Rest Basics

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Rest Basics

Use Rest with React and Concurrently. There are two applications joined by a proxy:

Both of these slide decks contain relevant information that might be useful when working through this assignment:

Motivation

Your assignment is to understand how and why a request from a client is processed and routed by your server. This is one of the key subjects that we must understand. Eventually, we must perform a complete meld with HTTP requests and how they are routed on a server.

Get Started

Merge your current work from weeks 1 and 2 into master. Fix any conflicts that might arise.

Create a branch called week03 based on your work in master.

Create the Project Directory

We are going to have two projects in this assignment, so lets set up a directory for them.

Create a directory called week03-rest-basics in your week03 branch by navigating to the root of your repository and entering the following commands:

mkdir week03-rest-basics
cd week03-rest-basics

Create Server

Make sure you have the latest copy of JsObjects and that your global npm packages are up to date:

jo
git pull
ncu -g

In the RestBasics directory, run the following command:

elf-express server
cd server

You have now created a fully functional, node based, express server that can handle HTTP requests.

NOTE: You can usually return to the directory you were in previously by typing cd -.

URLS

The following diagram provides us with some nomenclature.

           hierarchical part
         ┌─────────┴─────────┐
           authority       path
         ┌─────┴───────┐ ┌──┴──┐
  http://localhost:30025/api/foo?key=value&#qux
  └┬─┘   └───┬────┘└─┬─┘         └─────┬─┘    └─┬─┘
scheme     host    port              query   fragment

Some of the terms shown in this diagram are esoteric, obscure, and infrequently used. The key things you need to be able to recognize are the:

URLs and URNs are both URIs. Very loosely speaking, the technical difference looks something like this:

An analogy might be:

Another common analogy:

Don't confuse a path with a URN. For instance, the path the-great-gatsby in the following URL is not a URN:

HTTP Requests

An HTTP requests consists of URL like the one shown above:

We use a URL like this to send requests from a client to a server. If we want to tell our server to do something, we send it a request in the form a URL.

Routing Middleware

Let's write code that will handle a request such as the one defined above. We will put our custom server code in its own file.

In the routes folder create a file called test-routes.js:

var express = require('express');
var router = express.Router();

/* Set up a route called foo. */
router.get('/foo', function(request, response) {
    var message = { 'result': 'success', 'status': 'bar', 'file': 'test-routes.js' };
    console.log('Foo called:\n' + JSON.stringify(message, null, 4));
    response.send(message);
});

module.exports = router;

As explained above, this method is designed to be called whenever the user enters, or sends via HTTP, the following URL:

Application Middleware

To get started, we can describe how to link this new file to our express project. This involves using two commands:

Let's talk some more about middleware. Our server uses a library called express that is designed in part to handle requests sent to our server from a client. When a request is sent to our server:

Remember, if we don't ever set up any middleware for handling a request, then we never see the request and it will probably be, for all intents and purposes, ignored by our server.

Set the Port

Now set the port, in bin/www, to 30026. Two ways to do that are explained in our Concurrently assignment.

Be sure to modify listen like this:

server.listen(port, () => console.log("Listening on", port));

We are going to run the client on port 30025, so we are setting the port for the server to some other number. I've chosen 30026 in the hopes that it will be easy to remember.

Take a moment to understand what we have done.

For instance, the following HTTP request would call the foo method in our new file:

Here is how to break it down:

Application vs Router Middleware

Notice that we have made two similar calls:

The first use Express application middleware. The second uses Express router middleware.

The router object is designed to help you set up a dedicated portion of your server for handling a particular kind of URI. For instance, all URI's that begin begin /test-routes are handled in the file called routes/test-routes.js.

Test your Work

If you want, go ahead and start your new server:

npm start

NOTE: Most of the time, just typing npm start is sufficient to start your project. However, if you get an error, the first thing to check is that all the packages your app needs are properly installed. To do this, type npm install (npm i) and then try npm start again.

Once your app has started without errors, enter the following URL in the address bar of your browser:

You should see very plain output that looks a bit like this:

{"result":"success","status":"bar","file":"test-routes.js"}

Second Terminal Tab

Our server is running in your bash shell. Because of the way we started it, it completely takes up the shell making it unusable for any other purpose:

$ npm start

> server@0.0.0 start /home/charlie/Git/isit320-calvert-2017/RestBasics/server
> nodemon ./bin/www

[nodemon] 1.11.0
// NODEMON OUTPUT OMITTED HERE
Foo called:
{
    "result": "success",
    "foo": "bar",
    "file": "test-routes.js"
}
GET /test-routes/foo 304 7.572 ms - -

Because we can't use this terminal, we need to create a new one. In particular, you'll need to create a new terminal tab, by entering this command: CTRL-SHIFT-T. You can also create tabs from the terminal menu. In either case, you should end up with two bash shells available.

