System designLevel 2
Messaging and streams
Easynot started
Mediumnot started
Hardnot started
Real-time & fan-outDelivering live updates to millions of connected users instantly.3 minQueues & streamingDecouple services by passing messages asynchronously through an append-only log.3 minScaling & load balancingDistribute incoming traffic across a fleet of stateless servers to handle infinite growth.3 minNotifications and fan-outOne event happens, and now you owe a notification to everyone who cares. Where do you pay the cost: when it's written, or when it's read?4 minServer-sent events, long-poll, and fan-outInstead of clients nagging "anything new yet?", the server holds the line open and pushes each event out to everyone at once.4 minAPI designDesigning robust interfaces means planning for changing data and edge cases.3 minWebhook delivery gatewayPush events to someone else's server you don't control, and keep retrying calmly until it answers or you give up.4 minPresence and online statusA user is "online" only as long as their heartbeats keep arriving, silence for too long means away.4 minCarrier SMS routing protocolHow a text message actually finds your phone across the world.3 minDesigning team chatA message is a letter that must be filed before it's delivered, write it down first, then fan it out to everyone in the room.3 minEnd to end encryption (E2EE)Making sure even the server hosting the app can't read your messages.3 minPush notification gatewayOne logical "send" becomes thousands of tiny deliveries, the gateway is the part that turns a message and a token list into the right call to the right platform.3 minReal-time personalizationTurn a click you made one second ago into the next thing you see.4 minRealtime matchmakingWhy multiplayer lobbies use WebSockets instead of normal HTTP requests.3 minSecondary index serviceThe base table is filed by id; a secondary index is a second filing cabinet keyed by some other attribute, so a query doesn't have to read every row to find its matches.4 minThe notification senderA patient mail carrier: it takes your message off a durable queue, knocks on the provider's door, and if no one answers, waits a little longer each time before knocking again.5 minWebhook ordering and retry protocolDeliver every event at least once, let duplicates be harmless, and never let a later event for the same key slip past one that hasn't landed yet.4 min