Creating an API-Centric Web Application

Planning to start working on a new web application? In this tutorial, we'll discuss how to create an API-centric web application, and explain why this is essential in today's multi-platform world.

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For those who are unfamiliar with the term, API is short for Application Programming Interface. According to Wikipedia:

An application programming interface (API) is a source code based specification intended to be used as an interface by software components to communicate with each other. An API may include specifications for routines, data structures, object classes, and variables.

In simpler terms, an API refers to a set of functions built into an application, which can be used by other applications (or by itself, as we'll see later), to interact with the application. An API is a great way to expose an application's functionality to external applications safely and securely, since all functionality that these external applications can do is limited with what functionality is exposed in the API.

What's an “API-Centric” Web Application?

An API-Centric Web Application is a web application that basically executes most, if not, all its functionality through API calls.

An API-Centric Web Application is a web application that basically executes most, if not, all its functionality through API calls. For example, if you were to log in a user, you would send his credentials to the API, and the API would return to you a result saying if the user provided the correct user-password combination.

Another characteristic of an API-Centric Web Application is that the API will always be stateless, meaning it can't recognize API calls by session. Since API calls will be made by usually via the backend code, it will be hard to implement session handling, since there are usually no cookies involved in that. This limitation is actually good — this “forces” a developer to build an API that works not based on the state of the current user, but rather on functionality, which in turn, makes it easier to test, since the current state of a user doesn't need to be recreated.

Why go through all this trouble?

As web developers, we've seen technology evolve first hand. It's common knowledge that people today don't just use applications via a browser, but through other gadgets, like mobile phones and tablets. For example, this article on Mashable, entitled “Consumers Now Spending More Time on Mobile Apps Than the Web”, states:

Consumers are spending more time on mobile apps than on the web for the first time, a new report claims.

Flurry compared its mobile data to stats from comScore and Alexa, and found that in June, consumers spent 81 minutes per day using mobile apps, compared to 74 minutes of web surfing.

Here's a more recent article from ReadWriteWeb, entitled “More People Browse On Mobile Than Use IE6 & IE7 Combined:

The latest data on browser trends from Sitepoint show that more people browse the Web on smartphones than use Internet Explorer 6 and 7 combined. Those two old clunkers have been the bugbears of Web developers for years, requiring sites to degrade as nicely as possible to that least common denominator of browsers. But it's a new world now; 6.95% of Web activity in November 2011 was on mobile browsers, and only 6.49% was on IE 6 or 7.

As we can clearly see, more and more people get their news from alternative venues, specifically mobile devices.

What does this have to do with me creating an API-Centric Web Application?

This would inevitably lead to more usage of our application, since it can be used anywhere a person wants.

One of the main advantages of creating an API-centric application is that it helps you build functionality that can be used by ANY device, be it a browser, a mobile phone, a tablet, or even a desktop app. All you need to do is to create the API in such a way that all these devices can communicate with it, and voila! You'll have built a centralized application that can take input and execute functionality from any device that a person has!

API-Centric Application Diagram

API-Centric Application Diagram

By creating an application in this manner, we're able to easily take advantage of the different mediums used by different people. This would inevitably lead to more usage of an application, since it can be used anywhere a person wants.

To drive the point home, here's an article about Twitter's new redesigned website, which tells us about how they now use their API to power, essentially making it API-centric:

One of the most important architectural changes is that is now a client of our own API. It fetches data from the same endpoints that the mobile site, our apps for iPhone, iPad, Android, and every third-party application use. This shift allowed us to allocate more resources to the API team, generating over 40 patches. In the initial page load and every call from the client, all data is now fetched from a highly optimized JSON fragment cache.

In this tutorial, we'll be creating a simple TODO list application that is API-Centric and create one front-end client on the browser that interacts with our TODO list application. By the end, you'll know the integral parts of an API-Centric application, and at the same time, how to facilitate secure communication between the two. With that in mind, let's begin!

Step 1: Plan the Application's Functions

The TODO application we'll be building in this tutorial will have the basic CRUD functions:

  • Create TODO Items
  • Read TODO Items
  • Update TODO Items (rename, mark as done, mark as undone)
  • Delete TODO Items

Each TODO item will have:

  • a Title
  • a Date Due
  • a Description
  • a flag to tell if the TODO Item Is Done
  • Let's mockup the application as well so we have a guide on how it should look like afterwards:

    SimpleTODO Mockup

    SimpleTODO Mockup

    Step 2: Create the API Server

    Since we're developing an API-Centric application, we'll be creating two “projects”: the API Server, and the Front-end Client. Let's begin by creating the API server first.

    On your web server's folder, create a folder named simpletodo_api, and create an index.php file. This index.php file will act as a front controller for the API, so all requests to the API server will be made through this file. Open it up and put the following code inside:

What we've essentially built here is a simple front controller that does the following:

  • Accept an API call with any number of parameters
  • Extract the Controller and Action for the API call
  • Make the necessary checks to ensure that the Controller and Action exist
  • Execute the API call
  • Catch errors, if any
  • Send back a result to the caller

Besides the index.php file, create three folders: a controllers, models and data folder.

API server folders
  • The controllers folder will contain all the controllers we'll be using for the API server. We'll be building it using the MVC architecture to make the structure of the API server cleaner and more organized.
  • The models folder will contain all the data models for the API server.
  • The data folder will be where the API server saves any data

Go into the controllers folder and create a file called Todo.php. This will be our controller for any TODO list related tasks. With the functions we'll be needing for our TODO application in mind, create the necessary methods for the Todo controller:

Now, add the necessary functionality to each action. I'll provide the code for the createAction method and I'll leave it up to you to create the code for the other methods. If you're not in the mood though, you can just download the source code for the demo and copy it from there.

