HLDintermediatePremium

WhatsApp HLD: Real-Time Chat for Billions, Online or Off

A WhatsApp / chat-at-scale system design: persistent connections and a routing registry, store-and-forward for offline users, idempotent ordered delivery, and the delivery-tick receipts.

By fiveyearsdevJune 14, 202623 min read

You type "on my way" as the metro dives into the tunnel. The bubble sits there with a little clock icon. Three stops later the signal comes back and — without you touching a thing — the clock turns into one grey tick, then two, and by the time you reach the escalator they're blue. Your phone was offline the whole time. Nobody re-sent anything by hand, nothing arrived twice, and the message landed after the "leaving now" you'd typed a minute earlier, not before it. That tiny animation is the entire system design. The clock means your phone hasn't handed the message over yet. One tick means a se…

What’s inside

Let's start nowhere near a computer

Where this exact shape shows up

Step 1 — Functional requirements (sentences first)

Step 2 — Non-functional requirements

Step 3 — Connections and the routing registry

Step 4 — The data model, and which datastore

Step 5 — Verbs become the wire protocol

Step 6 — The core: dedup, sequence, store-and-forward

Step 7 — Receipts, and the per-device cursor

Step 8 — Presence: the requirement that is allowed to be wrong

Step 9 — Groups: where one send becomes N deliveries

Step 10 — Trade-offs (each one keeping an NFR)

The complete implementation

Step 11 — Only now, the boxes

Step 12 — Scaling the design, one bottleneck at a time

Step 13 — When a piece fails: designing for failure

The interview corner

Where to go from here

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