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App monitor location on background service

I would like to make an app that always works in the background (from boot up), which sends GPS coordinates to a server. This app should ALWAYS be active and should never close.
Should I use the services? I would like to use UDP sockets to send coordinates but I accept alternatives. I would also like to avoid using the google API.
Thanks a lot :)

If your app need to run in the background , you need Service and you need to make it a foreground service which means you need to show a notification to the user as long as your application is running.
To open app on device boot, from Android O, its not allowed. You will get an IllegalStateException.
The main reason for this is to prevent exactly what you are trying to achieve.
Its not good to keep running an app in the background and its especially bad to keep tracking users GPS coordinates and send it to the server.
Because it will drain the battery very soon.
However it is possible to keep a foreground service running which can take the GPS coordinates and send it to the server. But for that user has to open your App first.
Please refer to
https://developer.android.com/about/versions/oreo/background#services
Other alternative is to use JobIntentService which will schedule your tasks in smart ways to avoid draining users battery and data.
Regarding UDP sockets , it depends on your backend.

Best Practice for network operation at regular intervals?

I am making an application which contains performing network calls to look for updates. Can anyone help with some best practices to perform this task. I can make a service to run on background and perform network operation using Handler at regular intervals but this would consume a lot of data and battery. Is there any other way to do this?

Use an AlarmManager or JobScheduler (depending on API level), and pick a sane frequency. Doze mode will stop you from going too insane.

You could work with Push Notifications from any server when the backend recognizes an update. It's better for the server itself to listen to updates at a regular interval of time rather than the app, because of the reasons you listed.
When the app receives a push notification, it would mean that it needs updating.
There are a couple of options for you:
Azure Notifications Hub: https://learn.microsoft.com/en-us/azure/notification-hubs/
Firebase Cloud Messaging: https://firebase.google.com/docs/cloud-messaging/

Syncing the website account with android app

i was thinking if there is anyway to sync android app with a web/website account. Basically, if the user logs in through the web, i want the android app to log in/start up/show notification. Should i use a background service in android to monitor the account or something else
Please help
Thanks

As far as I can see, you have a couple of options:
Use a background service (as you suggested) which is constantly running and checking. This will be battery consuming.
Use the AlarmManager to schedule a repeating alarm to check on a scheduled interval. This will be less battery consuming but also less immediate (unless you set a low interval which will consume a lot of battery).
Use a push message. This will be the most accurate and least battery consuming for your phone, but it's also the most advanced (difficult) to set up.

Based on your requirement, optimized solution is to use GCM - Push Notification. Based on the whatever action occurs at web, you just need to send a push notification to clients. Based on the push notification IDs, client needs to perform appropriate action.

Android / iOS: schedule task while app is in background / off

When a Server wants to contact a client, though the corresponding app is inactive or off, he can do this via Google Cloud Messaging. My little application runs in combination with a webserver which I do not own and only runs php on, so actively contacting a client is impossible.
Now there are apps with probably similar problems, such as the Email apps. Mailservers never manually contact their clients, when a new message was received, so they check for new messages every, say, 30 minutes.
My question is: Is it possible to run such a background task? And is there a way to do this in iOS, too?
Thanks in advance!

If there is something you want android to do when the user is not interacting with the widget/application, you should use a service.
Android Service
It is meant to be used for tasks that require no user interaction and is especially great for checking something over and over. No guarantee that the os still wont kill it eventually, but it kept alive as long as possible. You can also create a service to be restarted anytime it dies, if you really want to do something long term.

Best way to have an Android app poll periodically in the background

Take the Gmail app as an example. Whether the phone is on or not, it polls every 10 minutes or so to download new emails which may have arrived since you last checked.
I know how to create a new service and bind to it. But I can see a few ways to accomplish this:
Bind once, and have the service run in an infinite loop, sleeping for 10 minutes between each loop
Bind and unbind right when it's done, scheduling the next bind somehow in 10 minutes
Using the AlarmManager class to schedule future polls
What are the trade offs? How does the Gmail app accomplish it?
Thanks!

Gmail app uses pushing, not polling. I suggest using this technique instead, polling is a battery killer in mobile devices.
To implement pushing, take a look at C2DM.
If you still want to poll, the recommended way would be to set up a periodic alarm in the AlarmManager.
UPDATE: Google has deprecated C2DM and replaced it with Google Cloud Messaging (GCM)
UPDATE: Google has deprecated GCM and replaced it with
Firebase Cloud Messaging (FCM)

For a continuous, but not intensive poll like the one you comment (in the range of minutes between polls), I would implement it with AlarmManager. That way you make sure the phone wakes up to poll without the need for a wakelock, which would destroy your battery. As CommonsWare pointed out, you will still need to implement a wakelock for the time your code is executing, but you can release it as soon as the code is done, avoiding keeping the phone on while just waiting. See his comment for an example on how to implement it.
I would use a Service if, instead, you need faster polls during a shorter period of time (seconds between each poll), since setting alarms does not make sense to such short periods, and the battery would drain anyway.