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Build the mental models behind modern software.

Already building applications, but unsure what happens underneath? Trace a web request through the layers it uses, locate a failure, and decide what to check next. Begin with the guided path or open one concept at a time. New to these systems? The lessons start with the essentials.

01 · Start here

See the whole request before studying the pieces.

A guided sequence for understanding how the layers of a web application cooperate—and where they fail.

Guided path Start here

How a Web Application Actually Works

Trace a web request across browser, network and server boundaries. Understand when caches, application logic or data storage enter the picture—and how to investigate a failure.

10 published lessonsAbout 3 hr 20 min

Open the path overview

The request journey

  1. Frontend, Backend, and DatabaseRead lesson →
  2. What happens when you enter a URLRead lesson →
  3. DNS and IP addressesRead lesson →
  4. TCP, HTTP, and HTTPSRead lesson →
  5. APIs and JSONRead lesson →
  6. How a backend processes a requestRead lesson →
  7. How a database stores and retrieves dataRead lesson →
  8. Authentication and sessionsRead lesson →
  9. Deployment and cloud serversRead lesson →
  10. Reverse proxies, logs, and debugging failuresRead lesson →

02 · Foundations

A working map of the software stack.

Categories organize the territory. Each one begins with a small set of concepts that make the next layer easier to reason about.

  1. How Software Works

    Build a systems-level picture of how programs use machines, communicate, and persist information.

    • Client and server
    • Processes and threads
    • Memory, CPU, and storage
    Published lessonFrontend, Backend, and Database: How They Work Together Published lessonWhat Happens When You Enter a URL? Published lessonDNS and IP Addresses: How a Domain Finds a Server Published lessonTCP, HTTP, and HTTPS: The Layers Behind a Web Request Published lessonAPIs and JSON: How Applications Communicate Explore How Software Works

  2. Frontend

    Understand how browsers turn code and data into interactive interfaces.

    • How Browsers Turn HTML and CSS Into Pixels
    • The DOM, Events, and User Interaction
    • The JavaScript Runtime, Event Loop, and Async Code
    Follow the frontend learning path Published lessonHow Browsers Turn HTML and CSS Into Pixels Published lessonThe DOM, Events, and User Interaction Published lessonThe JavaScript Runtime, Event Loop, and Async Code Explore Frontend

  3. Backend

    Trace what happens after a request reaches the application server.

    • Request lifecycle
    • APIs and webhooks
    • Authentication and authorization
    Published lessonHow a Backend Processes a Request Published lessonAuthentication and Sessions: How Applications Remember Users Explore Backend

  4. Databases

    Learn how software organizes, finds, protects, and changes durable data.

    • Data modeling
    • Indexes
    • Transactions
    Published lessonHow Databases Store and Retrieve Data Explore Databases

  5. Cloud and Deployment

    See how an application moves from source code to reliable running infrastructure.

    • Virtual machines and containers
    • DNS and reverse proxies
    • Regions, Availability Zones, and CDNs
    Published lessonDeployment and Cloud Servers: From Source Code to a Running Application Published lessonReverse Proxies, Logs, and Debugging Production Failures Explore Cloud and Deployment

  6. System Design

    Reason about the tradeoffs that keep software responsive, resilient, and understandable.

    • System Design Starts With Requirements and Constraints
    • Latency, Throughput, Capacity, and Bottlenecks
    • Scaling, Load Balancing, and Stateless Services
    Follow the system-design learning path Published lessonSystem Design Starts With Requirements and Constraints Published lessonLatency, Throughput, Capacity, and Bottlenecks Published lessonScaling, Load Balancing, and Stateless Services Explore System Design

  7. AI for Developers

    Understand the software primitives behind useful AI systems and developer tools.

    • Tokens and context windows
    • Embeddings and RAG
    • Agents and MCP
    Published lessonAI Coding Assistants and Agents Published lessonReviewing AI-Generated Code Explore AI for Developers

03 · Companion path

Software Development in the AI Era

A focused guide to using coding assistants while retaining the judgment to review, test, and ship their output. Separate from the software-fundamentals curriculum.

Eight concise lessons

Start with what an assistant can do. Continue through code review, testing, data boundaries, useful context, reliable workflows, and honest measurement.

Open the AI-era learning path

04 · Connected concepts

Understanding grows through connections.

Follow these links to connect the ideas. This is a learning progression, not a universal request trace: authentication may be unnecessary, and a cache can answer before application logic runs.

  1. DNSFind destination information
  2. HTTPDefine the exchange
  3. APIsShape the interface
  4. AuthenticationEstablish identity when needed
  5. CachingAvoid repeated work
  6. ScalingHandle growing demand
Planned: Behind the Technology & a Codewise letter

Behind the Technology is a future analysis series. No breakdowns have been published yet.

Read about the planned series