Illustration by Virginia Poltrack
Illustration by Virginia Poltrack
Illustration by Virginia Poltrack

WorkManager Periodicity

Pietro Maggi
Jun 14 · 9 min read

Welcome to the fourth post of our WorkManager series. WorkManager is an Android Jetpack library that makes it easy to schedule deferrable, asynchronous tasks that must be run reliably. It is the current best practice for most background work on Android.
If you’re been following thus far, we’ve talked about:

In this blog post, I’ll cover:

  • defining periodic work
  • cancelling work
  • customizing WorkManager configuration

Repeating work

In a previous blog, we’ve seen that you can schedule work using a OneTimeWorkRequest but if you want that your work is repeated periodically, you can use a PeriodicWorkRequest.

First, let’s see what are the differences between these two types of WorkRequest:

  • Minimum period length is 15 minutes (same as JobScheduler)
  • Worker classes cannot be chained in a PeriodicWorkRequest
  • Before v2.1-alpha02 it’s not possible to create a PeriodicWorkRequest with an initial delay.

Some of the most common questions I get at conferences are around periodic work. In this article I’ll cover the basics of periodic work, some common use cases and some common errors. Plus we will cover a couple of ways to write tests for your Worker classes.

API

The call to build a periodic work request is not very different from the one we’ve seen for the one time work. We have an additional parameter that specifies the minimum repeat interval:

val work = PeriodicWorkRequestBuilder<MyWorker>(1, TimeUnit.HOURS)
.build()

It’s called the minimum interval because of Android’s battery optimizations and because you can have constraints that extend the time between repetitions. For example, if you specify that the Work only runs when the device is charging, even if your minimum interval passes, if the device is not charging, this Work will not run until the device is plugged in.

PeriodicWorkRequest with a charging constraints

In this case, we are adding a charging constraint to the PeriodicWorkRequest and we are enqueueing it:

val constraints = Constraints.Builder()
.setRequiresCharging(true)
.build()

A note regarding how to retrieve the WorkManager instance.

WorkManager v2.1 has deprecated WorkManager#getInstance() and there’s now a new WorkManager#getInstance(context: Context) that works in the same way but supports the new on-demand initialization. In this article I’m going to use this new syntax that expects that we pass a context to retrieve the WorkManager instance.

A reminder about the “minimal interval”. WorkManager is balancing two different requirements: the application with its WorkRequest, and the Android operating system with its need to limit battery consumption. For this reason, even if all the constraints set on a WorkRequest are satisfied, your Work can still be run with some additional delay.

Android includes a set of battery optimization strategies: when a user is not using the device, the OS minimizes the activities to preserve the battery. This has the effect that running a Work can be delayed to the next maintenance window if your device is in Doze mode.

Interval and FlexInterval

As we saw, WorkManager is not about executing at exact intervals. If that is your requirement, you’re looking at the wrong API. Given that the repetition interval is in reality a minimum interval, WorkManager makes available an additional parameter that you can use to specify a window where Android can execute your Work.

In a nutshell, you can specify a second interval that controls when your periodic Worker will be allowed to run inside a portion of the repetition period. This second interval (the flexInterval) it’s positioned at the end of the repetition interval itself.

Let’s look at an example. Imagine you want to build a periodic Work request with a 30 minutes period. You can specify a flexInterval, smaller than this period, say a 15 minute flexInterval.

The actual code to build a PeriodicWorkPequest with this parameters is:

val logBuilder = PeriodicWorkRequestBuilder<MyWorker>(
30, TimeUnit.MINUTES,
15, TimeUnit.MINUTES)

The result is that our worker will be executed in the second half of the period (the flexInterval is always positioned at the end of the repetition period):

PeriodicWorkRequest with a 30' Interval and a 15' flexInterval

Keep in mind that these timings are always dependant on the constraints included in the work request and on the status of the device.

To learn more about this feature you can read the PeriodicWorkRequest.Builder documentation.

Daily Jobs

Because periodic intervals are not precise, you cannot build a periodic work request that is executed each day at a specific time. Not even if we relax the precision.

You can specify a 24 hours period, but because the work is executed respecting Android’s battery optimization strategies, you can only expect your Worker to be executed around that time. You can then have an execution at 5:00AM the first day, 5:25AM the second day, 5:15AM the third, then 5:30AM the following one and so on. With the error compThis is my new defaultounding over time.

The possibility to have a daily job run at around a specified time, is sometimes requested by developers coming from the Evernote Android-Job library.

At the moment, if you need to execute a worker at roughly the same time, every day, your best option is to use a OneTimeWorkRequest with an initial delay so that you execute it at the right time:

This is my new defaultval currentDate = Calendar.getInstance()
val dueDate = Calendar.getInstance()

This will take care of the first execution. We need then to enqueue the next execution of this work when we complete successfully:

class DailyWorker(ctx: Context, params: WorkerParameters) : Worker(ctx, params) {

Please, remember that the exact time that the worker is going to be executed depends on the constraints that you’re using in your work request and on the optimizations done by the Android platforms.

Status for Periodic Work

We saw that one of the differences that periodic work has when compared with one time work is that is not possible to build chains of work with a periodic work request. The reason for this constraint is that in a chain of work you transition from one worker to the next one in the chain when a worker transitions to the SUCCEEDED status.

