How to use

In order to test solution:

0. Open docker-compose.yml
1. Update LIST_OF_SERVERS if needed
2. Run everything by docker-compose up -d
3. Query API Gateway with docker exec -it <gateway_container_id> curl -X GET localhost:9669/get-fortune. See "Faced issues", can't trigger APIs from outside Docker network on Windows.

Testing

• No unit tests - zero-code approach, everything is configured with Spring Boot automatically. Instead of mocking Spring internals, and making tests brittle and complex, I use integration tests. Moreover Zuul, Ribbon and Hystrix projects are covered with unit tests (see github).
• Integration tests - start up API Gateway container, mock all service dependencies (fortune) services with WireMock. Simulate different edge-cases: delays, failures, etc.
• End to end tests - coverage could be achieved with integration tests, running all Docker services for test purposes would be an overkill. End-to-end performance could be tested manually.

Solution Details

I use Spring Boot and Netflix OSS stack to implement API gateway with load balancing functionality. Regret my choice :) because Spring Boot + Netflix OSS has poor and sometimes obsolete documentation. Netflix OSS could be replaced with Spring Cloud analogies (i.e. Spring Cloud Gateway).

API Gateway

Role of API Gateway is played by Zuul. Unlike Netty-based async Zuul2, Zuul 1 use blocking communication backed by Apache HTTP Client. Spring Cloud doesn't support Zuul 2, for non-blocking behaviour use Spring Cloud Gateway.

Spring Boot automatically include Zuul embedded proxy after adding dependency to spring-cloud-starter-netflix-zuul in your pom.xml and annotating main class with @EnableZuulProxy. Or by using Spring Initializr with Zuul option choosed.

All Zuul configs could be found at application.properties file.

Load Balancing

Role of layer 7 client-side load balancer is played by Ribbon. Ribbon is used by Zuul under the hood.

All Ribbon-related configurations could be found at application.properties file.

Service registry & scaling services

Ribbon could be easily integrated with service registries like Eureka, but Eureka requires clients (get-fortune services) to register themselves on start-up by calling Eureka server API.

I treat get-fortune services as third-party that can't be changed. That means I have to hard code list of services on application startup, see application.properties property called ribbon.listOfServers.

I don't want to re-build the Docker image each time list of servers is changed, so I pass the list as environment variable from docker-compose. You can add new get-fortune services by re-starting gateway with new value of LIST_OF_SERVERS variable.

I haven't used Spring Cloud Config or Spring Netflix Archanius since it would be an over engineering for this task.

Circuit Breaker

Zuul use Hystryx commands under the hood.

Faced issues

1. Windows 10 v1607 b14393.3326 doesn't support Docker Desktop, instead you have to run Docker Tools, and Docker containers within Oracle VM VirtualBox. See GitHub ticket
2. Docker on older versions of Windows 10 doesn't map ports to localhost. See GitHub ticket
3. Zuul/Ribbon/Hystrix documentation is obsolete, it lacks one very significant property - retry interval (exponential or at least fixed). This cause Zuul to fire retries immediately, and with latencies up to 5 seconds it makes situation even worse. Take a look at ApiGatewayHttpServerBasedIntegrationTest