circuit-breaker-demo

Demo of circuit breaker pattern in microservices

Circuit breaker pattern is a way to handle the request load when the microservices are not responding or is down completely. Here the client can either timeout the requests or handle in a way where the server is not overloaded with the requests. Here client can do either of the following

• throw an exception
• provide a dummy response
• respond with values from cache

We are going to use Netflix-Hystrix to do the circuit-breaker implementation.

This implemenation is an extension over the service-discovery implementation. Please refer to this repo for service-discovery related details.

Steps

• Create a spring-boot app with the below dependency

<dependency>
	<groupId>org.springframework.cloud</groupId>
	<artifactId>spring-cloud-starter-netflix-hystrix</artifactId>
	<version>2.2.2.RELEASE</version>
</dependency>

• Add @EnableCircuitBreaker annotation to your application class.

• Add @HystrixCommand annotation to your method where you want to implement the circuit breaker. Provide a fallback method which will return the response whenever circuit breaks.

@GetMapping("/hi")
@HystrixCommand(fallbackMethod = "fallbackHi")
public String hi() {
	ResponseEntity<String> exchange = restTemplate.exchange("http://service/hello", HttpMethod.GET, null, String.class);
	return exchange.getBody();
}
public String fallbackHi() {
	return "Service is not responding! Please try after sometime.";
}

Decide on when to break a circuit

When does the circuit should break? and when should the calls be sent again? We can configure this by taking into consideration of different parameters as listed below

• Timeout a request after p seconds
• Consider for last n request
• m out of n requests are failed
• After t seconds(sleep window) the client should agian start sending the requests

commandProperties = {
	@HystrixProperty(name = "execution.isolation.thread.timeoutInMilliseconds", value = "2000"),
	@HystrixProperty(name = "circuitBreaker.requestVolumeThreshold", value = "5"),
	@HystrixProperty(name = "circuitBreaker.errorThresholdPercentage", value = "50"),
	@HystrixProperty(name = "circuitBreaker.sleepWindowInMilliseconds", value = "5000") }

The above example considers a request timeout of 2secs. If among the last 5 requests, 50% of requests are failed to respond then circuit is broken. The client will again start sending the request after 5 secs(sleep window). You can test this behavior by bringing down the service and making the calls to client application.

Configure Hystrix Dashboard

To monitor all these requests, hystrix provides a dashboard. To configure the dashboard add the below dependency

<dependency>
	<groupId>org.springframework.cloud</groupId>
	<artifactId>spring-cloud-starter-netflix-hystrix-dashboard</artifactId>
	<version>2.2.2.RELEASE</version>
</dependency>

Make sure your pom has actuator dependency as well. Hystrix dashboard uses actuator

<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-actuator</artifactId>
</dependency>

Add @EnableHystrixDashboard annotation to the application class.

Add management.endpoints.web.exposure.include=* to the application.properties file.

To access the dashboard hit the app end point with /hystrix. http://localhost:8083/hystrix. As we just using a single hystrix app and not in a cluster. Copy the URL http://localhost:8083/actuator/hystrix.stream to the textbox and hit Monitor Stream button. This will give the api call details.

Bulk head pattern

Here in this pattern we can give each api with a dedicated threadpool which will not collide with other apis. To configure bulk head, you can provide below properties to @HystrixCommand

@HystrixCommand(fallbackMethod = "fallbackHi", threadPoolKey = "hiServicePool", threadPoolProperties = {
	@HystrixProperty(name = "coreSize", value = "20"),
	@HystrixProperty(name = "maxQueueSize", value = "10") })

Each threadpool can again be configured with the coreSixe(max threads allocated) and maxQueueSize(requests that can be saved to the queue)