1.2 Web App Architecture
Web Application Architecture.
Web application architecture refers to the structure and design of a web-based software application, including its components, interactions, and deployment mechanisms. A well-designed architecture is essential for ensuring the scalability, reliability, security, and maintainability of web applications. Here are some key components typically found in web application architecture:
Client-Side Components:
User Interface (UI): The UI is the front-end component of the web application that users interact with. It includes elements such as HTML, CSS, and JavaScript, which are rendered in the user's web browser.
Client-Side Frameworks/Libraries: Modern web applications often utilize client-side frameworks or libraries like React.js, AngularJS, or Vue.js to facilitate the development of dynamic and interactive user interfaces.
API Clients: Web applications may interact with external APIs (Application Programming Interfaces) to retrieve data or perform specific actions. API clients on the client-side facilitate communication with these APIs.
Server-Side Components:
Web Server: The web server hosts the web application and handles incoming HTTP requests from clients. Popular web servers include Apache, Nginx, and Microsoft IIS.
Application Server: The application server executes the business logic of the web application and interacts with databases or external services. Common application servers include Node.js, Django, Ruby on Rails, and Java Servlet Containers (e.g., Tomcat).
Database: The database stores and manages the application's data. Relational databases like MySQL, PostgreSQL, or Oracle, as well as NoSQL databases like MongoDB or Redis, are commonly used in web applications.
Middleware: Middleware components, such as caching servers (e.g., Redis), message queues (e.g., RabbitMQ), or load balancers, help improve the performance, scalability, and reliability of web applications.
Data Storage and Persistence:
Database Management System (DBMS): The DBMS is responsible for managing the storage, retrieval, and manipulation of data in the database. It provides mechanisms for querying, updating, and securing data.
Data Access Layer: The data access layer of the application interacts with the database, abstracting away the underlying database operations and providing a consistent interface for accessing and manipulating data.
Security Layers:
Authentication and Authorization: Authentication mechanisms verify the identity of users accessing the application, while authorization controls determine the actions users are allowed to perform based on their roles and permissions.
Encryption and Data Protection: Encryption techniques, such as SSL/TLS, protect data transmitted between the client and server, while encryption at rest secures data stored in databases or on disk.
Security Controls: Implementing security controls, such as input validation, output encoding, and access controls, helps prevent common vulnerabilities like SQL injection, cross-site scripting (XSS), and cross-site request forgery (CSRF).
Deployment and Scalability:
Deployment Infrastructure: Web applications can be deployed on physical servers, virtual machines, cloud platforms (e.g., AWS, Azure, Google Cloud), or container orchestration platforms (e.g., Kubernetes).
Scalability Patterns: Scalability patterns, such as horizontal scaling (adding more instances) or vertical scaling (increasing resources of existing instances), are employed to handle increasing user loads and ensure high availability and performance.
How Web Pages Are Rendered
Web pages are rendered by web browsers, which interpret HTML, CSS, and JavaScript code to display content to users. The rendering process involves several steps:
HTML Parsing:
The browser begins by parsing the HTML document received from the web server. Parsing involves breaking down the HTML code into a structured tree-like representation called the Document Object Model (DOM).
DOM Tree Construction:
Based on the parsed HTML code, the browser constructs the DOM tree, where each element in the HTML document becomes a node in the tree structure. The DOM represents the hierarchical structure of the web page's content, including elements like headers, paragraphs, images, and links.
CSS Parsing and Styling:
After parsing the HTML, the browser parses any linked or embedded CSS (Cascading Style Sheets) files associated with the web page. It constructs the CSS Object Model (CSSOM), which represents the styles applied to each element in the DOM tree.
The browser then combines the DOM tree with the CSSOM to create the Render Tree. The Render Tree contains only the elements and styles needed to render the visible portion of the web page.
Layout (or Reflow):
Once the Render Tree is constructed, the browser performs layout calculations to determine the size, position, and arrangement of each element on the web page. This process is also known as reflow or layout.
Layout involves calculating the dimensions of each element based on its content, padding, border, margin, and any applied CSS styles (e.g., width, height, position).
Painting:
After layout is complete, the browser proceeds to paint or render the web page onto the screen. It converts the visual representation of the Render Tree into pixels on the display device.
Painting involves filling in the pixels for each element based on their styles, colors, borders, and other visual properties.
JavaScript Execution:
In parallel with the rendering process, the browser executes any JavaScript code included in the web page. JavaScript can dynamically modify the DOM, update styles, handle user interactions, and fetch data from servers.
JavaScript execution may trigger additional DOM manipulations, style changes, or layout recalculations, which can affect the rendering of the web page.
Asynchronous Rendering:
Modern browsers often employ techniques like asynchronous rendering and incremental loading to improve the perceived performance of web pages. They prioritize rendering visible content first while continuing to load and render non-visible content in the background.
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