10 · HTTP APIs with FastAPI

When a program wants something from another program on a network, such as a model's prediction or a list of experiments, it usually asks over HTTP, the same protocol a browser uses. It sends a request: what to do, to what, and any data. The other program, the server, sends back a response: how it went, and any data.

Here's one exchange, with a small experiment tracker this lesson builds. curl is a command-line tool that makes HTTP requests; -i shows the response's headers too.

$ curl -i localhost:8000/health
HTTP/1.1 200 OK
date: Wed, 04 Nov 2026 09:30:00 GMT
server: uvicorn
content-length: 15
content-type: application/json

{"status":"ok"}
$ curl -i -X POST localhost:8000/runs -H 'Content-Type: application/json' -d '{"name": "baseline", "score": 0.81}'
HTTP/1.1 201 Created
date: Wed, 04 Nov 2026 09:30:00 GMT
server: uvicorn
content-length: 39
content-type: application/json

{"name":"baseline","score":0.81,"id":1}

And here's the second exchange as it travelled, in plain text (curl's -v shows this). The request: a method and a path, then headers, a blank line, and a body. The response: a status code, headers, a blank line, and a body. The body is JSON, text that maps straight onto Python dicts, lists, strings and numbers.

POST /runs HTTP/1.1
Host: localhost:8000
User-Agent: curl/8.7.1
Accept: */*
Content-Type: application/json
Content-Length: 35

{"name": "baseline", "score": 0.81}

HTTP/1.1 201 Created
date: Wed, 04 Nov 2026 09:30:00 GMT
server: uvicorn
content-length: 39
content-type: application/json

{"name":"baseline","score":0.81,"id":1}