How do iOS Push Notifications work?

W

5

134

How do iOS "push" notifications get delivered to a particular device without that device needing to poll a server?

For example, let's say I have received a new message on Facebook. Facebook notifies Apple that my device should receive a notification as such. But how does Apple know which device/IP to push the message to?

Woodwaxen answered 23/6, 2013 at 15:53 Comment(5)

@Wain I am looking for a detailed explanation at a technical level. That article is extremely high level. So my device maintains an open connection with Apple's push servers at all times? – Woodwaxen 23/6, 2013 at 15:58

Im more familiar with android push, but typically, when you install the app it registers the device with a unique ID with Apple. Then when facebook sends a message to apple, apple will forward it to the devices unique ID. – Stipple 23/6, 2013 at 15:58

@Pjack yep, but with my device constantly changing different cell towers, different wifi networks, how does Apple know the IP for my unique ID? – Woodwaxen 23/6, 2013 at 16:0

@whydna Thanks for asking this question. I wanted to understand precisely the same thing. I believe this answers my question here: #18860232 – Cantaloupe 18/9, 2013 at 9:53

@AndyHin The device OS (iOS) knows which APNs server(s) are valid. The device is connecting to these servers and keeping a connection open. This connection is likely associated with the device-id and other meta-data (phone number). When a message needs to be "pushed", the APNs finds the right connection and sends it down that already-open connection. So the APNs does not try to establish a connection to a device, it's always the device that opens that connection. One connection per device. – Pourparler 17/8, 2020 at 3:51

S

68

It was too much for me to put in a comment so.

From the documentation.

Apple Push Notification service (APNs) propagates push notifications to devices having applications registered to receive those notifications. Each device establishes an accredited and encrypted IP connection with the service and receives notifications over this persistent connection. Providers connect with APNs through a persistent and secure channel while monitoring incoming data intended for their client applications. When new data for an application arrives, the provider prepares and sends a notification through the channel to APNs, which pushes the notification to the target device..

I suggest reading the documentation for more information and how to use and configure. It's all there.

Push Notifications

Stipple answered 23/6, 2013 at 16:9 Comment(8)

So Apple maintains a static IP to your device? – Kaoliang 12/6, 2014 at 8:39

I was wondered when understood that my server have to send a notification to APNS FOR EACH TARGET DEVICE! :( – Combings 19/6, 2014 at 9:37

@Kaoliang The device establishes the connection, not the server. – Thorton 22/6, 2016 at 1:8

@Panama jack hello sir how long will the message remain in the server if i put note.expiry = Math.floor(Date.now() / 1000) + 43200 sec than will it remain up to 12 hrs . i just want to know does expiry time has limit or it goes our way. – Migdaliamigeon 6/12, 2016 at 5:27

so the devices maintain a long polling/socket kind of connection always with APN service? If such is the case then how does APN service handle so many connections - If a server can hold 50k connections and there are 500 million users always, service at least needs 10000 servers right. Is this a practical scenario? – Gingerly 20/7, 2018 at 11:16

@Gingerly That is for Apple to handle, but I would estimate 50k connections per server to be extremely underestimated. I would expect more in the order of 10 million connections per server. It all depends on the amount of traffic that they need to handle. – Indiscriminate 5/4, 2019 at 12:3

@Gingerly this was why I Google this question. I find it doubtful that 'long poll' would be the best solution Apple could think of. Just observing how wobbly my data connection gets when I use the phone on a train. I guess anyone who actually knew would have signed an NDA. It would be fun to guess. – Nivernais 16/4, 2019 at 2:7

@Gingerly I haven't dug deeply into push notifications in particular, but long poling/socket/etc isn't necessary except for browsers that are talking to web servers. The "regular" internet, i.e. TCP/IP, was designed to support exactly this kind of communication. There are some tricky aspects, like the device moving from network to network. My guess is that the device just handles reconnecting as needed. Also, these days a lot of consumer routers and maybe ISP routers don't deal well with long-lived, "quiet" TCP/IP connections, so there is probably some sort of keepalive. – Cormac 1/9, 2023 at 0:2

N

118

Each device can be updated with data using their own unique device tokens. This picture explains everything . .

enter image description here

Neelyneeoma answered 3/6, 2015 at 10:46 Comment(8)

Any idea what is the usage of the Device Token? Is it used for encrypting communication between the APNS server and the App or elsewhere? – Vitrics 27/4, 2016 at 8:15

@Mugen: Device token is used by APNS to identify which device it is supposed to forward the payload sent by provider! – Presbyterate 6/1, 2017 at 13:24

@Karan - Is there a distinction between notifications which are for a user (e.g. "Hey user you have a message!") and those that are meant for to wake up an app and silently tell it to do some background processing? – Insomnolence 23/2, 2017 at 0:47

@Insomnolence If you are using push notification, you will get notified whenever a notification has been received if the app is running. Also the active notification details are accessible , when user taps the notification from notification center and launches the app. During this time you can perform the processes that you wish to do. – Neelyneeoma 16/5, 2017 at 9:17

@KaranAlangat, How do Badge Count handled here? How server knows about it? – Scheller 20/7, 2018 at 11:27

@Scheller Usually we will get badge information from Notification Payload. We can change it from app also. – Neelyneeoma 20/7, 2018 at 13:33

how server knows unseen messages in app?did app needs to call anything? – Scheller 20/7, 2018 at 18:39

I think step 5 is the confusing one and triggered this question. It's a bit unclear to me as well how that part works, especially since your phone a lot of times can be behind a NAT router that typically rejects incoming traffic that wasn't requested. – Phenica 14/3, 2019 at 9:55