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System Design Interview: Facebook Messenger
Rumors say Facebook likes to ask candidates how they would design messenger. That’s probably why there are so many materials online for this topic. Utilitarian reasons aside, I think it’s indeed a good system design question to practice. After all, there is no shortcut to system design ability; only practice makes perfect. Facebook messenger has a rich feature set. I can’t cover all of them in one post, so I am going to willfully pick the areas I see as the core of messenger and dive into those.
Let’s clarify the requirements of a simple messenger system. I’ve annotated each feature with a number suffix corresponding to the visual on the figure.
A user has a list of contacts (1), from which she can see who is online/offline (2). The user can pick any one or multiple of her contacts and start a chat (3). In a chat, all participants can send messages (4). The messages will be displayed in chronological order (5). A message can be “liked” by any participant, whose avatar is then attached to the “liked” message (6). A participant’s avatar is also displayed after the last message she read, signaling her progress to other participants (7). A participant can delete any message in her own chat view (8). Only the sender of a message can delete it for all participants (9). Any user can leave a chat (10).
Let’s start by walking through the requirements from the angle of the messenger system’s backend (the backend for short) and see how that translate into information model and storage design. Then we’ll repeat the same journey from the perspective of the messenger app (the app) to see how things integrate together. In the end, I want to dive in the communication methods between the backend and the app as I believe that’s the meat of this system design question.
The Messenger Backend
First of all, the user information. This requires the backend to have a user table. Each user has a unique uid (and other primary user info), a contact list, a list of chats the user is actively involved in, and metadata. For simplicity’s sake, the messenger system doesn’t allow users to add or remove contacts. But it’s reasonable to expect some out of band contact edit (maybe through Facebook friends) to modify the user’s contact list. The chat list stores the active chat ids…
