Linked devices: what a session is, and why sending stops
A linked device holds its own session and its own keys. Since multi-device it keeps working with the phone switched off - but the link is not permanent, and when it ends, sending stops with no error that explains itself.
What ends a session
| Cause | Warning | Fix |
|---|---|---|
| Logged out from the phone | None | Link again |
| Number re-registered on a new handset | None | Link again |
| Phone offline for a long period | None | Bring the phone online, link again |
| Server-side reset | None | Link again |
| The phone factory reset | None | Link again, and check the PIN |
The column that matters is the middle one. There is no warning for any of these, which is why the operational answer is monitoring rather than reaction.
What does not end it
- The phone being switched off or out of battery.
- The phone being out of coverage for a while.
- The phone being on a different network.
- Restarting your own server.
This is worth internalising because the reflex when sending stops is to go and charge the phone. Since multi-device that is usually not the problem, and the minutes spent on it are minutes the queue keeps growing.
What to monitor
- Connection state per number, surfaced somewhere a person looks, not only in a log.
- Time since the last successful send. A number that normally sends every few minutes and has sent nothing for an hour is the alert, and it fires for every cause in the table above.
- Queue depth. Messages piling up unsent is the same signal from the other side.
The wider diagnostic order - everything or some things, the number, the session, the recipient, the window, the payload - is in messages stopped sending.
Audit the list, because it is a list of who can read everything
Every linked device can read every conversation on the number. A laptop in a repair shop, a device linked by somebody who has left, a session from a machine nobody recognises - each of those is full access to your customer history, and none of them appear anywhere except the linked devices screen.
- Open it monthly and read the list. It takes thirty seconds and nothing else surfaces this.
- Remove anything you do not recognise, immediately. The cost of being wrong is that somebody links again; the cost of being right and doing nothing is unbounded.
- Log out everything when a handset is lost before you do anything else, then re-link what you need.
- Do it as part of offboarding. Removing somebody's email and leaving their linked laptop is the version of this that actually happens.
Logging out all devices ends every session including your own integration's, so sending stops until it is linked again. Do it anyway when a device is lost - the alternative is a stranger reading your inbox - but do it knowing you are about to have an outage rather than discovering it afterwards.
Questions people actually ask
Does my phone need to be online for WhatsApp Web to work?
No, not since multi-device. Linked devices keep working with the phone off, though the phone has to connect periodically or the link eventually expires.
What logs out a linked device?
Logging out all devices from the phone, re-registering the number elsewhere, a long period with the phone never coming online, and occasionally a server-side session reset.
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