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WebSocket vs Push notification

WebSocketRajat has WhatsApp open
D
DKonline

Yesterday

Did you see Virat's cover drive tonight?10:14
Unreal. Timing was 🔥10:15
Nets at 6 tomorrow?10:31
Done. Bring the new bat 🏏10:32
Will do 👍10:34
Chat server Bholds Rajat
Chat server ADK’s server
RedisRajat → B
DK
Push notificationRajat’s phone is locked

Chat server Bno socket
Chat server ADK’s server
RedisRajat: away
NotificationWhatsApp
Apple pushAPNs / FCM
DK

One WhatsApp message, two ways to reach Rajat: down the socket his open app keeps, or a push that wakes his locked phone.

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REST vs GraphQLREST vs gRPC

WebSocket or Push notification?

Use both, for different moments. A WebSocket while the app is open: instant, and both ways. A push when it’s closed: the system wakes the phone, and the app opens a socket to fetch the rest.

See it in a real system

Watch WhatsApp’s real designEvery call when Rajat is away

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REST vs GraphQLREST vs gRPC
DifferenceWebSocketPush notification
Works whenThe app is openThe app is closed or the phone is locked
DirectionBoth waysOne way: to the phone
SpeedInstantUsually seconds; the system can delay it
CarriesMessages, receipts, typingA small wake-up; here, an encrypted payload the phone decrypts
CostA connection per open app, plus batteryOne shared connection per phone
In WhatsAppDelivery while the chat is openA banner, then a short fetch

WebSocket vs Push notification, explained step by step

1. A message arrives

One WhatsApp message, two ways to reach Rajat. On the left, a WebSocket: a live connection his open app keeps to WhatsApp. On the right, a push notification: Apple or Google wake his phone when the app is closed. Let’s put both through three situations.

First, DK sends: Dinner at 8? Rajat has WhatsApp open, so his phone keeps a WebSocket to chat server B. The message goes straight down it and appears in his chat at once.

His phone confirms on the same connection, and DK sees two grey ticks.

On the right, Rajat’s phone is locked, and his app holds no connection. The message waits in his inbox, and WhatsApp asks Apple’s push service to wake the phone. A banner appears.

The push wakes the app for a few seconds. It opens a short connection and fetches the message, and only then does DK see two grey ticks.

2. Rajat replies

Next, Rajat reads it and replies. The read receipt and his reply both go up the same open socket, instantly, because a WebSocket works both ways.

On the right, a push only goes one way: to the phone. Rajat taps the banner, WhatsApp opens and connects a socket, and only then can he read and reply.

3. Millions of phones

Finally, scale. Every open app holds its own socket, so a chat server keeps hundreds of thousands of them, and each phone spends battery keeping its socket alive. That’s why phones drop the socket when the app goes to the background.

On the right, the phone keeps one connection to Apple or Google, shared by every app. WhatsApp uses it only to say: wake up, there’s something new.

In short

So which should you pick? Both, for different moments. A WebSocket while the app is open: instant, and both ways. A push notification when it’s closed: the phone’s system wakes it, and the app opens a socket to fetch the rest.