Purple Cheetah

Introduction

Purple Cheetah is not a framework but rather a utility set for ExpressJS, written in Typescript. It was developed to resolve issues in our company and give us tools with a small amount of external dependencies for creating Web APIs (REST and/or GraphQL).

Table of contents

• Getting started
• Creating Purple Cheetah application
• Controller
  • Controller setup
  • Pre request handler
• Middleware
• Modules

Getting started

• The Easiest way to get started is to clone the starter GitHub repository: git clone [email protected]:becomesco/purple-cheetah-starter.
• Install the project dependencies by running: npm i.
• With this completed, development server can be started by running: npm run dev.
• By opening a browser and going to http://localhost:1280/hello/world, JSON response from the server can be seen.

Creating Purple Cheetah application

To create Express application powered by Purple Cheetah tool set, createPurpleCheetah function is used. This is also the main entry point for the application which is used to configure it. Application configuration is pretty simple. There are 4 important properties in configuration object (see Snippet 1):

• port - Specify the port on which Express server will be available,
• controllers - Array of controller objects which will be mounted in order,
• middleware - Array of middleware object which will be mounted in order,
• modules - Array of module objects which will be mounted in order,

const app = createPurpleCheetah({
  port: 1280,
  modules: [
    /* List of modules */
  ],
  controllers: [
    /* List of controllers */
  ],
  middleware: [
    /* Middleware list */
  ],
});

Snippet 1 - Create a Purple Cheetah application.

This will be explained in more detail.

• First step is mounting modules. They are mounded in FIFO order and once 1 module is mounted, it will trigger a callback which will mount the next module, and so on.
• With all modules mounted, if start function is present in the configuration, it will be called.
• Next step is mounting middleware objects which have flag after equal to false.
• After that, is middle function is present in the configuration, it will be called.
• In next step, all controller objects will be mounted in FIFO order.
• With all controllers mounted successfully, all middleware objects, with flag after equal to true, will be mounted.
• If finalize function is present in the configuration, it will be called.
• With all above steps completed successfully, HTTP server will be started, and it will print message like one shown below:

Purple Cheetah - Started Successfully
-------------------------------------
PORT: 1280
PID: 24720
TTS: 0.007s

Controller

Most important tools for creating REST APIs are tools for connecting HTTP requests with some business logic, doing a required work and creating a response. This is as easy as creating an HTTP route handler for specified method. In pure Express application this could be done like shown in Snippet 2.

app.get('/user', (request, response) => {
  // Get user from the database
  // ...
  response.json(user);
});

Snippet 2 - Creating an endpoint using ExpressJS

This is all very nice but writing a code this way can be messy and organizing it can be a challenge. Because of this, abstracts like Controller, Controller method and Middleware exist in the Purple Cheetah tool set. In this section, Controller abstract will be covered.

Controller is an abstraction which provides clean and unified way for creating a group of REST endpoints. Controller object is created by calling createController function which accepts configuration object as a parameter. Controller by itself if just a "placeholder" and does not hold any complex logic. To implement a logic, and to add REST endpoints, Controller method is used.

By using the Purple Cheetah Controller approach, code from Snippet 2 can be rewritten lite shown in Snippet 3.

const UserController = createController({
  name: 'User',
  path: '/user',
  methods() {
    return {
      getUser: createControllerMethod<unknown, User>({
        type: 'get',
        async handler() {
          // Get user from the database
          // ...
          return user;
        },
      }),
    };
  },
});

Snippet 3 - Create an endpoint using the Purple Cheetah controller/method approach.

Much more code is written in Snippet 3 compared to 2, so why is this better? Second example provides structure, consistency (which is not easy to spot on such a short example) and unified way to create REST endpoints. This means that navigation in project is much quicker, and it is easier to understand what is the end result of each endpoint. In addition to that, return type of the method can be specified.

