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Getting background tasks to play nicely with deployments

This article discusses the shutdown of background tasks during a rolling deployment.

The backstory

Early on, when our team was still small, we would occasionally run into the following error in production:

java.lang.IllegalStateException: ServiceLocatorImpl(__HK2_Generated_0,0,1879131528) has been shut down 
at org.jvnet.hk2.internal.ServiceLocatorImpl.checkState(ServiceLocatorImpl.java:2288) 
at org.jvnet.hk2.internal.ServiceLocatorImpl.getServiceHandleImpl(ServiceLocatorImpl.java:629) 
at org.jvnet.hk2.internal.ServiceLocatorImpl.getServiceHandle(ServiceLocatorImpl.java:622) 
at org.jvnet.hk2.internal.ServiceLocatorImpl.getServiceHandle(ServiceLocatorImpl.java:640) 
at org.jvnet.hk2.internal.FactoryCreator.getFactoryHandle(FactoryCreator.java:103) 
... 59 common frames omitted 
Wrapped by: org.glassfish.hk2.api.MultiException: A MultiException has 1 exceptions. They are:

We quickly realized that this error was due to a long-running job that was in the middle of its run during deployment. A service would shut down, the DI framework would kill various components and the job would end up trying to use a decomissioned instance of a class.

Since the team was still small and everyone in the (single) room was aware of the deployment, solving this was put on the back burner – a known issue that was under our control. We had bigger fish to free *.

Eventually, the combination of team size growth and the move to continuous deployment meant that we could no longer afford to ignore this error. An engineer on shift who is no longer familiar with the entire system cannot be relied on to “just know” that this is an “acceptable” error (I would add several more quotes around “acceptable” if I could). The deploys also reached a pace of dozens every day, so we were much more likely to run into the error.

Let’s dive into some more detail about the issue we faced.

The graceful shutdown problem

During deployment, incoming external connections are handled by our infrastructure. Connections are drained using the load balancer to route connections and wait for existing connections to close.

Scheduled tasks are not triggered externally and are not handled by the infrastructure. They require a custom applicative solution in order to shut down gracefully.

The setting

The task we’re talking about is a background task, triggered by Quartz and running in Dropwizard with an infrastructure of AWS Elastic Beanstalk (EBS) that manages deployments. Our backend code is written in Kotlin, but is simple enough to understand even if you are not familiar with the language.

Rolling deployments

Rolling (ramped) deploys are a deployment method where we gradually take a group of servers that run a service offline, deploy a new version, wait for the server to come back online and move on to the next server.

This method allows us to maintain high availability while deploying a new version across our servers.

In AWS’ Elastic Beanstalk (EBS), you can configure a service to perform a rolling deploy on the EC2 instances that run a service. The load balancer will stop routing traffic to an EC2 instance and wait for open connections to close. After all connections have closed (or when we reach a max timeout period), the instance will be restarted.

The load balancer does not have any knowledge of what’s going on with our service beyond the open connections and whether the service is responding to health checks.

Shutting down our background tasks

Let’s divide what needs to be done into the following steps:

  1. Prevent new background jobs from launching.
  2. Signal long running jobs and let them know that we are about to shut down.
  3. Wait for all jobs to complete, up to a maximum waiting period.

Getting the components to talk to each other

The first challenge that we come across is triggering the shutdown sequence. EBS is not aware of the EC2 internals, and certainly not of Quartz jobs running in the JVM. All it does, unless we intervene, is send an immediate shutdown signal.

We need to create a link between EBS and Quartz, to let Quartz know that it needs to shut down. This needs to be done ahead of time, not at the point at which we destroy the instance.

Fortunately, we can use Dropwizard’s admin capabilities for this purpose. Dropwizard enables us to define tasks that are mounted via the admin path by inheriting the abstract class Task. Let’s look at what it does:

class ShutdownTask(private val scheduler: Scheduler) : Task("shutdown") {
 
  override fun execute(parameters: Map<String, List<String>>, output: PrintWriter) {
    // Stop triggering new jobs before shutdown
    scheduler.standby()

    // Wait until all jobs are done up to a maximum of time
    // This is in order to prevent immediate shutdown that may occur if there are no open connections to the server
    val startTime = System.currentTimeMillis()
    while (scheduler.currentlyExecutingJobs.size > 0 && System.currentTimeMillis() - startTime < MAX_WAIT_MS) {
      sleep(1000)
    }
  }
}

Some notes about the code:

  1. The task receives a reference to the Quartz scheduler in its constructor. This allows it to call the standby method in order to stop the launch of new jobs
  2. We call standby, not shutdown, so that jobs that are running will be able to complete their run and save their state in the Quartz tables. shutdown would close the connection to those tables.
  3. We wait up to MAX_WAIT_MS before continuing. If there are no running jobs, we continue immediately.
  4. EBS does not have a minimum time window during which it stops traffic to the instance. If there are no open connections to the process, EBS will trigger a shutdown immediately. This is why we need to check for running jobs and wait for them, not just call the standby method and move on.