Create TodoItem.php inside the models folder so we can create the “item creation” code. Take note that I won't be connecting to a database, rather, I'll be saving the information into files. It should be relatively easy though to make this work with any database.

The createAction method calls two functions on the TodoItem model:

  • save() - this saves the TodoItem into a file, as well as set the todo_id for the TodoItem if necessary
  • toArray() - this returns an array version of the TodoItem, where the variables are the array's indexes

Since the API is called via HTTP requests, let's test that API call by calling it through the browser:


If everything worked, you should see a new folder inside the data folder, and inside that folder, you should see a file with the following content:

createAction result

createAction() result

Congratulations! You've successfully created an API server and made an API call!

Step 3: Secure the API Server with an APP ID and APP SECRET

Currently, the API server is set to accept ALL API requests. We'll need to limit it to our own applications only, to ensure that only our own front-end clients are able to make API requests. Alternatively, you can actually create a system wherein users can create their own applications that have access to your API server, similar to how Facebook and Twitter applications work.

Begin by creating a set of id-key pairs for the clients that will be using the API server. Since this is just a demo, we can use any random, 32 character string. For the APP ID, let's say it's application APP001.

Open the index.php file again, and then update it with the following code:

What we've done here is actually implement a very simple way of authenticating our front-end clients using a system similar to public-private key authentication. Basically, here is the step-by-step breakdown of how the authentication happens:

Public-key encryption

Public-key encryption
  • an API call is made, in it an $app_id and $enc_request is provided.
  • the $enc_request value is the API call parameters, encrypted using APP KEY. The APP KEY is NEVER sent to the server, it's only used to hash the request. Additionally, the request can only be decrypted using the APP KEY.
  • once the API call arrives to the API server, it will check its own list of applications for the APP ID provided
  • when found, the API server attempt to decrypt the request using the key that matches the APP ID sent
  • if it was successful in decrypting it, then continue on with the program

Now that the API server is secured with an APP ID and APP SECRET, we can begin programming a front-end client to use the API server.

Step 4: Create the Browser Front-end Client

We'll begin by setting up a new folder for the front-end client. Create a folder called simpletodo_client_browser on your web server's folder. When that's done, create an index.php file and put this code inside:

That should look something like this:

SimpleTODO Login Page

SimpleTODO Login Page

Take note that I've included 2 JavaScript files and 2 CSS files here:

Next, let's create the login.php file so we store the username and password inside a session on the client.

Here, we simply start a session for the user, based on the username and password combination the user will provide. This acts as a simple combination key, which will allow a user to access stored TODO items for a specific combination of both the username and password. We then redirect to todo.php, where we start interacting with the API server. Before we start coding the todo.php file though, let's first create an ApiCaller class, which will encapsulate all the API calling methods we'll need, including encrypting the requests.

Create apicaller.php and put the following inside:

We'll be using the ApiCaller class to send requests to our API server. This way, all the necessary encryption and cURL initialization code will be in one place, and we won't have to repeat our code.

  • the __construct function takes in three parameters:

    1. $app_id - the APP ID for the client (which is APP001 for the browser client)
    2. $app_key - the APP KEY for the client (which is 28e336ac6c9423d946ba02d19c6a2632 for the browser client)
    3. $api_url - the URL of the API server, which is http://localhost/simpletodo_api/
  • the sendRequest() function:

    1. encrypts the request parameters using the mcrypt library in the same manner that the API server decrypts it
    2. generates the $_POST parameters to be sent to the API server
    3. executes the API call via cURL
    4. checks the result of the API call was successful or not
    5. returns the data when everything went according to plan

Now, let's begin with the todo.php page. First off, let's create some code to retrieve the current list of todo items for the user nikko with the password test1234 (this is the user/password combination we used earlier to test the API server).

Go to the index.php page, login as nikko/test1234, and you should see a var_dump() of the TODO item we created earlier.

TODO item var_dump

Congratulations, you've successfully made an API call to the API server! In this code, we've:

  • started the session so we have access to the username and userpass in the $_SESSION
  • instantiated a new ApiCaller class, giving it the APP ID, APP KEY and the URL of the API server
  • send a request via the sendRequest() method

Now, let's reformat the data so it looks better. Add the following HTML to the todo.php code. Don't forget to remove the var_dump()!

It should now look something like this:


Pretty cool huh? But this currently does nothing, so let's begin adding some functionality. I'll provide the code for new_todo.php, which will call the todo/create API call to create a new TODO item. Creating the other pages (update_todo.php and delete_todo.php) should be very similar to this one, so I'll leave it up to you to create those. Open up new_todo.php and add the following code:

As you can see, the new_todo.php page uses the ApiCaller again to facilitate the sending the todo/create request to the API server. This basically does the same thing as before:

  • start a session so it has access to the $username and $userpass saved in the $_SESSION
  • instantiate a new ApiCaller class, giving it the APP ID, APP KEY and the URL of the API server
  • send the request via the sendRequest() method
  • redirect back to todo.php

Congratulations, it works! You've successfully created an API-centric application!


There are so many advantages to developing an application that's built around an API. Want to create an Android application version of SimpleTODO? All the functionality you would need is already in the API server, so all you need to do is just create the client! Want to refactor or optimize some of the classes? No problem — just make sure the output is the same. Need to add more functionality? You can do it wihtout affecting any of the client's code!

Though there are some disadvantages like longer development times or more complexity, the advantages of developing a web application in this manner greatly outweight the disadvantages. It's up to us to leverage on this kind of development today so we can reap the benefits later on.

Are you planning to use an API server for your next web application, or have you already used the same technique for a project in the past? Let me know in the comments!



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