Controller setup

Setup is a method inside of the controller configuration object which is called when a controller is mounted. It can be asynchronous and it is designed to some work with will be required for all (or majority) of method in specified controller. Output from the setup method is piped into methods and made available in that way.

const MyController = createController<{ baseMessage: string }>({
  name: 'My controller',
  path: '/hello',
  setup() {
    return {
      baseMessage: 'Hello, ',
    }
  }
  methods({ baseMessage }) {
    return {
      world: createControllerMethod<unknown, { message: string }>({
        path: '/:name',
        type: 'get',
        async handler({ request }) {
          return {
            message: `${baseMessage}${request.params.name}`
          };
        },
      }),
    };
  },
});

Snippet 4 - Setup method in a controller

Code from Snippet 4 is trivial but it illustrates how the setup methods works and how it is used. Some more relevant example would be to get security keys from other application which are required in controller methods.

Pre request handler

It is a method inside of a Controller Method configuration object. It is executed before each required to specified route and output from it is piped to handler method. Example for Snippet 4 can be modified to use pre request handler to which will convert a first letter of request parameter in upper-case like shown in Snippet 5.

const MyController = createController<{ baseMessage: string }>({
  name: 'My controller',
  path: '/hello',
  setup() {
    return {
      baseMessage: 'Hello, ',
    }
  }
  methods({ baseMessage }) {
    return {
      world: createControllerMethod<{ name: string }, { message: string }>({
        path: '/:name',
        type: 'get',
        async preRequestHandler({request}) {
          return {
            name: 
              request.params.name.substring(0, 1).toUpperCase 
              + request.params.name.substring(1).toLowerCase(),
          }
        }
        async handler({ name }) {
          return {
            message: `${baseMessage}${name}`
          };
        },
      }),
    };
  },
});

Snippet 5 - Controller method pre request handler

Again, example from Snippet 5 is trivial but it illustrates how pre request handler is used. As it can be seen, pre request handler functions like a middleware but only for specific route. It is useful when different routes use different mechanisms for security and resource protection.

Middleware

Middleware is similar to a controller, but it is usually used to transform incoming or outgoing data in some shape or form. Because of this, middleware is triggered for all methods on all routes which are starting with a specified path.

Like the createController function, createMiddleware function returns the Middleware object which is used in Purple Cheetah configuration object in the middleware array property. Example for creating a middleware object is shown in Snippet 6.

createMiddleware({
  name: 'Test middleware',
  path: '/test',
  handler() {
    return async () => {
      // Middleware logic.
    };
  },
});

Snippet 6 - Creating a simple middleware object.

It is important to know that handler method can be asynchronous and will be called only once when the middleware is mounted. So before returning a request handler function, setup of the middleware can be done.

Purple Cheetah comes with few predefined middleware objects:

• Body parser - Is a middleware which uses the functionality of the body-parser package. This middleware is not added to the Purple Cheetah application by default. To add it to the application, call createBodyParserMiddleware in middleware array of the Purple Cheetah configuration object.
• CORS - Is a middleware which uses the functionality of the cors package. This middleware is not added to the Purple Cheetah application by default. To add it to the application, call createCorsMiddleware in middleware array of the Purple Cheetah configuration object.
• HTTP exception handler - Is a simple middleware which checks if the next function was called with error. If the error is of type HttpException, error payload will be sent to the client, and warning log message will be printed. If the error is the native JavaScript error, error log message will be printed and the client will receive response with status 500 and no details. This middleware is added to the Purple Cheetah by default and can be overwritten in the configuration object.
• Route not found (404) - Is a simple middleware which return JSON object to the client if requested route does not exist. This middleware is added to the Purple Cheetah by default and can be overwritten in the configuration object.
• Request logger - Is a simple middleware which uses logger to log all incoming requests. This middleware is not added to the Purple Cheetah application by default. To add it to the application, call createRequestLoggerMiddleware in middleware array of the Purple Cheetah configuration object.

Modules

Module is the core abstract in the Purple Cheetah which allows external code to access the pipe. Modules are passed to the configuration object in modules array.

To learn more about modules visit modules repository