Stop 08 · Technical Fluency for PMsTopics · 2/11

Client-Server Basics

In short

Your app on the phone is only half the product. The other half runs on servers you never see. Knowing which half does what explains most "why is this hard?" answers.

The client (app, browser) requests something, the server processes it and responds — the mental model behind almost every feature you'll ever spec.

The purple blob hands a paper slip through a bank counter window to a friendly teller.

Think of it like this

The bank counter

You fill a slip and hand it to the teller. You can't open the vault yourself. The teller checks your details, updates the ledger and hands back cash or a receipt. You're the client, the bank's back office is the server.

Watch it happen

Opening a shopping app

Phone app

Server

draw loading screen

1/5The client draws what it can right away: layout, logo, a loading state.

The big ideas

What you need to know

Client shows, server decides

Clients handle display and input. Servers hold the data and the rules everyone must follow.

Never trust the client

Prices, discounts and permissions must be checked on the server. Clients can be tampered with.

Networks fail

Every request can be slow or fail. Good specs say what happens offline or on timeout.

App updates are slow

Server changes go live quickly. Mobile app changes wait for users to update.

Save yourself the pain

Common mistakes

Oops

Specifying a discount rule that only runs inside the app.

Do this instead

Rules that affect money or access belong on the server, where users can't change them.

Oops

Expecting a mobile fix to reach all users the same day.

Do this instead

Many users update late. Plan server-side fixes or flags for urgent changes.

Your mission

Watch a client talk

~20 min

Quick check

Where should the check "user can only apply one coupon per order" live?

Go deeper

1Diagram what happens when a user taps 'submit' on a form, from client to server and back.
2Learn where 'the frontend' and 'the backend' each live in this model.
3Understand why some bugs are client-side and others are server-side, and why that matters for triage.