One terminal, two tabls

NODE: For those who know about such things, I'll add that an express server is probably not a candidate to be run in the background with the & symbol. Later on we will see other ways to solve this problem, but for now, let's just start two bash terminal tabs, one for the server, and one for the client.

Create Client

Navigate to the second terminal tab, the one you are not using. From the root of the week03-rest-basics directory, create your client:

create-react-app client
cd client

This uses a global NPM package found in ~/npm/bin to create a new react based client application for use in the browser. It uses ES6 syntax, and has many fancy features that we will explore over time. create-react-app is built by Facebook, the same people who create React.

NOTE: At the time of this writing, April 2019, create-react-app uses an older version of Jest that has a lot of vulnerabilities. This is scheduled to be fixed in version 3.0 of create-react-app. Type create-react-app -- version to check the version number, and type ncu -g to see if an update is available.

It can be easier to use create-rect-app than to build a program by hand as we did in our ReactBasics assignment. The output from create-react-app is, however, much more feature rich and much more complex. Also, it commits you to a react based style of development quite different from traditional ES5 development that uses jQuery, Angular or some other library. In this course, we are going to spend considerable time working with older applications that have subsystems built with React and ES6.

Because our application uses ES6 (ES2015), you want to tell WebStorm to expect modern JavaScript syntax. In particular, set the File | Settings | Languages and Frameworks | JavaScript to React JSX.

JSX and React

Also set up ESLint support at File | Settings | Languages and Frameworks | JavaScript | Code Quality Tools | ESLint. Set the ESLint package to ~/npm/lib/node_modules/eslint. If that doesn't work, try "Detect package from configuration". You should be able to pick either option from drop down.

Webstorm and eslint

If you get an error about eslint: In the client, we used create-react-app and it installed eslint 9.0.0. Now back in the root of our project, we have concurrently itself set up, and it also installs eslint. Only it is perhaps using eslint 10.0.1. We have to change the version of eslint that is in the package.json that is in the root of our project so it is sync with the one in the client. To do this. You can open package.json and manually set eslint to 9.0.0. Remember, do this not in the client, but in the root of the project. Then, just to be safe, while still in the root of the project, delete node_modules and package-lock.json:

rm -r node_modules
rm package-lock.json.

Then reinstall: npm i

Open up App.css and change the logo height to 10vmin or smaller:

.App-logo {
  animation: App-logo-spin infinite 20s linear;
  height: 10vmin;
  pointer-events: none;
}

Set the min-height for the App-header to some small value like 15vh. Or build your own header. Be creative and do something unique. All I ask is that we can actually see your button when we open the app.

To learn about viewport units (vmin and vh), run this search:

In your client application, in package.json, just before the dependencies section, add this:

"proxy": "http://localhost:30026",

This tells our react app to forward requests for REST calls to our Express Server that is running on port 30026. See code in scripts/start.js for more details.

NOTE: Experiences has taught me that this is one of the most frequently neglected steps. Don't omit it! It is very important.

Make HTTP Request

We will use the native JavaScript call fetch in lieu of the jQuery calls named $.ajax or $.getJSON. The fetch call is part of the ES6 standard, and is now finalized.

Rewrite the Client

Here is the heart of our application, which we construct in client/src/App.js:

import React, {Component} from 'react';
import logo from './logo.svg';
import './App.css';

class App extends Component {
    constructor() {
        super();
        this.state = {            
            file: 'unknown',
            result: 'unknown',
            status: 'unknown'
        };
    }

    queryServer = () => {
        const that = this;
        fetch('/test-routes/foo')
            .then(function(response) {
                return response.json();
            })
            .then(function(json) {
                console.log('parsed json', json);
                that.setState(foo => (json));
            })
            .catch(function(ex) {
                console.log('parsing failed, URL bad, network down, or similar', ex);
            });
    };

    render() {
        return (
            <div className="App">
                <div className="App-header">
                    <img src={logo} className="App-logo" alt="logo"/>
                    <h2>Welcome to React</h2>
                </div>

