Distributed Systems

A customer clicks Pay. Your server charges the card, and then the connection drops before the response reaches the browser. The client cannot tell whether the c…

An order-ingestion API writes to an in-memory queue, a worker pool drains it, and a payment provider on the other end gets slow. The queue keeps accepting every…

A shopping cart write to cart-77 needs to land on enough replicas that a later read is guaranteed to see it — but "enough" is a number you choose, and the numbe…

A 4-node session cache scales to 5 nodes to handle more traffic. With hash(key) % N, that single node addition changes almost every key's target node, and the c…

A user changes their display name, refreshes the page a second later from a phone on a different network, and sees the old name. Nothing crashed. No request fai…

This guide is for backend engineers who know HTTP and database transactions but need to decide where six microservice patterns fit. By the end, you can trace on…

A trip booking needs a flight, a hotel room, and a card charge to either all succeed or all unwind — but each lives in a different service with its own database…

A signup service inserts a new User row, then calls the broker to publish UserRegistered so the welcome-email and loyalty services can react. The database commi…