Push Notifications

Say you don't know when a package will arrive, so you walk down to the mailroom every ten minutes. Your legs get tired and it eats up your time. Worse, nine out of ten trips are wasted.

Apps used to check for news in much the same way. An app would ask the server at set times, "Any new messages?" This is called polling. The more apps you have, the more wasted trips there are, and the faster your battery drains.

Push notifications work the other way around. The side with the news speaks up first, so you never have to go check. It's like the front desk accepting your package and calling you on the intercom first.

Thanks to this, apps just rest and do nothing most of the time. Even when news comes in, an app only needs to wake up briefly if it's needed.

For the front desk to call you, it has to know which apartment is yours. The first time you launch an app, your phone gets an address that belongs only to that app from the notification server. Then it registers that address with the app company's server. You also have to allow notifications, or news sent to this address can't show up on your screen.

When a new message comes in, the app company's server doesn't contact your phone directly. Instead, it asks the notification server, "Please deliver this message to this address." It's just like leaving a package with the front desk.

While your phone is on, it keeps one thin channel open to the notification server. When news arrives, it comes straight down this channel, and the operating system displays it on your screen. If needed, the app wakes up briefly at that moment to fill in the details.

Because apps share a single channel instead of each opening its own, it uses far less battery and data.

The front desk analogy also explains why notifications are sometimes late. When your phone falls into deep power-saving sleep, the channel closes for a while. Notifications that arrived in the meantime may come in a bunch once it reconnects, but some may get dropped and only the most recent one comes through. The same thing happens when your internet is down.

What a notification carries is very small. It isn't a photo or a whole conversation. It's more like a short note saying "you have a new message." When you tap the notification and open the app, the app then pulls the rest from the server. A notification is a summary of the news.

Turning notifications off doesn't make your messages disappear. The front desk just didn't call you, but the package is still sitting there. When you open the app later, all the news that came in is right there.

Notifications from websites borrow the channel in the same way. But this channel only delivers the news, so what actually gets shown is decided by the app and the user's settings.