                <p className="App-intro">
                    state: {this.state.status} file: {this.state.file}
                </p>
                <button onClick={this.queryServer}>Bar</button>
            </div>
        );
    }
}

export default App;

The key call here is to fetch, found in the method named queryServer. featch uses JavaScript promises. The promise uses two .then methods. If the call could not be made because of a problem on the client side, then the first .then method throws an exception. This first call to the .then method is there to check if the HTTP call was able to send a request to the server;

It will succeed even if the server reported an error such as 404 Not found or 500 Internal Server error. In other words, the first .then can succeed even if the server throws a 404 or 500 error. However, response.ok will be set to set false, and status and statusText will contain the error number and string describing the error. Thus one could write in the first then method:

if(response.ok) {
    return response.json();
}
const errorString = response.status + ' ' + response.statusText;
throw new Error(errorString);

If you did this, the second then would never be called.

The second .then method gives us the result if the method is able to make a call to the server. If the server sent us back some JSON, then the JSON will be found here in the second .then clause. Unless we explicitly throw an exception the second then will be called even on a 404 or 500 error. Sometimes the server correctly sends back JSON or some other entity, but we fumble our handling of it, thereby throwing an exception in the second .then call.

All errors should end up in the .catch block.

Let me try to summarize that:

NOTE: fetch can be used for retrieving a multitude of objects, not just JSON. But in our course, we usually use it retrieve JSON, so I'm emphasizing that functionality.

fetch('/test-routes/foo')
    .then(function(response) {
        // DID HTTP TALK TO THE SERVER? BLOWS UP IF SYNTAX BAD, NETWORK DOWN, URL BAD, ETC.
        // CHECK response.ok TO SEE IF THE CALL SUCCEEDED ON THE SERVER SIDE.
        // response.ok will be false if we return a 404 or 500 error.
    })
    .then(function(json) {
        // IF WE SUCCEED, WE GET RESULT HERE. TYPICALLY SOME JSON.
    })
    .catch(function(ex) {
        console.log('parsing failed, URL bad, network down, or similar', ex);
    });

Run the Application.

Assuming you have already run (npm install) the call to start the client could not be simpler:

npm start

Your application will magically open in the browser. Updates should also appear as you make them in your code.

NOTE: Remember, we typically need to run npm install only once per project on a particular machine. We run npm start every time we want to start or restart the application. We can use Ctrl-C to shut down either the client or the server.

Run Concurrently

Make sure you set up the project the same way we did in the Concurrently assignment. We should be able to issue a single command and start both projects.

Turn it in

Don't forget concurrently.

Push, submit. Specify:

Eject Option

Though I don't recommend it, and in fact recommend that you don't do it, if you are terminally curious, you can run this command in the root of your client application:

npm run eject

The command exposes the inner workings of create_react_app. In particular, it creates a scripts directory and modifies package.json. Look at package.json to see the relationship between the two. Rather than do this with a project that you might want to keep, I would do create a temporary project, and try this command there. It's interesting to learn more about create-react-app.

Manual Step

Don't do this in this application because everything is handled for us by the code auto-generated by create-react-app. But in a manual ReactBasics type of project, I believe you also need to add code to webpack.config.js in the entry property:

entry: [
     './public/javascripts/control.js'
]

Cloud 9

This is optional, but if you want to run on Cloud9 you might find these notes helpful.

Also, when on Cloud 9 be sure bower is installed:

npm -g install bower

Then make sure you have processed both package.json and bower.json:

npm install && bower install

Then run the application:

npm start

Then Preview | Preview running application from the menu items near the top right of the Cloud 9 IDE.

Putting Server Side Code in a Separate File

We don't do this anymore in this assignment. But sometimes, we want to move code out of routes/index.js into its own file. For instance, if had several methods in index.js for testing our ability to call a route, we might move that code into a file called routes/test-routes.js.

To set up our middleware for handling requests to testRoutes from the server we take two simple steps. In app.js around lines 10 and 28, we write the following:

var testRoutes = require('./routes/test-routes');  // on line 10
app.use('/test-routes', testRoutes);               // on line 28

The first line just allows us to gain access to our new file: it links it into our project. The second line states what we want to do: we want to use the code in our file as middleware. We are telling Express that if there is a request from the server that ends with /test-routes, then it should be passed to our code in ./routes/test-routes.js.