public class ActivationEventConsumer {

    private Logger log = LoggerFactory.getLogger(ActivationEventConsumer.class);

    @Inject
    public ActivationEventConsumer(Config cfg, MailService mailService, UserService service) {
        Util.setup(cfg);
        final DecodedJWT tok = Util.decode(cfg.getString("jwt.clienttoken"));
        service.activationEvents()
            .subscribe()
            .withGroupId("auth-ActivationEventConsumer")
            .atLeastOnce(Flow.<UserEvent>create()
            .map(this::toMail)
            .mapAsync(1, optMail -> {
                if (optMail.isPresent()) {
                    return mailService
                        .send()
                        .handleRequestHeader(authenticate(tok))
                        .invoke(optMail.get())
                        .thenApply(r -> {
                            if (!r.messageId.isPresent()) {
                                log.error("Mail could not be sent to "+optMail.get().to+" -- Reason: "+r.errorMessage.get());
                            }
                            return Done.getInstance();
                        });
                } else {
                    return CompletableFuture.completedFuture(Done.getInstance());
                }
            })
        );
    }

    private Optional<Mail> toMail(UserEvent event) {
        if (event instanceof UserEvent.NewActivationCode) {
            UserEvent.NewActivationCode ev = (UserEvent.NewActivationCode) event;
            log.info("Sending Activation-Mail for user "+ev.uuid.toString());
            return Optional.of(toActCodeMailing(ev));
        } else {
            return Optional.empty();
        }
    }

    //TODO: Replace with a template.
    private Mail toActCodeMailing(UserEvent.NewActivationCode e) {
        StringBuilder body = new StringBuilder();
        body.append("Hello ");
        body.append(e.firstname);
        body.append(",\n\nwelcome to our System! To activate your account please click on the following link:");
        body.append("\nhttp://blahblah");
        body.append("\n\nAlternatively, use the following Activation-Code: ");
        body.append(e.activationCode);
        body.append(" to enable your Account.\n\n");
        body.append("Your M2M Team");
        return new Mail("test@test.com", e.mail, "Account Activation", body.toString());
    }