System design interview question
"Design Gmail. Users send and receive email, search their entire mailbox instantly, and the system interoperates with the rest of the world over SMTP. 2 billion users, 15 GB of mail each."
a system constrained by a 40-year-old protocol you do not control, plus per-user search at enormous scale. The distinguishing features are: SMTP's asynchronous store-and-forward semantics, spam as an adversarial problem, and the fact that every user needs a private full-text index — 2 billion small indexes rather than one large one.
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