Important Points to remember

1. Concept of versioning in Npm [https://docs.npmjs.com/about-semantic-versioning ]
2. Server side rendering is faster than cliet side rendering , but former is useful only when client is Browser.
3. Concept of Middleware in node.js [https://expressjs.com/en/guide/using-middleware.html]
4. (Docker Notes) [https://github.com/Sagar-Chowdhury/Docker-Notes]
5. (Rabbit Mq) [https://github.com/Sagar-Chowdhury/RabbitMq-Notes]
6. (Quick JS Revision)namaste-javascript-notes.pdf

Express.js: The Minimalist Web Framework

• Core Philosophy: Express is a lightweight, flexible, and unopinionated web framework for Node.js. It provides the essentials for building web servers and APIs without imposing a rigid structure.
• Key Features:
  • Routing: Powerful system for mapping HTTP requests (GET, POST, etc.) to specific handler functions.
  • Middleware: Functions that have access to the request and response objects, allowing you to modify or process data before it reaches your route handlers.
  • Templating Support: While it doesn't enforce a specific templating engine, Express seamlessly integrates with popular engines like EJS, Pug, and Handlebars.
  • Error Handling: Provides mechanisms for centralized error handling within your application.

MVC (Model-View-Controller) Architectural Pattern

• Separation of Concerns: The core principle of MVC is to organize code into three distinct layers:
  • Model: Represents your data and business logic. Interacts with databases (MongoDB, SQL, etc.).
  • View: Responsible for UI representation. Generates HTML output, often using templating engines.
  • Controller: Handles HTTP requests, interacts with models to process data, and selects the appropriate view to render.

Express and MVC: A Perfect Fit

• Flexibility: Express doesn't force you into a strict MVC structure. However, its design aligns well with implementing MVC principles.
• Common Structure: Here's how a typical Express + MVC setup might look:
  • routes/: Contains your controllers.
  • models/: Houses your Mongoose models (or other database interaction logic).
  • views/: Stores your EJS templates (or those for your chosen engine).

Additional Benefits of Express + MVC

• Maintainability: Clear separation makes the codebase easier to manage and scale.
• Testability: Independent layers can be tested in isolation.
• Collaboration: Developers with specialized skills can focus on their respective areas (frontend, backend, database).

Absolutely! Here are detailed notes on MongoDB, Mongoose, and their integration with Node.js:

MongoDB

1. Document Database:

   • Flexible Schema: Stores data in JSON-like BSON (Binary JSON) documents within collections.
   • Dynamic Schema: Allows you to modify the structure of documents without rigid table definitions.

2. Scalability:

   • Horizontal Scaling: Easily scale horizontally by adding more servers to distribute data and workload.
   • Sharding: Divide data across multiple machines for performance and storage capacity.

3. High Performance:

   • Indexing: Supports efficient querying using indexes on document fields.
   • Aggregation: Powerful aggregation framework for complex data analysis.

4. Use Cases:

   • Web Applications: Store user data, sessions, content, and more.
   • Real-Time Analytics: Capture and analyze event data in real time.
   • Content Management Systems: Manage articles, blog posts, and other content.
   • IoT Applications: Handle large volumes of data from sensors and devices.

Mongoose MongoDb and Mongoose

1. Object Data Modeling (ODM):

   • Schema Validation: Enforce data types, required fields, uniqueness, and custom validation logic.
   • Relationships: Define relationships between documents (one-to-one, one-to-many) for cleaner data organization.
   • Middleware: Hooks for pre- and post-save operations.

2. Abstraction Layer:

   • Simplified Interactions: Mongoose provides a higher-level API for working with MongoDB, making it easier to interact with documents, schemas, and queries.
   • Easier Data Modeling: Schemas allow you to model your application's data in a structured way.
   • Type Safety: Get compile-time error checking by defining data types in your schemas.

3. Key Features:

   • Models: Constructors for creating documents based on your schema.
   • Query Builder: Powerful and intuitive way to build MongoDB queries using JavaScript.
   • Validation: Built-in and custom validation rules to ensure data integrity.
   • Plugins: Extend Mongoose with additional functionality.

