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Automation

Do things in the background


Reading Time: 7 min

In the previous chapters, you learned ways to improve your project. Can it still be improved? Perhaps. Let's introduce some automation. For that, let's discuss:

  • Schedulable events.
  • Services.

Schedulable events

The SalesProposal now has an expiration date. And the seller, if they want to, can reject the offer, but only after the expiration date. What if the offer expires on Sunday at 3AM? Do you get up or do you have your node do it for you?

In Corda, achieving this schedule automation is simply a matter of a few lines of code. You already have a flow that can reject a proposal. More generally, if the flow has a handler, you need to pick an @Initiating one. After that, it is only a matter of making the link:

  1. Have the state implement SchedulableState.
  2. Implement the public ScheduledActivity nextScheduledActivity function by returning proper information to run the reject flow of your choice, at the time of your choice.
  3. Annotate your chosen reject flow with @SchedulableFlow.

And voila. Do this micro exercise on your own before looking at a solution in the next chapter.

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If you have a hard time with the tests, have a look at running the mock network, and in particular network.waitQuiescent().

Services

Services are single instance classes that are loaded on startup and run on the node, in the background. You already came across one, when dealing with accounts: AccountService, or rather KeyManagementBackedAccountService. Predictably, schedulable events work thanks to a NodeSchedulerService.

Services can be used for multiple purposes, for instance:

  • To start flows. You can track a certain state type (remember trackBy?), then initiate a flow when you get updates, like in this automatic payment example. Don't forget to annotate your flow with @StartableByService.

  • Connect to the node's database. You can use the node's JDBC connection to create custom tables and do CRUD (Create, Read, Update, and Delete) operations on those tables as in this example. Note its use of the widely copied DatabaseService.

    As a side note, it's much easier to use JPA (Java Persistence API) to do CRUD operations on your custom entities instead of writing SQLs (see JPA support here).

  • Query the vault. The service has access to AppServiceHub, giving you access to many operations like querying the vault.

  • Implement an Oracle. More on that in the next chapter.

What do you need to declare a service? Simple. A Corda service:

  1. Should have the @CordaService annotation. This signals to the node that it should initialize it on startup.
  2. Should extend the abstract SingletonSerializeAsToken. Services, and large objects in general, shouldn't be serialized when a flow is check-pointed. Instead, a token that references the running service is serialized and is used to link back to the object, i.e. service, when the flow resumes. See the detailed explanation here.
  3. Should have a constructor that takes a single parameter of type AppServiceHub. Having this service-hub, grants the service access to privileged operations (e.g. start a flow).

Aside from those requirements, what the service does is up to you.

Service exercise

Recall that a SalesProposal Offer transaction has 2 reference states:

  • The StateAndRef<CarTokenType> instance.
  • The StateAndRef<NonFungibleToken> instance.

And, both have to be unconsumed for the Offer transaction to be notarized successfully. Similarly, an Accept transaction consumes the NonFungibleToken but also has 1 reference state:

  • The StateAndRef<CarTokenType> instance.

Do you see a problem here? It's a minor issue, admittedly. The seller is in control of the NonFungibleToken, but the CarTokenType is controlled by its maintainers, i.e. the DMV.

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If the DMV updates the car mileage, a new StateAndRef<CarTokenType> instance is created and the previous one is consumed. Therefore, the buyer can no longer notarize an Accept transaction.

So, the buyer would be in breach of the terms of the offer through no fault of their own. Perhaps it is a rare issue, but not so minor after all for participants who get caught in this scenario. The first thought is to, perhaps, have the Accept flow ask the seller to send the latest StateAndRef<CarTokenType>. Surely, it will avoid a notary exception. The problem with this solution, given it is all automated, is that it does not give the opportunity to the seller to reconsider the offer in light of new information.

Fortunately, there are services and services can help resolve this situation.

You will remedy this situation with the help of a ProposalService that, in short, proactively pushes new information to the potential buyer:

  • Tracks new instances of SalesProposal.
  • Extracts the StateAndRef<CarTokenType> instances out of these proposals.
  • In turn tracks updates on CarTokenType.
  • When such an offered StateAndRef<CarTokenType> has been consumed, it promptly informs the buyer of this.
  • Stops tracking when proposals are consumed.

With this service, the buyer is assured of always having the latest CarTokenType in their vault, the latest updated facts, such that should they Accept, their transaction will indeed notarize. After all, when the mileage has increased, perhaps the offer is not as interesting as it was initially and possibly a Reject is in order.

Go!

You will find an example solution in the next chapter.

Service Lifecycle Observer

Let's digress a bit here with a peek ahead.

Corda 4.4 introduced a new feature that allows your service to listen to node lifecycle events, such as state machine started or before node stop, and execute an action when that event is dispatched. You can also give priorities to your lifecycle observers so certain actions execute before others.

As an example, the code below does the following:

  1. Create an observer.

    class MyServiceLifecycleObserver implements ServiceLifecycleObserver
  2. Listen to the STATE_MACHINE_STARTED event. That is meaningful because once that event is dispatched, it means that AppServiceHub is available for your service to start flows and query the vault.

    @Override
    public void onServiceLifecycleEvent(@NotNull ServiceLifecycleEvent event)
    	throws CordaServiceCriticalFailureException {
    
    	// This event is dispatched when the State Machine is fully started, and
    	// AppServiceHub becomes available for use.
    	if (event == ServiceLifecycleEvent.STATE_MACHINE_STARTED) {
  3. Register the observer in the constructor of the service, so it starts listening to events when the service is instantiated.

    appServiceHub.register(1000, new MyServiceLifecycleObserver());
  4. Give the observer priority 1000, i.e. high. You might create another observer in this service (or another one) with a higher (e.g. 1001) or lower (e.g. 999) priority depending on which action (e.g. start a flow) should happen first. Observers with higher priority are started ahead.

    appServiceHub.register(1000, new MyServiceLifecycleObserver());

Here's the full code:

@CordaService
public class MyService extends SingletonSerializeAsToken {

	private final AppServiceHub appServiceHub;

	public MyService(AppServiceHub appServiceHub) {
		this.appServiceHub = appServiceHub;

		// Listen to node lifecycle events and execute actions.
		appServiceHub.register(1000, new MyServiceLifecycleObserver());
	}

	class MyServiceLifecycleObserver implements ServiceLifecycleObserver {

		@Override
		public void onServiceLifecycleEvent(@NotNull ServiceLifecycleEvent event)
			throws CordaServiceCriticalFailureException {

			// This event is dispatched when the State Machine is fully started, and
			// AppServiceHub becomes available for use.
			if (event == ServiceLifecycleEvent.STATE_MACHINE_STARTED) {
				// Query the vault.
				appServiceHub.getVaultService().queryBy();
				// Start a flow.
				appServiceHub.startFlow();
			}
		}
	}
}
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