starting and stopping services?

[Kevin Burton]

I have a bunch of services that need to be started and stopped.  ActiveMQ , Cassandra, Jetty, etc... 

They use files, start ports, etc.

I imagine the best strategy to just have a leif Node service just start its dependencies in its start() method.. then stop them.

So the could would look something like the following...

but it would be nice if the start/stop methods were auto-wrapped ...

This way on shutdown all threads and resources can be properly released.

This is also helpful in testing so that if you have an impl that creates threads they can be released.

public class Main {

    public static void main( String... args ) {

      MyService service = injector.getInstance( MyService.class )

    }

}

public class MyService {

    @Inject

    MyService( WebserverService webserverService ) {

        ...

    }

    

    public void start() {

        webserverService.start();

    }

    public void stop() {

        webserverService.stop();

    }

}

public class Webserver {

    @Inject

    MyService( CassandraService cassandraService ) {

        ...

    }

    public void start() {

        cassandraService.start();

    }

    public void stop() {

        cassandraService.stop();

    }

    

}

[Sam Berlin]

One decent approach is to use Guava's Service & ServiceManager, then bind your services using a multibinder and @Provides a ServiceManager constructed with the result of that multibinder.  Then you can just start/stop the ServiceManager, which will start/stop each service.

sam

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[Nate Bauernfeind]

I've spoken about my approach to this elsewhere, apologies if you've already come across this. I use Dropwizard which has a concept of Managed classes which get started and stopped by dropwizard. Though, you could do something like this yourself too if you are opposed to dropwizard.

My abstract module looks like this (please excuse the Scala; it simplifies some of the verbosity otherwise):

trait DropwizardModule[B <: Binder] extends Module {
  self: InternalModule[B] =>
 
  private[this] var injectorProvider: Provider[Injector] = null
 
  final def configure() {
    binderAccess.requireExplicitBindings()
    injectorProvider = getProvider[Injector]
    doConfigure()
  }
 
  protected[this] def injector = injectorProvider.get()
 
  def doConfigure()
  def install(env: Environment)
}
 
abstract class DropwizardPublicModule extends ScalaModule with DropwizardModule[Binder]
 
abstract class DropwizardPrivateModule extends ScalaPrivateModule with DropwizardModule[PrivateBinder]

Note how I always create a reference to a provider of the module's injector; this makes working with PrivateModule's a breeze as you will always get the local module and you don't need to expose things that simply get registered with dropwizard/jersey (or started/stopped by your code).

Then a very simple Module might look like this:

class TestModule extends DropwizardPrivateModule {
  def doConfigure() {
    bind[TestResource].asEagerSingleton()
    bind[TestService].asEagerSingleton()
  }
 
  def install(env: Environment) {
    env.addResource(injector.instance[TestResource])
    env.manage(injector.instance[TestService])
  }
}

Note: You could easily have your install method pass in an array list of Managed objects if you wanted to start / stop all on your own (and not use dropwizard).

And then in my main Service's run method I have code that looks like this:

  def run(cfg: TestApplicationConfiguration, env: Environment) {
    val modules: List[DropwizardModule[_]] = List(
      new TestModule
    )
 
    val injector = Guice.createInjector(modules: _*)
    modules.foreach(_.install(env))
  }

I keep reusing this approach over and over and have really found it to work extremely well. It allows you to delegate more responsibilities to each module; In addition to configuring some specific sub-graph of your application it's now responsible for starting, stopping, installing, registering whatever it needs in your applications framework. Most importantly, it enables a bit more freedom when you need to mix-n-match modules more freely without the hassle of dealing with updating your main method to comment/uncomment start()/stop() calls.

This is more or less similar to the multi-binder approach but also works across private modules and is more flexible for dropwizard (and other frameworks where you might want to do more than just start/stop instances in your injector).

Anyways, that's my food for thought.

Nate

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[Kevin Burton]

OK... getting closer to this.

My issue now is that most of my objects are requiring custom initialization.

For example, with cassandra I have to read the config in, create a cluster builder, set the options for the connection, then build a new session and cluster object.

I could use @Provides methods I think.. BUT then I don't get service start and stop.  Because I'd need a way to stop them.

What I was thinking of doing, was having a harness like a service manager to start all my services.

Then I would have this return a new binding... which I can use to create a new Injector which can inject all my newly created objects.

I just can't figure that part out yet. 

what I need is the ability to bind a class to an already instantiated object.. since I've just instantiated it.

Maybe one way to do it is to make each Service a Module and @Provides a member... 

Or make my own custom binding that has a bunch of @Provides and then I can just have it return the newly created object.  And I would then just use a bunch of bindings.. 

Actually.. I think that would work fine.

So every object I want to inject could be done by adding a new binding like?

class ProvideByObject<T> extends AbstractModule {

    private T value;

    ProvideByObject(T value) {

        this.value = value;

    }

    @Override

    protected void configure() {

    }

    

    @Provides 

    T getValue() {

        return value;

    }

    

}

[Kevin Burton]

Hm.. only that won't work... I guess this is due to type erasure? fun!

com.google.inject.CreationException: Unable to create injector, see the following errors:

1) T cannot be used as a key; It is not fully specified.

[Martin Grajcar]

On Fri, Oct 3, 2014 at 8:26 AM, Kevin Burton <burto...@gmail.com> wrote:

Hm.. only that won't work... I guess this is due to type erasure? fun!

com.google.inject.CreationException: Unable to create injector, see the following errors:

1) T cannot be used as a key; It is not fully specified.

If @Provides T getValue()  could work, then it'd provide everything, or wouldn't it?

I might be completely wrong, but why don't you use com.google.common.util.concurrent?

- wrap every service as a subclass of AbstractService (just 2 simple methods)

- write a class requiring all of them

- and passing them to a ServiceManager

- call ServiceManager#startAsync to start them all

- call ServiceManager#stopAsync to stop them all

- do whatever else it offers

[Sam Berlin]

As mentioned earlier, doing this is as simple as:

// In each module that has a service:

Multibinder<Service> multibinder = Multibinder.newSetBinder(binder(), Service.class);

multibinder.addBinding().to(MyService.class);

// In one main module:

@Provides ServiceManager provideServiceManager(Set<Service> services) {

   return new ServiceManager(services);

}

// And in your app:

@Inject ServiceManager serviceManager;

void start() {

   serviceManager.startAsync().awaitHealthy();

}

sam

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