From source code to an API

Learning outcome

By the end of this lesson, you will be able to trace how source code becomes a running API — through interpreters, compilers, databases, and HTTP.

The journey from code to product

When you write a program and put it on the internet, it goes through several transformations:

flowchart TD
  A[Source code] --> B{Interpreted or compiled?}
  B -->|Interpreted| C[Python / JavaScript<br/>Read and run line by line]
  B -->|Compiled| D[C / Rust / Go<br/>Translated to machine code]
  C --> E[Running process]
  D --> E
  E --> F[Listens for HTTP requests]
  F --> G[Queries database]
  G --> E
  E --> H[Returns JSON or HTML]

Interpreted vs compiled

Interpreted languages

Python, JavaScript, and Ruby are interpreted. A program called an interpreter reads your source code and runs it directly, one line at a time. This makes them easier to debug and faster to iterate, but slower at runtime.

Compiled languages

C, Rust, and Go are compiled. A compiler translates your entire source code into machine code before you run it. The result is a binary file that runs directly on the CPU. This makes them faster but harder to change quickly.

What is an API?

API stands for Application Programming Interface. It is a set of rules that one program uses to talk to another.

A web API listens for HTTP requests (like the ones your browser sends) and returns data, usually in JSON format:

GET /api/users
Response: [{"id": 1, "name": "Alice"}, {"id": 2, "name": "Bob"}]

What is a database?

A database stores structured data and lets you query it efficiently. Most web applications use a relational database like PostgreSQL (which this site uses).

Your server code sends SQL queries to the database:

SELECT name, email FROM users WHERE id = 1;

The database returns matching rows, and your server sends them to the browser as JSON.

How they fit together

1. You write Python source code.
2. Python interprets it and starts a server process.
3. The server listens on a port (like :8000) for HTTP requests.
4. A browser sends GET /api/users.
5. The server runs SELECT * FROM users against PostgreSQL.
6. The database returns rows.
7. The server formats them as JSON and sends the response.

Common beginner mistakes

• Confusing a compiler with an interpreter. Python does not compile your code to machine code. It reads and runs it live.
• Thinking the database lives in the same process. The database is a separate program, often on a different machine. Your server talks to it over the network.
• Writing business logic in the database. Keep logic in your application code. The database should store and retrieve data.

Practice task

Install a simple Python web framework like Flask. Write a route GET /hello that returns {"message": "Hello, world!"}. Run it and visit http://localhost:5000/hello in your browser.

Recap

• Interpreted languages run line by line. Compiled languages become machine code first.
• An API is a set of rules for programs to communicate.
• A database stores structured data that your server queries.
• A web application connects all three: source code → server → database → HTTP response.

Next step

Now that you understand the full stack, the next lesson guides you through shipping a small useful project to test which development direction fits you.