes:
- 200 — Success.
- 201 — Created. Returned on successful creation of a new resource.
- 400 — Bad request. Data issues such as invalid JSON, etc.
- 404 — Not found. Resource not found on GET.

3.3. Setting Up the Database

To get MySQL running in the Cloud9 environment, type at the terminal:

mysql-ctl install

The output will be something like this:

MySQL 5.5 database added.  Please make note of these credentials:

Root User: username
Database Name: c9

Because MySQL has a blank password by default, setup is significantly easier for users. Additionally, since developers generally don’t (and often shouldn’t) put sensitive/important data in a development environment like Cloud9, having no password on MySQL is rarely an issue.

To start MySQL, at the terminal type:

mysql-ctl start

To stop MySQL, type:

mysql-ctl stop

Our database will only have one table, called quotations, with three columns (see Table 2).

Table 2. Quotations Table
Column Name	Description
`id`	The unique integer quotation ID.
`author`	A text value with the name of the quotation author.
`excerpt`	A text value with the quotation body.

Let’s download a script that will simplify the creation of the database. Type at the terminal:

wget -O quotations.sql http://goo.gl/k9Uix2

To execute the quotations.sql script, open the Cloud9 MySQL command tool. At the terminal type:

mysql-ctl cli

At the MySQL monitor (the shell with mysql> prompt) type:

source quotations.sql

This command creates the database table and populates it with some 50 quotations. Press Ctrl-D to exit the MySQL monitor.

This is the Node code to connect to the database:

const mysql = require('mysql');

const db = mysql.createConnection({
  host: 'localhost',
  user: 'username', // <-- Update this line with your username.
  database: 'c9'
});

db.connect((err) => {
  if (err) {
    console.error('Unable to connect to the database.');
    throw err;
  } else {
    console.log('Connected to the database.');
  }
});

The following snippet is an example on how to use the database connection to execute a SQL statement. In this case we select all columns from the quotations table:

db.query(
  'SELECT * FROM quotations',
  (err, rows) => {
    if (err) throw err;
    rows.forEach((row) => { console.log(row.author); });
    db.end(); (1)
  }
);

1	The `end()` method closes the database connection. We usually don’t do this in our server code because it’s more convenient to keep the connection open indefinitely.

Check these links to get more information regarding on how to use a MySQL database:

3.4. Adding Middleware to the Server

We need our server to have some additional functionality:

We want to be able to parse the body of client requests as JSON objects.
We want to eliminate any possible issues regarding cross-origin resource sharing (CORS) when programming Ajax clients.
We want to serve static resources (static HTML pages, images, cascading style sheets, client-side JavaScript, etc.).

Any functionality can be added to our server using middleware. Express middleware allows us to plug in a stack of actions that need to be executed.

To do this we first import the modules that provide the needed functionality:

const bodyParser = require('body-parser');
const cors = require('cors');

We then add them to the app object:

app.use(bodyParser.json());
app.use(cors());
app.use(express.static('public'));

The last line tells the server that anything inside the 'public' directory is a static resource. Thanks to this, we don’t need to install and configure a full-blown web server such as Apache.

3.5. GET /quotations

Let’s first write a web service that returns a list with partial information of all the quotations available in the database.

app.get('/quotations', (req, res) => { (1)
  db.query('SELECT id, author FROM quotations', (err, rows) => { (2)
    if (err) {
      res.status(500).json(err); (3)
    } else {
      let result = rows.map((row) => { (4)
        return {
          id: row.id,
          author: row.author,
          url: makeUrl(row.id)
        };
      });
      res.json(result); (5)
    }
  });
});

1	The URI for our web service endpoint is `'/quotations'`.
2	Extract from our table the id and author information of every single quotation.
3	Any SQL/database error will be dealt as an “internal server error” (status code 500). We convert the error object to JSON and send it back as the body of the response.
4	Map every element of the result set into an object with the information (`id`, `author`, and `url`) we wish to provide. The `url` is the web link we’ll use to retrieve single quotation objects later on (see: `GET /quotations/{ID}`).
5	Respond with an implicit status 200, returning the resulting array as a JSON object.