Given a reference to Dropwizard’s environment on startup, we can call

environment.admin().addTask(ShutdownTask(scheduler))

to initialize the task. Once added to the environment, we can trigger the task via a POST call to 

http://<server>:<adminport>/tasks/shutdown

The last thing we need to do is add this call to EBS’ deployment. EBS provides hooks for the deploy lifecycle – pre, enact and post. In our case we will add it to the pre stage. In the .ebextensions directory, we’ll add the following definition in a 00-shutdown.config file:

files:
  "/opt/elasticbeanstalk/hooks/appdeploy/pre/00_shutdown.sh":
    mode: "000755"
    owner: root
    group: root
    content: |
      #! /bin/bash
      echo "Stopping Quartz jobs"

      curl -s -S -X POST http://<server>:<adminport>/tasks/shutdown

      echo "Quartz shutdown completed, moving on with deployment."

So far we have accomplished steps #1 & #3 – Preventing new jobs from launching and waiting for running jobs to complete. However, if there are long running jobs that take more than MAX_WAIT_MS to complete, we will reach the time out and they will still terminate unexpectedly.

Signaling to running jobs

We put a timeout of MAX_WAIT_MS to protect our deployment from hanging due to a job that is not about to terminate (we could also put a -max-time on the curl command).

Many long running jobs are batch jobs that process a collection of items. In this step, we would also like to give them a chance to terminate in a controlled fashion – Give them a chance to record their state, notify other components of their termination or perform other necessary cleanups.

Let’s give these jobs this ability.

We’ll start with a standard Quartz job that has no knowledge the state of the system:

class SomeJob: Job {
  override fun execute(context: JobExecutionContext) {
    repeat(1000) { runner ->
      println("Iteration $runner - Doing something that takes time")
      sleep(1000)
    }
  }
}

We want to be able to stop the job. Let’s define an interface that will let us do that:

interface StoppableJob {
  fun stopJob()
}

Now, let’s implement the interface:

class SomeStoppableJob: Job, StoppableJob {

  @Volatile
  private var isActive: Boolean = true

  override fun execute(context: JobExecutionContext) {
    repeat(1000) { runner ->
      if (!isActive)  {
        println("Job has been stopped, stopping on iteration $runner")
        return
      }

      println("Iteration $runner - Doing something that takes time")
      sleep(1000)
    }
  }

  override fun stopJob() {
    isActive = false
  }
}

This setup allows us to pause execution. Notice that our flag needs to be volatile. Now we can modify ShutdownTask to halt the execution of all running tasks:

class ShutdownTask(private val scheduler: Scheduler) : Task("shutdown") {
 
  override fun execute(parameters: Map<String, List<String>>, output: PrintWriter) {
    // Stop triggering new jobs before shutdown
    scheduler.standby()

    scheduler.currentlyExecutingJobs
        .map { it.jobInstance }
        .filterIsInstance(StoppableJob::class.java)
        .forEach {
          it.stopJob()
        }

    // Wait until all jobs are done up to a maximum of time
    // This is in order to prevent immediate shutdown that may occur if there are no open connections to the server
    val startTime = System.currentTimeMillis()
    while (scheduler.currentlyExecutingJobs.size > 0 && System.currentTimeMillis() - startTime < MAX_WAIT_MS) {
      sleep(1000)
    }
  }
}

Conclusion

Looking at the implementation, we now have the ability to control long-running tasks and have them complete without incident.

It reduces noise in production that would otherwise send engineers scrambling to see whether there is a real problem, or just a deployment artefact. This enabled us to focus on real issues and improve the quality of our shifts.

Overall, well worth the development effort.

[update] For a discussion on how to implement shutdown flags in Kotlin, see https://galler.dev/kotlin-code-styles-for-stopping-loop-iterations/