What Is Last Seen

What "last seen" is — and the two events that drive it

A timestamp showing when Bob was last active on WhatsApp. Generated from two system events: app open and app close.

What Alice sees¶

When Alice opens a chat with Bob, she sees one of two things at the top of the screen:

Online                        ← Bob has WhatsApp open right now
Last seen today at 9:42am     ← Bob last had WhatsApp open at 9:42am

"Online" means Bob's WebSocket is live at this moment. "Last seen" means it isn't — and this is when it last was.

The two events¶

Last seen is driven by exactly two events in the system:

Event 1 — App opens

Bob opens WhatsApp. The app establishes a WebSocket connection with the WS server. The moment that connection is live, Bob is considered online.

Bob opens WhatsApp
→ WebSocket established
→ Redis registry: SET ws:bob → ws_server_3
→ Bob's status: ONLINE
→ Alice sees: "Online"

Event 2 — App closes (or connection drops)

Bob closes WhatsApp — or his phone loses signal, locks, or the OS kills the app. The WebSocket connection breaks.

The WS server detects the disconnection — either via a graceful TCP close or a heartbeat timeout — and records the timestamp:

Bob closes WhatsApp
→ WebSocket disconnects
→ WS server detects: connection for bob gone
→ Records: last_seen[bob] = now()
→ Redis registry: DEL ws:bob
→ Bob's status: OFFLINE
→ Alice sees: "Last seen today at 9:42am"

What drives the timestamp — close, not open¶

The last seen timestamp is always the disconnect time, not the connect time.

If Bob opened WhatsApp at 9:30am and closed it at 9:42am, last seen shows 9:42am. Alice knows Bob was active until 9:42am — not when he first opened the app. The closing moment is what matters.

Why "online" and "last seen" are separate states¶

They feel like one feature but they're two distinct states:

ONLINE      → WebSocket is live right now
             → updated in real time as Bob uses the app
             → disappears the moment Bob closes the app

LAST SEEN   → timestamp of the most recent disconnect
             → persists after Bob goes offline
             → Alice can see it even hours or days later

"Online" is ephemeral — it exists only while the connection is live. "Last seen" is durable — it's stored and served on demand.