Dropwizard Health Check Configuration Example

We have already seen how dropwizard is so effective in developing self-contained REST APIs and even REST client services as well. Dropwizard contains almost all needed packages to build APIs in very easy way without making the things complex. One of the easily implemented features of dropwizard is health check service which can be used for monitoring the status of the application/components that you are creating – in runtime.

Implementing Dropwizard Health Check

DropWizard health check is implemented by extending the HealthCheck class and returning Result.healthy() if everything is alright and Result.unhealthy() if something is not working – as expected.

Health Check Configuration

public class AppHealthCheck extends HealthCheck 
{
	@Override
	protected Result check() throws Exception 
	{
		if(Check some condition == true){
			return Result.healthy();
		}
		return Result.unhealthy("Error message");
	}
}

To register this AppHealthCheck class with dropwizard application, use Environment.healthChecks() registry.

public void run(Configuration c, Environment e) throws Exception 
{
	//Application health check
	e.healthChecks().register("APIHealthCheck", new AppHealthCheck());
}

Validating System Health

Dropwizard will a HTTP resource endpoint on /healthcheck on the admin port (default 8081). Dropwizard also includes a check for deadlocks by default along with any custom health checks you define as in our case AppHealthCheck.

http://localhost:8081/healthcheck

Above health check URL will return some result as below:

{
	"APIHealthCheck": {
		"healthy": true
	},
	"deadlocks": {
		"healthy": true
	}
}

Custom REST Resource to Run Health Checks

If you do not want to use admin port then you can also create a custom REST resource which will run health checks for you, and return the result in your desired response format.

To run all health checks and get all it’s results, you will call registry.runHealthChecks() inside REST resource.

@Produces(MediaType.APPLICATION_JSON)
@Path("/status")
public class HealthCheckController 
{
	private HealthCheckRegistry registry;

	public HealthCheckController(HealthCheckRegistry registry) {
		this.registry = registry;
	}
	
	@GET
	public Set<Entry<String, Result>> getStatus(){
		return registry.runHealthChecks().entrySet();
	}
}

Now when we call this REST API using http://localhost:8080/status, we get response like below:

[
  {
    "APIHealthCheck": {
      "healthy": true,
      "message": null,
      "error": null
    }
  },
  {
    "deadlocks": {
      "healthy": true,
      "message": null,
      "error": null
    }
  }
]

You can customize the messages as per your need.

Dropwizard Health Check Example

To demonstrate the above both functionalities, I have modified the code given in dropwizard hello world application.

AppHealthCheck.java

package com.howtodoinjava.healthcheck;

import java.util.ArrayList;

import javax.ws.rs.client.Client;
import javax.ws.rs.client.Invocation;
import javax.ws.rs.client.WebTarget;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.Response;

import com.codahale.metrics.health.HealthCheck;

public class AppHealthCheck extends HealthCheck {
	private final Client client;

	public AppHealthCheck(Client client) {
		super();
		this.client = client;
	}

	@Override
	protected Result check() throws Exception {
		WebTarget webTarget = client.target("http://localhost:8080/employees");
		Invocation.Builder invocationBuilder =  webTarget.request(MediaType.APPLICATION_JSON);
		Response response = invocationBuilder.get();
		@SuppressWarnings("rawtypes")
		ArrayList employees = response.readEntity(ArrayList.class);
		if(employees !=null && employees.size() > 0){
			return Result.healthy();
		}
		return Result.unhealthy("API Failed");
	}
}

HealthCheckController.java

package com.howtodoinjava.healthcheck;

import java.util.Map.Entry;
import java.util.Set;

import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;

import com.codahale.metrics.health.HealthCheck.Result;
import com.codahale.metrics.health.HealthCheckRegistry;

@Produces(MediaType.APPLICATION_JSON)
@Path("/status")
public class HealthCheckController 
{
	private HealthCheckRegistry registry;

	public HealthCheckController(HealthCheckRegistry registry) {
		this.registry = registry;
	}
	
	@GET
	public Set<Entry<String, Result>> getStatus(){
		return registry.runHealthChecks().entrySet();
	}
}

App.java

package com.howtodoinjava.rest;

import io.dropwizard.Application;
import io.dropwizard.Configuration;
import io.dropwizard.client.JerseyClientBuilder;
import io.dropwizard.setup.Bootstrap;
import io.dropwizard.setup.Environment;

import javax.ws.rs.client.Client;

import com.howtodoinjava.healthcheck.AppHealthCheck;
import com.howtodoinjava.healthcheck.HealthCheckController;
import com.howtodoinjava.rest.controller.EmployeeRESTController;
import com.howtodoinjava.rest.controller.RESTClientController;

public class App extends Application<Configuration> {

	@Override
	public void initialize(Bootstrap<Configuration> b) {
	}

	@Override
	public void run(Configuration c, Environment e) throws Exception 
	{
		e.jersey().register(new EmployeeRESTController(e.getValidator()));
		
		final Client client = new JerseyClientBuilder(e).build("DemoRESTClient");
		e.jersey().register(new RESTClientController(client));
		
		//Application health check
		e.healthChecks().register("APIHealthCheck", new AppHealthCheck(client));
		
		//Run multiple health checks
		e.jersey().register(new HealthCheckController(e.healthChecks()));
	}

	public static void main(String[] args) throws Exception {
		new App().run(args);
	}
}

Validate Health Check URLs

1) http://localhost:8081/healthcheck

2) http://localhost:8080/status

Let me know of your questions in comments section.

Happy Learning !!

Reference:

Dropwizard Docs