How Bretzel works

Every web app has two halves: the browser (the client) and the server. Knowing where the code runs — and when one crosses from one to the other — is the frame that lights up everything else.

The two halves

Half	What it is
Client	the browser, on the user's machine — it displays and reacts
Server	your machine, where the Python runs — it holds the truth

Why on the server

The web's basic rule: the client is always wrong. Everything arriving from the browser can be tampered with. So what matters lives server side:

Security — the client is never trusted; the validation that protects happens here.

Source of truth — the data is stored and owned by the server, not by the tab.

Confidential logic — a price calculation, a business rule must not leave for the browser.

Sharing — a server state is seen by several users / tabs; a client state is not.

Why on the client

Going through the server costs a network request. For a purely visual interaction (opening a menu, ticking a box), it is waste. Staying client side means:

No request — it works with no round trip, hence instantly.

Lightening the server — it is the user's machine doing the work, not yours.

UI = f(state)

The interface is a function of the state. One does not manipulate the DOM by hand: one describes what the UI must be for a given state, one mutates the state, and the UI is recomputed.

Bretzel's cycle

An interaction happens (a click, a keystroke).

If it changes the truth, it goes server side: a Python function runs.

This function mutates the state.

The part of the interface that depends on that state is re-rendered and sent back to the browser.

An interaction that does NOT change the truth (opening a panel) short-circuits the server step and stays in the browser.

An event's routes

Concretely, an event takes one of these routes:

The event…	tourne	Round trip?
calls a Python function	serveur	oui
drives a component / a client state	client	non

It is the same boundary, applied everywhere: state, actions and reactivity each have a server side and a client side. The rest of the docs follows that plan.