Backend

Backend Development

Javascript Backend Roadmap

• Server :
  • It is a software serving something.
• Two Major Component:
  • A Programming Language
    • Java, Js, PHP, Golang, C++ (on a framework)
    • Master one programming language
  • A Database
    • Mongo, MySQL, Postgress, Sqlite(ORM, ODM)
    • Master one database
• A Javascript based backend.
• A JS Runtime : Nodejs / Deno / Ban

• API - nothing but a response.

• Directory Structure:
  • Src
    • index - DB Connects
    • App - Config , cookie , urlencode
    • Constants - enums , DB-name
  • DB
    • Database connection
  • Models
    • Structure of data
  • Controllers
    • Functionality , Takes data and process it
  • Routes
    • /Signup , /login etc
  • Middlewares
  • Utils
    • Utility , To send mail , upload file
  • More(depends)

How to deploy backend code in production

• Download NodeJs Enviornment

• Computer <-express-> Server

• npm init
  • It is a utility.
  • Tell a process to create package.json file.
• create a script
  • "start": "node index.js"
• Express Js
  • Web Framework

Connect frontend and backend | Proxy and CORS

• Two types to import files
  1. CommonJS - require (sync)
  2. ModuleJS - import (async)
• Rename the terminal name and changed the color
• Used axios library
• Used CORS (Cross origin resource sharing)
• Used proxy

  •       export default defineConfig({
          server:{
              proxy: {
              '/api' :'http://localhost:3000'
              },
          },
          plugins: [react()],
          })
    

• append the url http://localhost:3000 before /api whenever server get url start from /api

• Bad practice to used frontend dist folder in backend .

Setup a professional backend project

• Model Link

• .gitkeep standard procedure to track Empty folder.
• .gitignore used to not track and push the files.
• Used git ignore generators to generate git ignore file.
• created .env.sample to give env variables to others.
• Dev dependency are those dependency which are only used in development not in production. (npm i -D nodemon )
• Create important folder in src directory
  • controllers - Functionality.
  • db - Database connection.
  • middlewares - Code run in between.
  • models - Database schema
  • routes - Handle different routes
  • utils - Utility (File Upload , Mailing , Tokens)
• prettier - To maintain formatting in a team.
• Included .prettierrc file to define formatting.
• Included .prettierignore file to ignore the formatting in defined files.

Connecting database in MERN with debugging

• Created MongoDb Atlas account.
• Created a Database and connect I.P Address to access database from anywhere.
• Used dotenv , mongoose , express packages.

• Two important points about database connectivity:

  1. When connecting to databases, handling potential data-not-found scenarios is essential. Employ try/catch blocks or promises to manage errors or we can also use promises.

     • key to remember : ( wrap in try-catch )

  2. Database operations involve latency, and traditional synchronous code can lead to blocking, where the program waits for the database query to complete before moving on. So, we should async/await which allows for non-blocking execution, enabling the program to continue with other tasks while waiting for the database response.

     • key to remember : ( always remember the database is in another continent, so use async await)

• Used two approach to connect the database - 1. In Index File, 2. In Seprate DB file

• Since Node v20.6.0, node is supporting env file without any 3rd party library (dotenv). You have to just add –env-file in your script tag in package.json. So, your script tag will look like:

  "scripts": {
    "start": "node --env-file=.env src/index.js",
    "dev": "nodemon --env-file=.env src/index.js"
  }

• Assignments -
  • console log connectionInstance
  • Read more about process.exit code - Blog

Custom api response and error handling

• Installed two packages cookie-parser & cors
• Defined origin for cors middleware.
• Defined middleware to accept json - app.use(express.json())
• express.static("public") used this middleware to staticlly use the files. (images, pdf, etc)
• Define asyncHandler and ApiError in utils Folder for handling async operation and Api Error.
• Define Api Response as well.

User and video model with hooks and JWT

• Created user model & video model
• Installed mongoose-aggregate-paginate-v2 package for Video Schema
• Installed bcrypt package to hash password.
• Installed JWT package to create tokens.
• Used pre middleware function to encrypt the password before save.
• Don’t use arrow function in pre middleware function because there is no context of this.
• JWT is a bearer token.
• Created two methods generateAccessToken & generateRefreshToken.

Upload file in backend | Multer

• Used cloudinary and multer to upload files.
• We will use multer to save file temporarily on local storage.
• and We will use cloudinary to take file from local storage and upload on cloud.
• Uses try catch block to handle file uploading.
• Made a middleware using Multer.
• Using multer to store the files in diskstorage.

HTTP crash course | Http Methods | Http Headers

What are HTTP Headers ?

