Design a collaborative document editor like Google Docs.
Whether you recognise concurrent editing as a conflict-resolution problem rather than a locking one.
System designHard
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What a strong answer does
The hard part in a Google Docs-style editor is not rendering text boxes, storing files, or opening WebSockets. It is that Alice and Bob can edit the same sentence at nearly the same time, each client can see a different temporary reality because of network delay, and the system still has to converge to one identical document everywhere. If Alice deletes a word while Bob inserts inside it, both edits may be locally valid when made, but they conflict once exchanged. The design is therefore primarily a replicated-state conflict-resolution system with a nice UI around it.