Performing CRUD Operations:

• Create: const user = new User({...}); user.save();
• Read: User.find({ ... });
• Update: User.findByIdAndUpdate(id, { ... });
• Delete: User.findByIdAndDelete(id);

Internal Working Deep-Dive

Node JS Single Threaded Event Loop Model As we have already discussed, Node JS applications uses “Single Threaded Event Loop Model” architecture to handle multiple concurrent clients. There are many web application technologies like JSP, Spring MVC, ASP.NET, HTML, Ajax, jQuery etc. But all these technologies follow “Multi-Threaded Request-Response” architecture to handle multiple concurrent clients. We are already familiar with “Multi-Threaded Request-Response” architecture because it’s used by most of the web application frameworks.

(Traditional Web Application Processing Model - **Multi-Threaded Request-Response Model**)

Here's a point-wise summary of the Multi-Threaded Request-Response Model :

Key Points

• Traditional Model: Prevalent in web applications built without Node.js.
• Stateless: Based on HTTP, which doesn't maintain state between requests.
• Multiple Threads: Used for handling concurrent client requests.

Processing Steps

1. Client Request: Client sends a request to the web server.
2. Thread Pool: Server maintains a pool of threads to serve requests.
3. Infinite Loop: Server waits for incoming requests.
4. Request Handling: Server receives a request.
5. Thread Assignment: A thread from the pool is assigned to the request.
6. Thread Processing: The thread performs the following:
   • Reads the client request
   • Processes the request
   • Performs blocking I/O operations (if needed)
   • Prepares the response
7. Response: Thread sends the response to the web server.
8. Server Delivery: Server sends the response to the client.
9. Repeat: Server loops back, handling requests for all clients.

Diagram Dynamics

• Concurrent Requests: The web server handles multiple client requests simultaneously.
• Thread Pool: The server has a limited pool of threads to serve requests.
• Thread Assignment: Each incoming request is assigned a thread from the pool.
• Processing:
  • Threads handle request reading, processing, and response preparation.
  • Blocking I/O operations can delay a thread's completion.
• Resource Management:
  • If all threads in the pool are busy, new requests must wait.
  • Threads need to release resources before becoming available again.

Drawbacks of the Multi-Threaded Request/Response Model

• Scalability Limits: Handling many concurrent requests can exhaust the thread pool, causing delays.
• Resource Overhead: Each thread consumes memory and other resources.
• Request Waiting: Clients may wait for available threads, especially if the pool is small.
• Blocking I/O Inefficiency: Blocking operations stall threads, slowing down the overall process.

(Node JS Architecture - Single Threaded Event Loop Model - Working and Functionality )

Let's break down the JavaScript callback mechanism, which is the foundation of Node.js's asynchronous handling:

Core Concepts

• Functions as First-Class Citizens: In JavaScript, functions are treated like any other data type. This means:

  • You can assign them to variables.
  • You can pass them as arguments to other functions.
  • You can return them from functions.

• Callback Functions: A callback function is a function that's passed as an argument to another function. The idea is that the callback will be executed later, often when a specific task or event has completed.

• Asynchronous Operations: Many operations in JavaScript (like network requests, timers, or some file I/O) don't give you results immediately. Instead, they take time to complete. Callbacks allow you to execute code after these operations finish.

Example

function fetchData(url, callback) {
  // Simulate a network request that takes time
  setTimeout(() => {
    const data = 'Some data fetched from the network';
    callback(data); // Execute the callback with the fetched data
  }, 1000); // Simulate a 1-second delay
}

function displayData(data) {
  console.log('Received data:', data);
}

fetchData('https://api.example.com', displayData); 

Explanation

1. fetchData: Simulates a network operation. It takes a URL and a callback function (callback) as arguments.
2. setTimeout: Imitates the asynchronous nature of network requests. After the delay, it executes the callback function, passing it the fetched data.
3. displayData: A simple function to log data.
4. Calling fetchData: You call fetchData and provide displayData as the callback.

