Implementing a new service

To demonstrate implementing a new service, this guide details implementing a new logging service. In most applications, it may be more desirable to utilise a persistent storage medium as opposed to an in-memory one. This tutorial details implementing a simple service that will achieve this functionality.

Services in UNDERSEA implement the AgentService interface and in turn, this interface extends from AgentAware. While creating a new service, the initialisation process may require a reference to the associated agent instance, however, this has not been fully constructed yet. The AgentAware interface solves this by moving initialisation process of a service to the initialise method that must be implemented and this provides a reference to the agent.

Lets start by creating a new DiskLogService that contains a BufferedWriter

public class DiskLogService implements LogService {

    private Agent agent;
    private BufferedWriter writer;

    @Override
    public void initialise(Agent parentAgent) {
        this.agent = parentAgent;
        try {
            this.writer = new BufferedWriter(new FileWriter("log-" + agent.name() + ".txt", true));
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

...

The initialise method is called by the ServiceManager during starting a service before calling the run method on the service. This ensures that every service is fully initialise beforehand.

Now we can implement the (very simple) add method so log entries are written:

@Override
public void add(LogEntry logEntry) {
    try {
        writer.write(logEntry.toString());
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}

The add method can be invoked directly on the instance of the DiskLogService or, normally, by acting directly on the instance of an agent through agent.log(logEntry). To register the new logging service with the ServiceManager:

serviceManager.registerService(new DiskLogService());