The implementation of the makeUrl() function used above is as follows:

function makeUrl(id) {
  return 'http://' + process.env.C9_HOSTNAME + '/quotations/' + id;
}

The following program puts together everything we’ve described so far:

File: quotations_app.js

'use strict';

const express = require('express');
const bodyParser = require('body-parser');
const cors = require('cors');

const app = express();
app.use(bodyParser.json());
app.use(cors());
app.use(express.static('public'));

app.listen(process.env.PORT, () => {
  console.log('Web server running as: ' + process.env.C9_HOSTNAME);
});

const mysql = require('mysql');
const db = mysql.createConnection({
  host: 'localhost',
  user: 'username', // <-- Update this line with your username.
  database: 'c9'
});

db.connect((err) => {
  if (err) {
    console.error('Unable to connect to the database.');
    throw err;
  } else {
    console.log('Connected to the database.');
  }
});

function makeUrl(id) {
  return 'http://' + process.env.C9_HOSTNAME + '/quotations/' + id;
}

app.get('/quotations', (req, res) => {
  db.query('SELECT id, author FROM quotations', (err, rows) => {
    if (err) {
      res.status(500).json(err);
    } else {
      let result = rows.map((row) => {
        return {
          id: row.id,
          author: row.author,
          url: makeUrl(row.id)
        };
      });
      res.json(result);
    }
  });
});

To test the server program, type at the terminal:

node quotations_app

Now run curl as a client. At at another terminal window, type:

curl localhost:8080/quotations

You should see a huge list containing all the objects representing the information of every quotation in the database, something like this:

[{"id":1,"author":"Harold Abelson","url":"http://host-name/quotations/1"},
 {"id":2,"author":"Alan Kay","url":"http://host-name/quotations/2"},
 {"id":3,"author":"Donald Knuth","url":"http://host-name/quotations/3"},

 ... (46 quotations omitted)

 {"id":50,"author":"Aristotle","url":"http://host-name/quotations/50"}]

3.6. Exercise B ★

Modify the GET /quotations web service so that it takes an optional query string parameter called sort. If this parameter is equal to false or if it wasn’t provided, the web service should behave as it currently does. On the other hand, if the parameter is equal to true, then the returned list of quotation objects should be sorted by author in ascending order. Use curl to test your modified web service.

These two commands should produce the same output that we had before:

curl localhost:8080/quotations?sort=false

curl localhost:8080/quotations

But this command:

curl localhost:8080/quotations?sort=true

should produce a different output, something like this:

[{"id":2,"author":"Alan Kay","url":"http://host-name/quotations/2"},
 {"id":27,"author":"Alan Kay","url":"http://host-name/quotations/27"},
 {"id":29,"author":"Alan Kay","url":"http://host-name/quotations/29"},

 ... (46 quotations omitted)

 {"id":46,"author":"Thomas Watson","url":"http://host-name/quotations/47"}]

3.7. Exercise C ★

Modify the GET /quotations web service so that each individual quotation object of the resulting list has an additional prelude property, comprised of the first three words of the excerpt followed by an ellipsis ('...').

For example, the curl command:

curl localhost:8080/quotations

should now produce this output:

[{"id":1,"author":"Harold Abelson","prelude":"Programs must be...",
  "url":"http://host-name/quotations/1"},
 {"id":2,"author":"Alan Kay","prelude":"Technology is anything...",
  "url":"http://host-name/quotations/2"},
 {"id":3,"author":"Donald Knuth","prelude":"Everyday life is...",
  "url":"http://host-name/quotations/3"},

 ... (46 quotations omitted)

 {"id":50,"author":"Aristotle","prelude":"We are what...",
  "url":"http://host-name/quotations/50"}]

This problem can be solved easily using these JavaScript functions: split(), slice(), and join().

3.8. Text Clients

We can write client programs that access our Quotations web service using any language with libraries that support HTTP GET requests. The following programs are simple text clients that call our web service and print to the standard output the id and author of all the quotations.

