Notification Reply

Replying from the notification banner — without opening the app

iOS and Android both support inline replies directly from the notification. WhatsApp doesn't need to fully launch for this to work.

What inline reply looks like¶

Bob sees the notification banner: "Alice: hello??"

On iOS and Android, he can long-press the notification and type a reply directly — without opening WhatsApp. The reply sends. WhatsApp never fully launches.

How it works under the hood¶

When Bob taps reply in the notification, the OS wakes WhatsApp as a background process — not a full app launch. The UI doesn't appear. Only a lightweight background task runs.

Bob types "on my way" in notification reply field
  → hits send

OS wakes WhatsApp background process
  → WhatsApp background task:
     1. Reads reply text and conversation_id from notification
     2. Makes a REST API call to the WhatsApp server:
        POST /messages
        { conversation_id: conv_abc123, content: "on my way", sender: bob }
     3. Server processes it normally:
        → INCR seq counter → write to DynamoDB → deliver to Alice
     4. Background task exits

No WebSocket. No full app session. Just one REST call.

What Bob does NOT have at this point¶

Bob replied from the notification banner. He has not opened WhatsApp. His client has:

✓ The notification preview: "Alice: hello??"   (from notification payload)
✗ Full conversation history                     (not synced yet)
✗ Messages seq=42 "hey" and seq=43 "where are you?"  (never received)

The notification only carried the latest message preview. The earlier messages in this offline session are not on Bob's device yet.

What happens when Bob opens WhatsApp after replying¶

Bob opens WhatsApp
  → WebSocket established
  → pending_deliveries queried: {conv_abc123, first_undelivered_seq=42}
  → DynamoDB query: PK=conv_abc123, SK >= 42
  → returns seq 42, 43, 44 (Alice's messages)
  → Bob's outgoing reply is already in DynamoDB as seq=45
  → Full conversation renders:

    seq=42  Alice: "hey"
    seq=43  Alice: "where are you?"
    seq=44  Alice: "hello??"
    seq=45  Bob:   "on my way"    ← his notification reply, already delivered

The full history appears as if nothing unusual happened. The notification reply slotted into the correct position in the conversation.

Why this matters for the design¶

Notification reply confirms that WebSocket is not the only message delivery path. REST works for low-frequency, non-realtime sends like notification replies. WebSocket is the primary path because it's persistent and low-overhead for real-time messaging — but the system is designed to handle both.

Two delivery paths

WebSocket → primary, real-time, persistent connection REST → fallback, notification replies, one-off sends when app is not open