Adding a new custom module to the Spring Boot application is not so complex. To demonstrate this feature, assume that if a service gets more than two transactions in a minute, then the server status will be set as Out of Service.

In order to customize this, we have to implement the HealthIndicator interface and override the health method. The following is a quick and dirty implementation to do the job:

class TPSCounter {
  LongAdder count;
  int threshold = 2;
  Calendar expiry = null; 

  TPSCounter(){
    this.count = new LongAdder();
    this.expiry = Calendar.getInstance();
    this.expiry.add(Calendar.MINUTE, 1);
  }
  
  boolean isExpired(){
    return Calendar.getInstance().after(expiry);
  }
  
  boolean isWeak(){
    return (count.intValue() > threshold);
  }
  
  void increment(){
     count.increment();
  }
}

The preceding class is a simple POJO class that maintains the transaction counts in the window. The isWeak method checks whether the transaction in a particular window reached its threshold. The isExpired method checks whether the current window is expired or not. The increment method simply increases the counter value.

For the next step, implement our custom health indicator class, TPSHealth. This is done by extending HealthIndicator, as follows:

@Component
class TPSHealth implements HealthIndicator {
  TPSCounter counter;

@Override
    public Health health() {
        boolean health = counter.isWeak(); // perform some specific health check
        if (health) {
            return Health.outOfService().withDetail("Too many requests", "OutofService").build();
        }
        return Health.up().build();
    }
    
    void updateTx(){
    if(counter == null || counter.isExpired()){
      counter = new TPSCounter();
      
    }
    counter.increment();
    }
}

The health method checks whether the counter is weak or not. A weak counter means the service is handling more transactions than it can handle. If it is weak, it marks the instance as Out of Service.

Finally, we will autowire TPSHealth into the GreetingController class and then call health.updateTx() in the greet method, as follows:

  Greet greet(){
    logger.info("Serving Request....!!!");
    health.updateTx(); 
    return new Greet("Hello World!");
  }

Go to the /health end point in the HAL browser and take a look at the status of the server.

Now, open another browser, point to http://localhost:8080, and fire the service twice or thrice. Go back to the /health endpoint and refresh to see the status. It should be changed to Out of Service.

In this example, as there is no action taken other than collecting the health status, even though the status is Out of Service, new service calls will still go through. However, in the real world, a program should read the /health endpoint and block further requests from going to this instance.

  @Autowired   
  CounterService counterService;

  @Autowired
  GaugeService gaugeService;

  this.counterService.increment("greet.txnCount");
  this.gaugeService.submit("greet.customgauge", 1.0);