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System Design and AI Engineering, by Roni Das. 984 interactive lessons.
Start LearningFoundation · Core Fundamentals
The foundational architecture of the web: clients (browsers, apps) send requests and servers process them and return responses. Every web interaction follows this pattern.
In short
The client-server model is a way of splitting a computer application into two roles: a client that asks for something and a server that holds the data or logic and answers. The client sends a request over a network, the server processes it and sends back a response, which is the pattern behind almost every website, mobile app, and API on the internet.
In short
The client-server model is a way of splitting a computer application into two roles: a client that asks for something and a server that holds the data or logic and answers. The client sends a request over a network, the server processes it and sends back a response, which is the pattern behind almost every website, mobile app, and API on the internet.
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The System Design course covers Client-Server Model across a run of lessons, not one page. These 4 alone are about 125 minutes of step-by-step reading, 1 of them with code you run in the browser, all with a quiz.
When network calls fail, and they will, retries turn transient failures into invisible hiccups.
Abstract data access behind a clean interface, swap databases without touching business logic.
Collect multiple server responses and deliver them together to reduce round trips.
Two writers update the same Delta Lake table at once: who wins, and what does the loser see? I ran real writer processes against 3,475,226 NYC taxi trips, 5 runs per case. Two appends both committed every time. Two updates of different rows clashed every time, because they shared the same 9 files: one committed, the other got CommitFailedError. Splitting the table by the filter column let both commit. A check-then-append job started twice stored 317,150 trips for a day of 158,575 in 5 of 5 runs. In delta-rs 1.6.6, a WriteSerializable setting took effect only when spelled writeSerializable.
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A client is any program that initiates a request: your web browser, a phone app, a command-line tool like curl, or even another server calling an API. A server is a program that waits for requests, does some work, and replies. The defining rule is that the client always speaks first and the server never starts the conversation on its own.
This split lets one server handle thousands of clients at once, and lets a client be a thin, simple program that does not need to store all the data or run all the heavy logic. The browser on your laptop does not contain Gmail's inbox; it just asks Google's servers for it and renders what comes back.
It contrasts with peer-to-peer, where every machine is both a client and a server and there is no central authority. BitTorrent and the original Skype were peer-to-peer. Almost everything else you use daily, from Amazon to your banking app, is client-server.
Say you open systemdesign.academy. Your browser (the client) first resolves the domain name to an IP address through DNS, then opens a TCP connection to that address on port 443 and runs a TLS handshake to encrypt the channel. Only then does it send an HTTP request like GET / with headers describing what it wants.
The server receives that request, often through a load balancer that picks one of many backend machines. The chosen server runs application code, maybe queries a database like PostgreSQL, builds an HTML or JSON response, and sends it back down the same connection with a status code such as 200 OK or 404 Not Found.
Each request is independent. Plain HTTP is stateless, meaning the server does not remember you between requests by default. To recognize a logged-in user, the server hands the client a session cookie or a token that the client attaches to every later request. That is how the model fakes continuity on top of one-off message exchanges.
The client-server model is the default for almost any system where data must be shared, secured, or kept consistent across many users. Centralizing data on a server means you control access, run backups, push updates in one place, and keep a single source of truth. That is why every SaaS product, online store, and multiplayer backend uses it.
The trade-offs are real. The server is a central point of failure: if it goes down, every client is stuck, which is why production systems run many server replicas behind a load balancer. The server is also a scaling bottleneck, since one machine can only handle so many simultaneous connections before you must add more capacity or shard the load.
There is also latency baked in. Every interaction is a network round trip, so a user in Sydney talking to a server in Virginia pays the speed-of-light cost on every request. Engineers fight this with CDNs, caching, and edge servers placed closer to users, but the underlying request-response shape never changes.
Gmail and Google Search
Your browser is a thin client that sends requests to Google's servers, which run the search or fetch your inbox and return the result.
Nginx
One of the most widely deployed web servers; it accepts client HTTP requests and either serves files directly or forwards them to application servers.
PostgreSQL
A database server that application servers act as clients to, sending SQL queries over a connection and receiving result sets back.
Amazon and online stores
The mobile app or website is the client; Amazon's backend servers handle catalog, cart, and checkout, keeping all real data server-side.
A client starts the conversation by sending a request and usually runs on a user's device. A server waits for requests, does the work or holds the data, and sends back a response. One server typically handles many clients at the same time.
Not quite. Three-tier is a common way to build the server side: a presentation tier, an application tier, and a data tier. Client-server is the broader idea of request and response between two roles. A three-tier app is still client-server from the browser's point of view.
In client-server there is a clear, central server that everyone talks to. In peer-to-peer every machine is both client and server and they talk directly to each other with no central authority. BitTorrent is peer-to-peer; almost every website is client-server.
Yes, and it is common. An application server is a server to the browser but a client to the database. An API gateway answers user requests while making its own requests to downstream services. The role depends on which side of a given request the program is on.
Because the server does not automatically remember anything about a client between requests. Each request must carry all the context it needs. To keep a user logged in, the server gives the client a cookie or token that the client resends on every later request.
The industry examples in “The next question”:
HTTP
The protocol powering the web. A request-response model where clients ask for resources and servers respond. Stateless by design.
REST API
An architectural style for building APIs using standard HTTP methods (GET, POST, PUT, DELETE). Resources are identified by URLs.
API
Application Programming Interface: a contract defining how two pieces of software talk to each other. The waiter between your frontend and your backend.
Latency
The time delay between sending a request and getting a response. Amazon found every 100ms of extra latency costs 1% in sales.
Throughput
The number of operations a system can handle per unit of time. Think of it as how many cars a highway can move per hour.
Bandwidth
The maximum amount of data that can be transferred over a network in a given time. It's the width of the pipe, not how fast the water flows.
This page explains the idea. The course covers it step by step, with diagrams. It is part of the System Design Masterclass course, which has 770 lessons. 18 of them are free to read. You pay once and keep it for life, with no subscription.
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Asked about Client-Server Model in an interview?
Each question has why it is asked, a full answer, what the interviewer listens for, the follow-ups, the common mistakes and a diagram (268 diagrams in all). A PDF you keep, junior to staff level.
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