Key Points

• fetchData doesn't block the execution of the rest of your code.
• When the simulated network request completes, the displayData callback is invoked, and the received data is logged.

Why Callbacks are Important in Node.js

• Non-blocking I/O: Node.js uses callbacks to avoid waiting for operations like file I/O or network requests to finish. This allows other code to run, keeping Node.js applications responsive.
• Event Loop: The event loop, at the heart of Node.js, frequently checks if any callbacks are ready to be executed.

Diagram Dynamics

• Concurrent Requests: Multiple clients send requests to the Node.js web server.
• Event Queue: Incoming requests are placed in the Event Queue.
• Event Loop: The core mechanism that picks up and processes requests one by one.
• Thread Pool: A limited set of threads to handle specific tasks.

Request Handling

1. Event Loop Processing: The Event Loop picks up a request from the Event Queue.
2. Complexity Check: The Event Loop assesses whether the request requires blocking I/O or intensive computation.

Scenario 1: Simple Requests

3. Direct Handling: If the request is simple, the Event Loop itself processes it entirely (reading, execution, response preparation).
4. Response: The Event Loop sends the prepared response to the client.

Scenario 2: Complex Requests

5. Thread Delegation: If the request is complex, the Event Loop offloads it to a thread from the internal thread pool.
6. Thread Processing: The assigned thread handles the complex logic, including any blocking I/O or computation.
7. Callback: Upon completion, the thread sends the response back to the Event Loop.
8. Event Loop Delivery: The Event Loop forwards the response to the client.

Key Point: Node.js prioritizes efficient processing of simple, non-blocking requests directly on the Event Loop. Complex tasks are strategically offloaded to the thread pool to prevent the Event Loop from stalling.

Node.js leverages a built-in internal thread pool to offload certain potentially blocking or computationally intensive tasks. Let's break down what this means and how it works:

Purpose of the Thread Pool

• Overcoming I/O Bottlenecks: JavaScript's single-threaded event loop is fantastic for handling many concurrent requests. However, operations like these can block the event loop:

  • File system I/O (reading/writing files)
  • Network operations
  • Database queries
  • Certain crypto functions

• Optimized Resource Use: Node.js uses a thread pool (managed by the libuv library) to execute these tasks on separate worker threads. This prevents the main event loop from stalling, enhancing responsiveness.

Key Points

• Limited Size: The thread pool typically has a default size of 4 threads. This can be adjusted using the UV_THREADPOOL_SIZE environment variable.
• Task Delegation: Node.js automatically determines which tasks are suitable for the thread pool.
• Asynchronous Handling: Even when using the thread pool, Node.js maintains asynchronous behavior. When a task completes on a worker thread, a callback is queued in the event loop for processing the results.

Common Operations Utilizing the Thread Pool

• fs module: File system operations.
• crypto module: Some cryptographic functions.
• dns.lookup() Domain Name System (DNS) lookups.
• zlib module: Compression and decompression functions.

Important Considerations

• Not All Operations: Not every operation in Node.js uses the thread pool. Pure JavaScript execution and most networking tasks happen directly on the event loop.
• CPU-Bound Tasks: While the thread pool can help with I/O-bound tasks, large CPU-intensive processes can still strain the system. Consider using Node.js's worker thread capabilities for such scenarios.

Why the "Single-Threaded" Label Still Holds

1. JavaScript Execution: Your core JavaScript code still executes within the single thread of the event loop. The thread pool doesn't create multiple threads to run your JS code concurrently.

2. Focus on Asynchronous I/O: Node.js is optimized for scenarios where many operations are asynchronous (mostly I/O bound). The thread pool acts as a crucial supporting mechanism, not a primary driver of execution.

In Summary

It's more accurate to describe Node.js as having a single-threaded architecture augmented by a thread pool to optimize specific operations. The single-threaded event loop remains the centerpiece of its execution model.