How to Build Scalable Web Applications: Key Principles and Architecture

Why Build Scalable Web Applications?

Imagine a marketing campaign that drives thousands of concurrent users; without proper design, the system will suffer random errors, slow loading, endless waiting, disconnections, and service outages, turning attracted users into attackers that exhaust server resources and cause crashes.

Overview of Scalable Application Architecture

The two main principles of a scalable architecture are:

Separation of concerns

Horizontal scaling

Separation of Concerns

Each type of task should have an independent server.

When a single server handles all work—processing requests, storing files, etc.—it becomes a bottleneck; overload affects the whole application, causing pages or images to fail to load. To avoid this, distinct servers should handle different responsibilities, such as an API server for real‑time client requests and a separate worker for image processing, which can be slower and does not require immediate feedback.

Horizontal Scaling

Horizontal scaling distributes load across multiple servers. Each server runs the application and can be enabled or disabled based on current load.

The load balancer controls the number of active servers, activating additional instances when demand rises and deactivating them when it falls. It also performs health checks and routes traffic only to healthy servers. Various algorithms—round‑robin, random, least latency, least traffic—can consider factors such as geographic location and server capacity.

Horizontal scaling does not require scaling the entire application; for example, when an API server reaches its limit, the load balancer can spin up more API servers without affecting other components.

Building a Scalable Application

Typical servers for different task types include:

API server

Database cluster

Static storage server

Worker for complex, non‑real‑time tasks

Each server can become a potential bottleneck, so we examine them individually.

API Server

The API server handles core functional requests, which increase with user volume.

Key point: do not store any user data; keep it stateless.

If a user uploads an image and server A stores it, a subsequent request handled by server B would not find the image. Load balancers can also terminate or pause any server at any time.

Static Storage Server

Static storage works together with a CDN (Content Delivery Network), a cache of servers that deliver content instantly to users.

When many users simultaneously access a video stored in a California static server, the load can become overwhelming. With a CDN, the first request uploads the video to the nearest CDN node, and subsequent users retrieve it from that node, preventing overload and providing fast loading speeds.

Worker

Not all user requests need an immediate response; some tasks can run in the background while the user continues other activities, such as video processing after upload.

These tasks are handled by Workers and a Message Queue. Workers run on independent servers and can scale with load, while the Message Queue acts as a task manager between the API server and Workers, holding tasks until a Worker is available and retrying if a Worker fails.