Is there any alternative for PeriodicWorkManager, if I would like to run a background work more frequently than 15 minutes?

The PeriodicTimeRequest has a minimum periodic time of 15 minutes. But I see, that for example Google Maps location sharing can update more frequently than that, and facebook messenger can also receive messages almost instantly.

I would like to send a notification to the user, when it got a new message. My application has to work on local network, so Firebase is not an option. I have to send a json request to the server, and if there is a new message, I show a notification to the user.

Regarding FCM:

FCM, which is available in all devices with Google Play takes the weight of subscribing to and receiving push events, under all the resource constraints Android has been ever introducing.

It's tightly coupled with the OS and is unified (one entity, one persistent connection for all apps in your device), which is why it works

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Regarding Frequency of your Work:

Given your requirement of more frequent pings to the server, you'd need to have a service which runs all the time, i.e. A Foreground Service.

It is resource consuming though, so good luck convincing the user with a good reason why it should stay alive all the time.

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I think you've managed to make the client-server interaction possible, since identifying a server in a local network is a huge task in itself.

use this in your service.

@Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        CountDownTimer timer = new CountDownTimer(15 * 60 * 1000, 1000) {
            @Override
            public void onTick(long millisUntilFinished) {
                // execute your task here, every sec
                //if you want increase the count down interval from 1000 to what you want
            }

            @Override
            public void onFinish() {
                this.start();
                // it will start again.
            }
        };
        timer.start();
        return START_STICKY;
    }

I am afraid it is not going to be possible without using a set of workarounds. Which means you might not get a consistent behavior.

@Arvind has done a very good job explaining the benefits of a Firebase Service and it is the recommended approach for achieving such task.

First I'd like to point out that such restrictions on the WorkManager exist because Android has been suffering (between other things) of developers trying to abuse some mechanisms to get their software working and at the end of the day, the battery of the users had been suffering from such abuses and since Android 6 Google has started trying to address these issues. There's a good read for you over here about Doze mode and how to work with it

I am pointing this stuff out because I've been trying to build a chat service that wouldn't rely on Firebase and I really don't want you to waste as much time as me banging your head against a wall. There are things that you simply can't fight. That means that if the device enters in a "deep-sleep" mode sometimes you can only accept it.

My approach

Please

keep in mind the user interests and the life of their batteries and try to be as smooth as you can with their devices and this is just a workaround over the restrictions that have been imposed upon us. And that I discourage this approach due to the amount of work that it takes to pull off and for how misused it can be.

My solution

Essentially, to get notified (ie getting your code running) in an Android App you're going to be wanting to receive system events or Broadcasts. This is where you set up a BroadcastReceiver and you get Intents delivered to it and you can act upon them accordingly. BUT YOU HAVE TO BE QUICK BECAUSE YOU HAVE ONLY 10 SECONDS OF RUNTIME BEFORE THE OS KILLS THE PROCESS AGAIN. Ideally you would have a really quick server so you can have very little IO times to ensure you can be within 10 second restriction as frequently as possible.

So essentially you would be using a combination various of services that you would like to be monitoring in order to get notifications (aka Broadcasts) whenever the state of those changes. Here are a few ideas:

• WiFi state (which will also be useful to see if you can reach your local server)
• Bluetooth Low Energy packets (or Nearby which may solve the entirety of your problem depending on Nearby's capabilities)
• WorkManager as you already pointed out.
• AlarmManager to schedule a broadcast of intents every so often.
• Geofencing (although it involves reading the user's location; you can set really small geofences around the office building and get notified by a Broadacast when users go through that geofence)

So whenever you receive a Broadcast of these sources you would handle such notifications from within the same BroadcastReceiver

From the implementation body of this Broadcast receiver you would poll the local network's server to check whether if your user has new messages or not and lift up a notification. And it's important to keep the amount of work and IO times the app has to do at a minimum since those add up and you've got only 10 seconds.

You can get around the 10 second mark if you launch a ForegroundService. Then, that period of time is going to be extended until a 10 minute mark and you will need a visible notification for the user stating something that you're checking if it's got any new messages.

Keep in mind

• Don't stress the user's battery too much. Or Android will penalise your app and you'll end up notified less often or even completely not notified.
• Be gentle with the user. If the user has to force-kill your app at some point it will stop receiving any sort of Broadcasts or running any sort of WorkTasks.
• This solution can behave differently accross devices. Since the decisions of notifying your app are made by the OS, different OS (redmi, samsung, meizu...) you are likely to not end up with a consistent behavior across all devices
• You don't have control over things, the OS does
• Within measure, try to time your Broadcasts to your BroadcastReceiver within spans of 3 minutes or so; so you are always receiving a Broadcast below the 15 minute mark.