• metadata - Key-value send along with request & response.
• Uses - caching, authentication, manage state

• x-prefix -> 2012(x-deprecated)

•   Request Headers -> from Client
    Response Headers -> from Server
    Representation Headers -> encoding / compression
    Payload Headers -> data
  
  

• Most Common Headers :

  •   Accept : application/json 
      User-Agent     (Request from which application)
      Authorization  (Bearer token)
      Content-Type   (Images, Pdf, etc)
      Cookie
      Cache-Control  (Expiry of Data)
    

• CORS :

  •   Access-Control-Allow-Origin
      Access-Control-Allow-Credentials
      Access-Control-Allow-Methpd
    

• Security :

  • Cross-Design-Embeddedr-Policy
    Cross-Design-Opener-Policy
    Content-Security-Policy
    X-XSS-Protection
    

HTTP Methods

• Basic set of operations that can be used to interact with server.

  • GET : retrieve a resource
  • HEAD : No message body (respose headers only)
  • OPTIONS : What operations are available
  • TRACE : Loopback test (get some data)
  • DELETE : Remove a resource
  • PUT : Replace a resource
  • POST : Intract with resource (mostly add)
  • PATCH : Change part of a resource

• HTTP Status Code :

  •   1XX  - Informational
      2XX -  Success
      3XX -  Redirection
      4XX -  Client Error
      5XX -  Server Error
    

  •   100 - Continue           400 - Bad Request 
      102 - Processsing        401 - Unauthorized
      200 - Ok                 402 - Payement required
      201 - Created            404 - Not Found
      202 - Accepted           500 - Internal ServerError
      307 - Temporary redirect  504 - Gateway timeout
      308 - Permanent redirect
    

Router and Controller with debugging

• Breaks the problem then find solution
• Made user controller
• Made user route
• Test the route with postman

Logic building | Register controller

• Steps to regsiter user
  1. get user details from frontend
  2. validation
  3. check if user already exists: username, email
  4. check for images, check for avatar
  5. upload them to cloudinary, avatar
  6. create user object - create entry in db
  7. remove password and refresh token filed from response
  8. check for user creation
  9. return response
• Written a middleware to upload images when users will register
• fetched the required field from frontend.
• Used diffrent way to vaildate error.
• Used operator to check user already exists or not.
• Assignment -: console log - req.files , existedUser
• Gets the files location.
• Uploded the files on cloudinary.
• Inserted the data of user in the database.
• Checked user created or not
• return response using ApiResponse of created user.

How to use postman for backend

• Uses Postman to register user.
• solve error of await.
• Written code to remove the local storage files.(async)
• console logged req.files.
• solve bug of cover image.
• created collection and variable in Postman.
• Used the created variable in defined collection.

Access, Refresh Token, Middleware and Cookies

• Access token expires soon.
• Refresh token expires in long term.
• Algorithm of loging user
• ```javascript /**
  • req body -> data
  • username or email
  • find the user
  • password check
  • access and refresh token
  • send token in cookie */ ```
• generated access and refresh token.
• saved refresh token in database.
• set the cookie to modifidable ony in server side.
• return and saved the refresh & access token in cookie.
• made a middlleware of auth.
• created route of login and logout.
• added middleware in logout route.
• cleared the cookie and logged out the user.

Access token and refresh token

• Fix the bugs and make a login request using Postman.
• created controller for refreshAccessToken

Writing update controllers for user

• Added subscription model.
• added changeCurrentPassword function in user controller.
• added getCurrentUser function to get current user info.
• added updateAccountDetailsto update fullname and email.
• added updateUserAvatar controller to update the user profile.
• added updateCoverImage controller to update cover image of user.

Understand the subscription Schema

• TODO - Delete old image on cloudnary during updating avatar and cover image.
• Subscription Schema -> Subscribers , Channel
• User -> a, b, c, d, e
• Channel - CAC , HCC , FCC
• When user will subscribe any channel then a document will be created.
• When a documents creates , It will contain two values Subscribers & Channel.
• When a user a subscribes channel CAC then a new document will be created. It will contain Channel -> CAC & Subscribers -> a. It will continues for others users.
• Subscribers - No. of documents where channel == CAC
• Channel - Extract channel name and No. of documents where subscriber == c (subscribed channel)

Mongodb aggregation

• The aggregation pipeline consists of one or more stages that process documents:
• Each stage perfroms an operation on input documents
• The documents that are output from a stage passed to the next stage.
• can return groups of documents. For ex - return the total, average, e.t.c
• written getUserChannelProfile contoller.
• written aggregation pipelines for getuserchannelprofile

Write sub pipelines and routes

• Written Pipelines and subpipelines to add getWatchHistory controller.
• Added new routes for user controller.

Summary of Backend Series

• Don’t depend upon packages .
• Do core work as much as you can.

•    Enviornment Variables -> Dependency (Packages) -> Database Connection -> Setting Path (index.js) -> Routing & Controller , Api Versioning -> Models -> Model Hooks -> Tokens -> Controllers (User) -> DB Query -> CRUD Operation -> Middlewares -> File Management (During Uplading) -> Aggregation Pipelines -> MongoDB Operators 
  

MongoDB models for like playlist and tweet

• Cheked all the routes and fixed the bugs.
• added models
  • comment.model.js
  • playlist.model.js
  • tweet.model.js
  • like.model.js