Design Real-Time Chat

This is the design that maps most directly onto a product Cisco owns, and it helps to name that before drawing a single box. Cisco runs Webex, its enterprise collaboration suite, and messaging sits at the center of it: one-to-one chats, group spaces, presence, the read receipt a colleague watches after asking a question. A backend that holds a live connection to every open client, delivers a message the instant it is sent, and keeps the whole history searchable for years is not a hypothetical here. It is the thing they operate. The interviewer is not checking whether you can recite a generic chat diagram. They want to see whether you understand the shape of a system their own teams keep running.

Where the design round lands depends heavily on the level in play. Under senior it stays light, a handful of concept questions tucked into a technical screen. From senior upward it becomes a real, level-gating discussion. It commonly begins with a walk through something you designed in the real world before turning to a prompt like this one, and the interviewer reviews your design as a peer. The prompt starts small and grows under you. Now it is not one chat but a space with hundreds of members. Now the recipient has been offline all weekend. Now presence has to work for everyone at once. Expect the push to land on the parts a collaboration platform feels first. A message that must never be lost. A conversation everyone sees in the same order. And a live connection to millions of clients that has to find exactly the right one.

The Cisco angle

A real-time messaging backend is not a whiteboard hypothetical at Cisco. It is Webex, and Webex is theirs. Cisco bought Webex and runs it as a flagship: enterprise chat, spaces, presence, calling, all of it real-time collaboration for companies that treat a dropped or reordered message as a support ticket. An interviewer on a collaboration or full-stack team has almost certainly worked on a piece of this, so a generic messaging sketch gets probed and a reliability-first one lands.