This document aims to describe how Element android displays notifications to the end user. It also clarifies notifications and background settings in the app.
`To read events, the intended flow of operation is for clients to first call the /sync API without a since parameter. This returns the most recent message events for each room, as well as the state of the room at the start of the returned timeline. `
## How does a mobile app receives push notification
Push notification is used as a way to wake up a mobile application when some important information is available and should be processed.
Typically in order to get push notification, an application relies on a **Push Notification Service** or **Push Provider**.
For example iOS uses APNS (Apple Push Notification Service).
Most of android devices relies on Google's Firebase Cloud Messaging (FCM).
> FCM has replaced Google Cloud Messaging (GCM - deprecated April 10 2018)
FCM will only work on android devices that have Google plays services installed
(In simple terms, Google Play Services is a background service that runs on Android, which in turn helps in integrating Google’s advanced functionalities to other applications)
De-Googlified devices need to rely on something else in order to stay up to date with a server.
Technically there is a difference between a push and a notification. A notification is what you see on screen and/or in the notification Menu/Drawer (in the top bar of the phone).
Notifications are not always triggered by a push (One can display a notification locally triggered by an alarm)
## Push in the matrix federated world
In order to send a push to a mobile, App developers need to have a server that will use the FCM APIs, and these APIs requires authentication!
This server is called a **Push Gateway** in the matrix world
That means that Element Android, a matrix client created by New Vector, is using a **Push Gateway** with the needed credentials (FCM API secret Key) in order to send push to the New Vector client.
If you create your own matrix client, you will also need to deploy an instance of a **Push Gateway** with the credentials needed to use FCM for your app.
This is defined by [**push rules**](https://matrix.org/docs/spec/client_server/r0.4.0.html#push-rules-).
`A push rule is a single rule that states under what conditions an event should be passed onto a push gateway and how the notification should be presented (sound / importance).`
There are different kind of push rules, it can be per room (each new message on this room should be notified), it can also define a pattern that a message should match (when you are mentioned, or key word based).
Of course, content patterns matching cannot be used for encrypted messages server side (as the content is encrypted).
That is why clients are able to **process the push rules client side** to decide what kind of notification should be presented for a given event.
## Push vs privacy, and mitigation
As seen previously, App developers don't directly send a push to the end user's device, they use a Push Provider as intermediary. So technically this intermediary is able to read the content of what is sent.
App developers usually mitigate this by sending a `silent notification`, that is a notification with no identifiable data, or with an encrypted payload. When the push is received the app can then synchronise to it's server in order to generate a local notification.
## Background processing limitations
A mobile applications process live in a managed word, meaning that its process can be limited (e.g no network access), stopped or killed at almost anytime by the Operating System.
In order to improve the battery life of their devices some constructors started to implement mechanism to drastically limit background execution of applications (e.g MIUI/Xiaomi restrictions, Sony stamina mode).
Then starting android M, android has also put more focus on improving device performances, introducing several IDLE modes, App-Standby, Light Doze, Doze.
In a nutshell, apps can't do much in background now.
If the devices is not plugged and stays IDLE for a certain amount of time, radio (mobile connectivity) and CPU can/will be turned off.
For an application like Element, where users can receive important information at anytime, the best option is to rely on a push system (Google's Firebase Message a.k.a FCM). FCM high priority push can wake up the device and whitelist an application to perform background task (for a limited but unspecified amount of time).
Notice that this is still evolving, and in future versions application that has been 'background restricted' by users won't be able to wake up even when a high priority push is received. Also high priority notifications could be rate limited (not defined anywhere)
It's getting a lot more complicated when you cannot rely on FCM (because: closed sources, network/firewall restrictions, privacy concerns).
The documentation on this subject is vague, and as per our experiments not always exact, also device's behaviour is fragmented.
It is getting more and more complex to have reliable notifications when FCM is not used.
* The Element android app published on F-Droid do not rely on FCM (all related dependencies are not present)
* The Element android app published on google play rely on FCM, with a fallback mode when FCM registration has failed (e.g outdated or missing Google Play Services)
As this mode does not need to live beyond the scope of the application, and as per Google recommendation, Element uses the internal app resources (Thread and Timers) to perform the syncs.
When a message should be notified to a user, the user's homeserver notifies the registered `push gateway` for Element, that is [sygnal](https://github.com/matrix-org/sygnal) _- The reference implementation for push gateways -_ hosted by matrix.org.
This sygnal instance is configured with the required FCM API authentication token, and will then use the FCM API in order to notify the user's device running Element.
As per [Google recommendation](https://android-developers.googleblog.com/2018/09/notifying-your-users-with-fcm.html), Element will then use the WorkManager API in order to trigger a background sync.
> We recommend using FCM messages in combination with the WorkManager 1 or JobScheduler API
> Avoid background services. One common pitfall is using a background service to fetch data in the FCM message handler, since background service will be stopped by the system per recent changes to Google Play Policy
If Element is able to detect one of this cases, it will notifies it to the users and when possible help him fix it via a dedicated troubleshoot screen.
Meanwhile, in order to offer a minimal service, and as per Google's recommendation for background activities, Element will launch periodic background sync in order to stays in sync with servers.
The fallback mode is impacted by all the battery life saving mechanism implemented by android. Meaning that if the app is not used for a certain amount of time (`App-Standby`), or the device stays still and unplugged (`Light Doze`) , the sync will become less frequent.
And if the device stays unplugged and still for too long (`Doze Mode`), no background sync will be perform at all (the system's `Ignore Battery Optimization option` has no effect on that).
Also the time interval between sync is elastic, controlled by the system to group other apps background sync request and start radio/cpu only once for all.
Usually in this mode, what happen is when you take back your phone in your hand, you suddenly receive notifications.
The fallback mode is supposed to be a temporary state waiting for the user to fix issues for FCM, or for App Developers that has done a fork to correctly configure their FCM settings.
Also Google's recommended background processing method cannot be applied. This is because all of these methods are affected by IDLE modes, and will result on the user not being notified at all when the app is in a Doze mode (only in maintenance windows that could happens only after hours).
Only solution left is to use `AlarmManager`, that offers new API to allow launching some process even if the App is in IDLE modes.
Notice that these alarms, due to their potential impact on battery life, can still be restricted by the system. Documentation says that they will not be triggered more than every minutes under normal system operation, and when in low power mode about every 15 mn.
F-Droid version will schedule alarms that will then trigger a Broadcast Receiver, that in turn will launch a Service (in the classic android way), and the reschedule an alarm for next time.
Depending on the system status (or device make), it is still possible that the app is not given enough time to launch the service, or that the radio is still turned off thus preventing the sync to success (that's why Alarms are not recommended for network related tasks).
That is why on Element F-Droid, the broadcast receiver will acquire a temporary WAKE_LOCK for several seconds (thus securing cpu/network), and launch the service in foreground. The service performs the sync.
