Score yourself

Give yourself 0, 1 or 2 points per row.

Area	0 points	1 point	2 points
Requirements	Started drawing	Send and receive	Order, duplicates, offline, multi-device, groups and encryption agreed
Estimates	None	Numbers, not used later	Connections and message rate drove gateways and sharding
API and data	Skipped	REST only	WebSocket messages with client_id, seq, delivered/read; cursors per device
High-level design	Server polls	Gateways without routing	Store-then-deliver, registry routing, push + sync walked
Deep dive	Hand-waved	One topic	Ordering and dedupe, ticks across devices, offline sync or groups done properly
Trade-offs	None named	Named when asked	Raised unprompted (per-chat order, retention, receipt cost, E2E limits)
Communication	Long silences	Mostly clear	Thought out loud, checked in, managed time

12 to 14: strong hire signal. 8 to 11: solid, polish the weak rows. Under 8: redo the round in a week, timer on.

The bar: fresher vs SDE-2

	Fresher	SDE-2
Transport	WebSocket	Gateway tier, registry, heartbeats, reconnect storms
Ordering	Timestamps	Per-chat sequence from one writer; client IDs for dedupe
Ticks	Sent and read	Stored / all devices / read, "up to N" acks, privacy setting
Offline	"Store and forward"	Per-device cursors, push nudges, deletion after delivery
Groups and media	Loop over members	Sender keys, receipt aggregation, encrypted blobs with keys in messages

Common mistakes

Ordering by client timestamps.
No client ID, so a retried send appears twice.
Delivering before storing, so a crash loses messages.
Two grey ticks after the first device acks, ignoring the user's other devices.
Sending big media through the chat socket instead of object storage.

Quick check

Five questions on this chapter. Pick an answer, then check it.

quiz

1[
2 {
3  "q": "What should happen before a message is delivered to the recipient?",
4  "options": [
5   "Nothing",
6   "It's stored durably and the sender gets an ack",
7   "It's shown as read",
8   "It's encrypted twice"
9  ],
10  "answer": 1,
11  "explain": "Store first, then deliver; ack with a sequence number."
12 },
13 {
14  "q": "What stops a resent message from appearing twice?",
15  "options": [
16   "The timestamp",
17   "A client-generated message ID the server de-duplicates on",
18   "The sequence number",
19   "Nothing"
20  ],
21  "answer": 1,
22  "explain": "The client ID stops repeats; the chat's sequence number sets the order."
23 },
24 {
25  "q": "What sets the order of messages in a chat?",
26  "options": [
27   "Each phone's clock",
28   "A per-chat sequence number assigned by the server",
29   "Message length",
30   "Arrival at the recipient"
31  ],
32  "answer": 1,
33  "explain": "Clocks on phones disagree; the chat's sequence number is the order."
34 },
35 {
36  "q": "A phone was offline for a day. How does it catch up?",
37  "options": [
38   "The server resends everything ever",
39   "It syncs from its last cursor (sequence number) per chat",
40   "It can't",
41   "Push notifications carry every message"
42  ],
43  "answer": 1,
44  "explain": "Store first, nudge the sleeper, sync from the cursor."
45 },
46 {
47  "q": "How is a photo sent in an encrypted chat?",
48  "options": [
49   "Inside the WebSocket message",
50   "Upload the encrypted file to storage; send its link and key in the message",
51   "Through SMS",
52   "As base64 text"
53  ],
54  "answer": 1,
55  "explain": "Upload the locked box, send the key in the message."
56 }
57]

38. Design a Chat App (WhatsApp)

Score yourself

Score yourself

The bar: fresher vs SDE-2

Common mistakes

Quick check

Platform

Company

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