Consuming a web service

Calling a web service from a program: asynchronous call, promise, reading the response, HTTP errors, timeouts, authentication token, pagination.

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HTTP client: what is it?
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The part of a program that builds a request (URL, method, headers, body), sends it to a server and reads the response.
Browser, curl, fetch in JavaScript, requests in Python: all are HTTP clients. The server only sees the request, not the language that wrote it.
In which order does this program print the letters, once the server has answered?

```javascript
console.log("A");
fetch(url).then(() => console.log("B"));
console.log("C");
`
- A, C, B
- A, B, C
- B, A, C
- C, A, B
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A, C, B
fetch returns at once with a promise: the program carries on and prints C. The function passed to .then only runs after the current code ends, when the response is there.
In a function declared …, the … keyword waits for a promise to settle before moving to the next line, without blocking the rest of the program.
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In a function declared async, the await keyword waits for a promise to settle before moving to the next line, without blocking the rest of the program.
await only exists in an async function (or at the top level of a module). While waiting, the program runs the code that follows the function call.
Asynchronous call: what is it?
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A call that returns immediately and whose result arrives later, while the program carries on.
A network response takes tens of milliseconds or more; waiting in place would freeze the interface. The code that depends on the response runs when it arrives.
A call has been answering 401 Unauthorized since this morning although the code has not changed. What is the most likely cause and the fix?
- The server is down: retry in a loop until it answers
- The URL is wrong: add a trailing slash
- The token has expired: get another one
- The method is wrong: switch from GET to POST
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The token has expired: get another one
401 says “I do not know who you are”: credentials missing, wrong or out of date. An expired token is the commonest case when nothing changed in the code. An outage would give a 5xx, a wrong URL a 404.

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