PeriodicWorkRequest states

Periodic work never ends up in a SUCCEEDED status; it keeps running until it is cancelled. When you call Result#success() or Result#failure() from a periodic worker, it transitions back to the ENQUEUED status waiting for the next execution.

For this reason, when you work with periodic work, you cannot build chains, not even with a unique work request. In this case, periodic work requests lose the capability to append work: you can only use KEEP and REPLACE, but not APPEND.

Data in and out

WorkManager lets you pass a Data object into your worker and return a new Data object with your success or failure call (there’s no data out option when you return a Result#retry() simply because the worker execution is stateless).

In a chain of one time workers, what is returned as output from one worker becomes the input of the next worker in the chain. As we’ve seen, periodic work cannot be used in work chain because it really never “successfully” finishes; it can only ends with a cancellation.

So, where can we see and use the data returned with a Result#success(outData)?

We can observe these Data through the WorkInfo of the PeriodicWorkRequest, but only until the next time the periodic Work is executed and we can only rely on the Worker to be in a ENQUEUED status to check it’s output:

val myPeriodicWorkRequest =
PeriodicWorkRequestBuilder<MyPeriodicWorker>(1, TimeUnit.HOURS).build()

This may not be your best option if you need to provide some results from your periodic Worker. A better option is to pass the data through another medium like a table in a database.

For more information on how to retrieve the status of your Work you can refer to the second blog post in this series and to the WorkManager documentation: Work States and observing work.

Unique Work

Some WorkManager use cases can fall into a pattern where the application enqueues some work as soon as it starts. This can be a background sync task that you want to run periodically, or a scheduled content download. Whatever it is, the common pattern is that it needs to be enqueued as soon as your application starts.

I’ve seen this pattern surfacing few times, in your Application#onCreate, the developer creates a work request and enqueues it. Everything is fine until you find that the same work is running twice or more times. This is especially relevant for periodic work, that never reaches a final state, if there are no cancellations.

We like to say that WorkManager guarantees the execution of your work even if your application is closed or the device is restarted. So, enqueueing your worker at each start of your application cause to add a new WorkRequest each time. If you’re using a OneTimeWorkRequest, it’s probably not a big deal. Once the work is run to completion, it’s done. But for periodic Work, “done” is a different concept, and you can easily end up with multiple periodic work request being enqueued.

The solution in this case is to enqueue your WorkRequest as unique Work using the WorkManager#enqueueUniquePeriodicWork():

class MyApplication: Application() {

This keeps you from enqueuing your work multiple times.

KEEP or REPLACE?

Which of these policies you choose is really dependent on what you’re doing in your workers. Personally, I default to the KEEP policy as it is more lightweight, not having to replace an existing WorkRequest. It also avoids the possible cancellation of an already running worker.

I switch to REPLACE policy only if there’s a good reason for it (for example, if I want to reschedule a Worker from its doWork() method).

If you choose a REPLACE policy, your Worker should gracefully handle stoppages because WorkManager may have to cancel a running instance if a new WorkRequest is enqueued while the Worker is running. But you should do this anyway, given that WorkManager may stop your work if a constraint is not met anymore during the Worker’s execution.

For more information on unique work, you can refer to the documentation: Unique work.

Testing Periodic Work

WorkManager’s documentation on testing is quite comprehensive and covers the basic scenarios. After WorkManager v2.1 release there are now two ways to test your workers:

Using WorkManagerTestInitHelper you can test your Worker classes simulating delays, meeting the constraints and the period requirements. The advantage of this way of testing your Workers is that it can handle the situation where a Worker enqueues itself or another Worker class (like in the sample we saw above where we implemented a “DailyWorker” that runs at roughly the same time every day. To know more about this, you can read WorkManager’s Testing documentation.

If you need to test a CoroutineWorker, RxWorker or ListenableWorker, using WorkManagerTestInitHelper has some additional complexity because you cannot rely on its SynchronousExecutor.

To make testing these classes more straightforward, WorkManager v2.1 includes a set of new WorkRequest builder:

  • TestWorkerBuilder to invoke directly a Worker class
  • TestListenableWorkerBuilder to invoke directly a ListenableWorker, RxWorker or CoroutineWorker

These have the advantage that you can test any kind of Worker classes because in this case you’re running it directly.

You can read more about these in the documentation and you can see an example of a test using these new builders in the Sunflower sample application:

import android.content.Context
import androidx.test.core.app.ApplicationProvider
import androidx.work.ListenableWorker.Result
import androidx.work.WorkManager
import androidx.work.testing.TestListenableWorkerBuilder
import com.google.samples.apps.sunflower.workers.SeedDatabaseWorker
import org.hamcrest.CoreMatchers.`is`
import org.junit.Assert.assertThat
import org.junit.Before
import org.junit.Test
import org.junit.runner.RunWith
import org.junit.runners.JUnit4

Conclusion

I hope that you find this article useful and I’d love to hear about how you’re using WorkManager or what WorkManager’s feature can be better explained or documented.

You can reach me on Twitter @pfmaggi.

WorkManager’s Resources

Android Developers

The official Android Developers publication on Medium

Pietro Maggi

Written by

Android Developer Advocate @Google

Android Developers

The official Android Developers publication on Medium

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