Task runner is a Scala library that provides an abstraction for running asynchronous tasks. Asynchronous tasks are useful in web programming, because it allows us to quickly respond with a succesful status to a users request, even though performing all the work the user requested may take a long time.
This library intends to solve the problems of retrying and persisting asynchronous tasks, that pop up when doing work asynchronously.
In your build.sbt:
resolvers += "Lunatech Releases" at "http://artifactory.lunatech.com/artifactory/releases-public/"
libraryDependencies += "com.lunatech.task-runner" %% "task-runner-core" % "0.3"
If you want the MySQL Squeryl task store, you can use:
libraryDependencies += "com.lunatech.task-runner" %% "task-runner-squeryl-mysql-store" % "0.3"
Suppose that we have a webservice, where people can register a user account. Upon registration, the webservice will do a request to an system to store the customer data.
At some point in time, our system should return a success status back to the client, indicating that the data was received. After giving this success status, our webservice is responsible for the data, until it succesfully sends it to the external system and retrieves a success status from that system.
The easiest way to solve this, is to make our client wait until our webservice registered the data at the external service. That means our service gets a success status from the external service before it sends one to the client. That's great, because it means that the data is never the responsibility of our service! Awesome!
One disadvantage of this approach that the speed with which our service can give a confirmation to the user is limited by how fast it can get one from the remote service.
Another one is that when the external system is unavailable, our webservice is effectively unavailable as well. If we can't get a confirmation, we won't give one to our client.
Often, we want the availability or performance of our service not to be limited by the availability or performance of the external service.
So what we can do is to take responsibility of the data, by sending a success status to our client as soon as we've received the data. Then, asynchronously, we'll send it to the external service.
This introduces two needs into our system:
- retrying: When the external service is down, we need to retry sending our data to it at a later point in time.
- persistence: Now that we're responsible for the data, we don't want it to be gone if our system is shut down or crashes. Often, we're trying to keep our service stateless, which it's not anymore if there's tasks hanging around in memory.
This library helps to deal with these needs by introducing an abstraction for running asynchronous tasks:
trait TaskRunner[A] {
def runTask(task: A, delay: Option[FiniteDuration]): Future[Unit]
}
It also provides two implementations of this trait. A BasicTaskRunner, which is capable of retrying, but doesn't do persistence. And a PersistentTaskRunner, which is capable of retrying and persisting.
This task runner can retry tasks, but doesn't persist them. They're gone if the application is stopped.
This task runner can run any sort of task, as long as it has the Task typeclass implemented:
trait Task[A] {
def run(a: A): Future[Unit]
}
The persistent task runner can retry tasks, and persists them in a pluggable storage backend.
This adds some additional constraints on the type of tasks that can be run, because the system must be able to persist and restore them.
To persist tasks, a PersistentTaskRunner uses a TaskStore, an abstraction for storing tasks. Naturally, this TaskStore must be able to serialize a task somehow and be able to restore it.
TODO, write about RetryStrategy