All these programs produce this exact same output:

1 - Harold Abelson
2 - Alan Kay
3 - Donald Knuth

... (46 quotations omitted)

50 - Aristotle

3.8.1. ECMAScript 6

File: quotations_client.js

'use strict';

const request = require('request');
const URL = 'http://localhost:8080/quotations';

request(URL, (err, response, body) => {
  if (err || response.statusCode !== 200) {
    console.error('ERROR: ' + (err || body));
  } else {
    let result = JSON.parse(body);
    result.forEach((q) => {
      console.log('%d - %s', q.id, q.author);
    });
  }
});

To run this program, type at the terminal:

node quotations_client

3.8.2. Python 3

File quotations_client.py

from http.client import HTTPConnection
from sys import stderr
from json import loads

URL = 'http://localhost:8080/quotations'

conn = HTTPConnection("localhost:8080")
try:
    conn.request("GET", URL)
    res = conn.getresponse()
    body = res.read().decode('utf-8')
    if res.status != 200:
        raise Exception(str(res.status) + ' '
            + res.reason + '. ' + body)
    for q in loads(body):
        print('{0} - {1}'.format(q['id'], q['author']))
except Exception as err:
    print('ERROR: ' + str(err), file=stderr)

To run this program, type at the terminal:

python3 quotations_client.py

3.8.3. Java 7

File: QuotationsClient.java

package sigcse2016.workshop;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.StringReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.Iterator;
import org.json.simple.JSONArray;
import org.json.simple.JSONObject;
import org.json.simple.parser.JSONParser;

class QuotationsClient {

    public static final String URL_STRING = "http://localhost:8080/quotations";

    public static void main(String[] args) throws Exception {
        URL url = new URL(URL_STRING);
        try {
            HttpURLConnection conn =
                (HttpURLConnection) url.openConnection();
            conn.connect();
            try (BufferedReader br =
                    new BufferedReader(
                        new InputStreamReader(conn.getInputStream()))) {
                StringBuilder body = new StringBuilder();
                String line;
                while ((line = br.readLine()) != null) {
                    body.append(line);
                }
                JSONParser parser = new JSONParser();
                JSONArray list =
                    (JSONArray) parser.parse(new StringReader(body.toString()));
                Iterator it = list.iterator();
                while (it.hasNext()) {
                    JSONObject q = (JSONObject) it.next();
                    System.out.printf("%d - %s%n",
                        q.get("id"), q.get("author"));
                }
            }
        } catch (Exception e) {
            System.err.println("ERROR: " + e);
        }
    }
}

To run this program, follow these steps:

Download the json-simple JAR file. This allows us to work with JSON in Java. At the terminal type:
```
wget http://json-simple.googlecode.com/files/json-simple-1.1.1.jar
```

Compile the program. Type:

javac -d . -cp json-simple-1.1.1.jar QuotationsClient.java

Execute the compiled program. Type at the terminal:

java -cp .:json-simple-1.1.1.jar sigcse2016.workshop.QuotationsClient

3.9. Exercise D ★

Modify any of the programs from the Text Clients section so that it displays the prelude property that was added to the objects of the result list of our web service in Exercise C.

The output of your modified client program should be like this:

1 - Harold Abelson: Programs must be...
2 - Alan Kay: Technology is anything...
3 - Donald Knuth: Everyday life is...

... (46 quotations omitted)

50 - Aristotle: We are what...

3.10. GET /quotations/{ID}

Let’s now write a web service for retrieving a single quotation with a particular ID:

File: quotations_app.js (add this code to the end of the file)

app.get('/quotations/:id', (req, res) => { (1)
  db.query('SELECT id, author, excerpt FROM quotations WHERE id = ?', (2)
    [req.params.id], (3)
    (err, rows) => {
      if (err) {
        res.status(500).json(err);
      } else {
        let row = rows[0]; (4)
        if (row) {
          res.json({ (5)
            id: row.id,
            author: row.author,
            excerpt: row.excerpt,
            url: makeUrl(row.id)
          });
        } else {
          res.type('text').status(404).send('Resource not found.\n'); (6)
        }
      }
    });
});

The code is quite similar to the previous web service, but with some important differences:

1	The URI for our web service endpoint is `'/quotations/:id'`. This allows us to use `:id` as named route parameter; its value can be obtained with the expression: `req.params.id`.
2	Extract from our table the id, author, and excerpt information of the specific quotation that has the same ID as the one indicated in the request.
3	Any value contained in this array will be correctly inserted in the previous SQL statement, replacing the corresponding placeholder symbol (`?`). This eliminates SQL injections vulnerabilities.
4	If the SQL statement result is a single row, that means we found the requested quotation.
5	In that case, respond with an implicit status code 200, returning the resulting row as a JSON object.
6	Otherwise `row` is `undefined`, so respond with a status code 404 and a message stating that the resource was not found.

