# xDS Service

Istio is deployed with a control plane and a data plane. Traditionally, the data plane uses sidecar containers with network proxies (such as Envoy), which increases network latency for microservice calls. In Kubernetes environments, Sermant uses the xDS protocol to communicate directly with Istiod, enabling service discovery. Using a Proxyless Service Mesh based on Istio+Sermant can significantly reduce microservice call latency and simplify system deployment. This article introduces how to use Sermant's xDS service in plugin development.

# Service Discovery Based on xDS Protocol

# Feature Introduction

Service discovery based on xDS protocol allows Sermant to connect to Istio's control plane to obtain Kubernetes service information and the corresponding specific service instance information.

Note: Using Sermant's xDS service discovery capability requires the service to be deployed in a Kubernetes container environment and istio is running.

# Development Example

This development example is based on the project created in the Creating Your First Plugin document and demonstrates how the plugin uses the xDS service discovery capability provided by the Sermant framework to obtain service instances:

1. Add the xdsServiceDiscovery variable in the com.huaweicloud.sermant.template.TemplateDeclarer class under template/template-plugin in the project to get the xDS service discovery service provided by the Sermant framework for obtaining service instances:

   XdsServiceDiscovery xdsServiceDiscovery = ServiceManager.getService(XdsCoreService.class).getXdsServiceDiscovery();
   

2. After obtaining the service discovery instance, you can call the APIs provided by XdsServiceDiscovery to perform corresponding actions. This example demonstrates how to directly obtain service instance information for a service named service-test:

   @Override
   public ExecuteContext before(ExecuteContext context) throws Exception {
     Set<ServiceInstance> serviceInstance = xdsServiceDiscovery.getServiceInstance("service-test");
     Iterator<ServiceInstance> iterator = serviceInstance.iterator();
     while (iterator.hasNext()) {
         ServiceInstance instance = iterator.next();
         System.out.println("ServiceInstance: [" + instance.getHost() + " : "
                 + instance.getPort() + "]");
     }
   }
   

   Note: service-test must be the name of the Kubernetes Service. The obtained service instance information corresponds to the Pod instance data of that service, an example Kubernetes Service looks like this:

   apiVersion: v1
   kind: Service
   metadata:
     name: service-test
   spec:
     type: ClusterIP
     ports:
     - name: http
     	port: 8080
       targetPort: 8080
       protocol: TCP
     selector:
       app: service-test
   

   After completing the development, refer to the Packaging and Building process in the Creating Your First Plugin document. Execute mvn package in the root directory of the project to generate the build artifact.

3. Enable the xDS service and set it to be enabled in agent/config/config.properties with the following configuration:

   # xDS service switch
   agent.service.xds.service.enable=true
   

4. After completing the execution, package the Sermant image and the host microservice image. Start the service-testservice in the k8s environment and create the corresponding service instances (implemented by the user). Finally, start the host application in Kubernetes and mount Sermant. Refer to the Sermant Injector User Guide for guidance on packaging Sermant and the host image in the Kubernetes environment, as well as mounting Sermant for startup.

5. After the host microservice pod with Sermant mounted starts successfully, you can execute the following command to obtain the host microservice logs and view the service instances obtained through the xDS service discovery capability:

   $ kubectl logs -f ${POD_NAME}
   ServiceInstance: [xxx.xxx.xxx.xxx : xxxx]
   

   Note: ${POD_NAME} must be the name of the pod running the host microservice, which can be viewed using the kubectl get pod command.

# xDS Service Discovery API

Obtain xDS Service Discovery Service:

XdsServiceDiscovery xdsServiceDiscovery = ServiceManager.getService(XdsCoreService.class).getXdsServiceDiscovery();

The xDS Service Discovery service has two interface methods, used respectively for directly obtaining service instances and obtaining service instances through subscription:

Set<ServiceInstance> getServiceInstance(String serviceName);

void subscribeServiceInstance(String serviceName, XdsServiceDiscoveryListener listener);

# Directly Obtain Service Instance

• Obtain the service instance information of the Kubernetes service, returning a java.util.Set containing all instances of the service.

  xdsServiceDiscovery.getServiceInstance("service-test")
  

# Subscribe to Obtain Service Instances

• Subscribe to the service instance information of the Kubernetes service and register a service discovery listener. The listener's process function can execute custom operations when service instances change.

  xdsServiceDiscovery.subscribeServiceInstance("service-test", new XdsServiceDiscoveryListener() {
              @Override
              public void process(Set<ServiceInstance> instances) {
                  // do something
              }
          });
  

• XdsServiceDiscoveryListener (opens new window)，contains the following interface methods:

  Method	Description
  void process(Set instances)	Callback interface for processing the latest service instance information

• ServiceInstance (opens new window), with the following methods:

  Return Type	Method Name	Description
  String	getClusterName()	Get the Istio cluster name the instance belongs to
  String	getServiceName()	Get the Kubernetes service name the instance belongs to
  String	getHost()	Get the Pod IP of the instance
  int	getPort()	Get the port of the instance
  Map<String, String>	getMetaData()	Get the metadata of the instance
  boolean	isHealthy()	Check if the service is healthy

# xDS Service Configuration

In the agent/config/config.properties file of the Sermant Agent package, you can enable the xDS service using the agent.service.xds.service.enable configuration item. Other xDS service configurations are also set in this file:

# xDS service switch
agent.service.xds.service.enable=false