Back-end development and APIs

Behind almost every page that shows your own data sits a program assembling it on demand. MDN's server-side module makes the point that this kind of work is more repetitive than it looks from outside: fetching records and rendering them, validating what a user submitted and storing it, checking who is logged in. Repetition is why frameworks dominate the field. Django (Python), Rails (Ruby), Laravel (PHP), Spring Boot (Java), ASP.NET Core (C#), and Express or Fastify on Node.js each package the recurring jobs so a developer can concentrate on the rules particular to one business.

What happens to one request

1. Routing. The web server or framework matches the method and path to the handler responsible for them.
2. Validation. Input is checked against what the handler expects, because anything arriving from a client can be forged.
3. Identity and permission. Authentication establishes who is calling; authorisation decides whether that caller may do this to this particular record. Passwords, where they exist, are stored only as a slow salted hash.
4. Work. Business rules run, usually with reads and writes to a database, sometimes with calls to other services or a queue for anything that can wait.
5. Response. A status code that tells the truth, a body in the agreed format, and headers that control caching.

Most beginner faults sit at one of those five stops: trusting input because the front end already checked it, confusing who someone is with what they may do, issuing a query per item in a loop instead of one query for the lot, and answering every failure with a success code and an error message tucked inside.

Contracts between services

An API is a promise to other programmers, and promises are expensive to change. A REST API treats data as resources, each at a fixed address, and leans on HTTP's own methods and status codes, which makes it easy to cache and to explore with ordinary tools. GraphQL exposes one endpoint and lets the client specify exactly which fields it wants, which suits interfaces assembling data from many sources and moves complexity into the server's query handling. gRPC trades readability for compact binary messages between internal services. Whichever style you use, the durable habits are the same: publish a schema, version deliberately, and never remove a field a client might still read.

Deployment has widened what back end means. A handler may run in a long-lived process behind a load balancer, in a container, or as a serverless function billed per call, and AWS's Developer Associate exam is largely about writing code that behaves well in those settings. freeCodeCamp's Back-End Development and APIs certificate covers Node.js, Express and REST design from the start. Security questions at this layer lead into application security, and the pipeline that ships the service into DevOps and CI/CD.