Let’s use curl again, now to retrieve a single quotation. Restart the server. At a different terminal type:

curl localhost:8080/quotations/42

The output should be:

{"id":42,"author":"Douglas Adams","excerpt":"A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.","url":"http://host-name/quotations/42"}

3.11. A Web Client: cowsay

We’ll now show the code for an HTML client that employs our two RESTful web services. This application is a combination of two classical Unix programs: fortune and cowsay. It’ll display an ASCII picture of a cow with a random quotation.

Follow these steps:

Create a directory called public. At the terminal type:
```
mkdir public
```

Create a CSS file called styles.css in the public directory:

File: public/styles.css

body {
  color: Gainsboro;
  background: #1D1D1D;
  padding: 0 2em;
  margin: 0;
  font-family: sans-serif;
  font-size: 20px;
}

h1 {
  color: White;
  font-size: 180%;
}

.invisible {
  display: none;
}

#author_paragraph {
  text-align: right;
  font-size: 80%;
  font-style: italic;
}

#cowsay {
  color: Chartreuse;
}

#cowsay pre {
  margin-top: 0px;
  font-size: 150%;
  font-weight: bold;
}

button {
  background-color: Cornsilk;
  border: none;
  color: black;
  padding: 10px 15px;
  text-align: center;
  text-decoration: none;
  display: inline-block;
  margin: 4px 2px;
  font-size: 110%;
  cursor: pointer;
  border-radius: 20px;
  margin-right: 20px;
}

#balloon {
  padding: 10px 30px;
  border: 5px dashed Chartreuse;
  border-radius: 20px;
  width: 500px;
}

In the public directory, create a JavaScript file called cowsay.js:

File: public/cowsay.js

var QUOTATION_URL = 'http://host-name/quotations/'; // <-- Update the host-name

function start() {
  $.ajax({
    url: QUOTATION_URL,
    method: 'GET'
  })
  .done(getAllQuotations)
  .fail(displayError);
}

function getAllQuotations (quotations) {
  var rndNum = (quotations.length * Math.random()) | 0;
  $.ajax({
    url: quotations[rndNum].url,
    method: 'GET'
  })
  .done(getOneQuote)
  .fail(displayError);
}

function getOneQuote (quote) {
  $('#excerpt').text(quote.excerpt);
  $('#author').text(quote.author);
  $('#cowsay').show();
}

function displayError (err) {
  alert(JSON.stringify(err));
}

$(start);

Create an HTML file called cowsay.html and place it in the public directory:

File: public/cowsay.html

<!DOCTYPE html>
<html>
  <head>
    <title>Cowsay</title>
    <link rel='stylesheet' href='/styles.css' />
    <script src='https://ajax.googleapis.com/ajax/libs/jquery/1.12.0/jquery.min.js'>
    </script>
    <script src='/cowsay.js'></script>
  </head>
  <body>
    <h1>cowsay @ SIGCSE 2016</h1>
    <div id="cowsay" class="invisible">
      <div id="balloon">
        <p id="excerpt"></p>
        <p id="author_paragraph">
          &mdash; <span id="author"></span>
        </p>
      </div>
        <pre>
       \
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||
        </pre>
      <button onclick="location.reload(true)"/>Refresh</button>
    </div>
  </body>
</html>

Now, place the URL host-name/cowsay.html in your browser’s address bar. You should see something like this:

3.12. PUT /quotations/{ID}

The code for a RESTful web service that updates a specific quotation would look like this:

File: quotations_app.js (add this code to the end of the file)

app.put('/quotations/:id', (req, res) => { (1)
  let id = req.params.id;
  let body = getBody(req);
  if (body) {
    db.query('UPDATE quotations SET author=?, excerpt=? WHERE id=?',
      [body.author, body.excerpt, id],
      (err, result) => {
        if (err) {
          res.status(500).json(err);
        } else {
          res.type('text').send(
            'Resource with ID = ' + id + ' updated.\n');
        }
      });
  } else {
    res.type('text').status(400).send('Bad data. No resource updated.\n'); (2)
  }
});

1	Notice that we use the HTTP `PUT` method and that we indicate in the URI the ID of the specific quotation to modify.
2	If the request body is not what’s expected, respond with a status code 400 and a “bad data” error message.

