Using Hystrix with Dropwizard

I've previously blogged about Hystrix/Tenacity and Breakerbox. The full code and outline of the example is located here.

Subsequently I've been using Dropwizard and Hystrix without Tenacity/Breakerbox and found it far simpler. I don't see a great deal of value in adding Tenacity and Breakerbox as Hystrix uses Netflix's configuration library Archaius which already comes with dynamic configuration via files, databases and Zookeeper.

So lets see what is involved in integrating Hystrix and Dropwizard.

The example is the same from this article. Briefly, it is a single service that calls out to three other services:

• A user service
• A pin check service
• A device service

To allow the application to reliably handle dependency failures we are going to call out to each of the three services using Hystrix commands. Here is an example of calling out using the Apache Http Client to a pin check service:

	public class PinCheckDependency extends HystrixCommand<Boolean> {
	private HttpClient httpClient;
	public PinCheckDependency(HttpClient httpClient) {
	super(HystrixCommandGroupKey.Factory.asKey("PinCheckService"));
	this.httpClient = httpClient;
	}
	@Override
	protected Boolean run() throws Exception {
	HttpGet pinCheck = new HttpGet("http://localhost:9090/pincheck");
	HttpResponse pinCheckResponse = httpClient.execute(pinCheck);
	int statusCode = pinCheckResponse.getStatusLine().getStatusCode();
	if (statusCode != 200) {
	throw new RuntimeException("Oh dear no pin check , status code " + statusCode);
	}
	String pinCheckInfo = EntityUtils.toString(pinCheckResponse.getEntity());
	return Boolean.valueOf(pinCheckInfo);
	}
	@Override
	public Boolean getFallback() {
	return true;
	}
	}

view raw PinCheckDependency.java hosted with ❤ by GitHub

To execute this command you simply instantiate an instance and call execute(), Hystrix handles creating a work queue and thread pool. Each command that is executed with the same group will use the same work queue and thread pool. You tell Hystrix the group by passing it to super() when extending a Hystrix command. To configure Hystrix in a Dropwizard like way we can add a map to our Dropwizard YAML:

	defaultHystrixConfig:
	hystrix.command.DeviceServiceDependency.execution.isolation.thread.timeoutInMilliseconds: 2000
	hystrix.threadpool.PinCheckService.maxQueueSize: 50
	hystrix.threadpool.PinCheckService.coreSize: 5

view raw hystrix.yml hosted with ❤ by GitHub

This will translate to a Map in your Dropwizard configuration class:

	@NotNull
	@JsonProperty
	private Map<String, Object> defaultHystrixConfig;
	public Map<String, Object> getDefaultHystrixConfig() {
	return defaultHystrixConfig;
	}

view raw AppConfig.java hosted with ❤ by GitHub

The advantage of using a simple map rather than a class with the property names matching Hystrix property names is this allows you to be completely decoupled from Hystrix and its property naming conventions. It also allows users to copy property names directly from Hystrix documentation into the YAML.

To enable Hystrix to pick these properties up it requires a single line in your Dropwizard application class. This simplicity is due to the fact that Hystrix uses Archaius for property management.

	@Override
	public void run(AppConfig appConfig, Environment environment) throws Exception {
	ConfigurationManager.install(new MapConfiguration(appConfig.getDefaultHystrixConfig()));
	...
	}

view raw IntegrationApplication.java hosted with ❤ by GitHub

Now you can add as any of Hystrix's many properties to your YAML. Then later extend the Configuration you install to include a dynamic configuration source such as ZooKeeper.

I hope this shows just how simple it is to use Hystrix with Dropwizard without bothering with Tenacity. A full working example is on github.