The getBody() function used in the above code verifies two things:

That the content type of the request body is declared to be application/json.
That the JSON request body actually contains two properties: author and excerpt.

If any of these conditions is not met, the function returns undefined. Otherwise it returns a new object with the corresponding values of author and excerpt provided by the client request. This is the code for this function:

File: quotations_app.js (add this code to the end of the file)

function getBody(req) {
  let body = req.body;
  let correctContentType =
    req.headers['content-type'].startsWith('application/json');
  let correctProperties =
    body.author !== undefined && body.excerpt !== undefined;

  if (correctContentType && correctProperties) {
    return { author: body.author, excerpt: body.excerpt };
  } else {
    return undefined;
  }
}

We’ll use curl to test our new web service. After restarting the server, type at another terminal:

curl localhost:8080/quotations/1 \
-X PUT \
-H 'Content-Type: application/json' \
-d '{"author":"Somebody","excerpt":"Some smart quotation."}'

Notice that we need to specify not only the URL, but also:

The HTTP method we want use (with the -X option).
A request header with the content type of the body (with the -H option).
The JSON request body itself (with the -d option).

The expected output is:

Resource with ID = 1 updated.

To verify that the quotation was actually updated use the GET /quotations/{ID} web service:

curl localhost:8080/quotations/1

You should now see the updated quotation with the new author and excerpt values:

{"id":1,"author":"Somebody","excerpt":"Some smart quotation.","url":"http://host-name/quotations/1"

3.13. Exercise E ★★

Write a web service for deleting a specific quotation. The endpoint definition of this service should look something like this:

app.delete('/quotations/:id', (req, res) => {
  // ...
});

The delete SQL statement that you need to use has the following syntax:

DELETE FROM table-name WHERE column-name = value

Use curl to test your code after restarting the server:

curl localhost:8080/quotations/7 -X DELETE

When the corresponding ID exists the output should be a text message like this:

Resource with ID = 7 deleted successfully.

If the ID doesn’t exist (for example if you try to delete the same quotation again), the output should be:

Resource with ID = 7 does not exist.

In the callback function for the db.query(), use the property result.affectedRows to check how many rows were affected when performing the delete operation. If it’s equal to zero, then no rows were deleted, meaning that the requested ID was not found. If it’s equal to one, then the deletion was successful.

3.14. Exercise F ★★

Write a web service for creating a new quotation. The endpoint definition of this service should look something like this:

app.post('/quotations', (req, res) => {
  // ...
});

The MySQL statement to insert a new row in the database has the following syntax:

INSERT INTO table-name SET column-name₁ = value₁, column-name₂ = value₂, …, column-name_n = value_n

Just like in the PUT /quotations/{ID} web service, the request body should contain a JSON object with property names and values for author and excerpt. If this is not the case, respond with a status code 400 and a “bad data” error message.

In the callback function for db.query(), use the property result.insertId to get the ID of the newly created quotation.

The response must include a header, called location, containing the URL of the new quotation.

Use the method res.set("header-name", "value") to add headers to the response.

If the insertion is successful, the response status must be 201.

Restart the server. Test your code using curl:

curl -i \
localhost:8080/quotations \
-X POST \
-H 'Content-Type: application/json' \
-d '{"author":"Yoda","excerpt":"Do. Or do not. There is no try."}'

The output should be:

HTTP/1.1 201 Created
X-Powered-By: Express
Access-Control-Allow-Origin: *
location: http://sigcse2016-workshop402-arielortiz.c9users.io/quotations/51
Content-Type: text/plain; charset=utf-8
Content-Length: 35
ETag: W/"23-ld6StBwFwiRTMacdtjicag"
Date: Fri, 04 Mar 2016 05:01:03 GMT
Connection: keep-alive

New resource created with ID = 51.

Specifically note the response status and the location header.

4. Resources

4.1. Useful Web Links

4.2. Books

Beginning Node.js by Basarat Syed
RESTful Web API Design with Node.js by Valentin Bojinov