# 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:

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();
```

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.

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
```
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.
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 three interface methods: used respectively for directly obtaining service instances, obtaining service instances through subscription and obtaining service instances through cluster.

Set<ServiceInstance> getServiceInstance(String serviceName);

void subscribeServiceInstance(String serviceName, XdsServiceDiscoveryListener listener);

Optional<XdsClusterLoadAssigment> getClusterServiceInstance(String clusterName);

# 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 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

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

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

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

# Get Service Instances through Cluster

Retrieve the service instance information of a Service Cluster. The return type is Optional<XdsClusterLoadAssignment>, which contains all service instances of the Cluster:
```
xdsServiceDiscovery.getClusterServiceInstance("outbound|8080||service-test.default.svc.cluster.local");
```

Cluster service instance XdsClusterLoadAssignment (opens new window) has the following fields:

Field Name	Field Type	Description
serviceName	String	The name of the service to which the Cluster service instance belongs
clusterName	String	The name of the Cluster
localityInstances	Map<XdsLocality, Set>	The service instances of the Cluster, consisting of service instances from different localities

Locality information XdsLocality (opens new window) has the following fields:

Field Name	Field Type	Description
region	String	Region information
zone	String	Zone information
subZone	String	Sub-zone information
loadBalanceWeight	int	Load balancing weight
localityPriority	int	Locality priority

# Route Configuration Service Based on xDS Protocol

# Feature Introduction

The Route Configuration Service Based on xDS Protocol allows Sermant to connect with Istio's control plane to retrieve the routing configuration information of Kubernetes Services.

Note: To use Sermant's xDS routing configuration service, the service must be deployed in a Kubernetes container environment and running Istio.

# Development Example

This development example is based on the project created in the Create Your First Plugin document, demonstrating how a plugin can retrieve routing configuration information for services using the xDS routing configuration service provided by the Sermant framework:

In the io.sermant.template.TemplateDeclarer class under template/template-plugin in the project, add the variable xdsRouteService to obtain the xDS route configuration service provided by the Sermant framework, which is used to get the routing configuration information of the services:
```
XdsRouteService xdsRouteService = ServiceManager.getService(XdsCoreService.class).getXdsRouteService();
```
After obtaining the route configuration service instance, you can call the API provided by xdsRouteService to get the routing configuration information of a specified service:
```
List<XdsRoute> serviceRoute = xdsRouteService.getServiceRoute("spring-test");
System.out.println("The size of routing config: " + serviceRoute.size());
```
Note: service-test must be the name of a Kubernetes Service. The retrieved routing configuration is delivered through Istio's DestinationRule and VirtualService. For the specific routing configuration fields and templates supported by Sermant, please refer to the section Routing based on xDS Service.

After development, you can follow the Packaging and Building steps from the first plugin creation to generate the build artifact by executing mvn package in the project root directory.
Enable the xDS service and configure agent/config/config.properties to enable the xDS service. The configuration example is as follows:
```
# xDS service switch
agent.service.xds.service.enable=true
```
After completion, package the Sermant image and the host microservice image. Start the host microservice in Kubernetes and mount Sermant. For guidance on packaging Sermant and host images and mounting Sermant to start host microservices in Kubernetes, please refer to the Sermant Injector User Guide. Use the DestinationRuleand VirtualService rules according to the Istio Routing Configuration Template provided by Sermant.
After the host microservice is successfully started with Sermant mounted, you can execute the following command to fetch the logs of the host microservice and check the number of routing configurations retrieved by the xDS route configuration service:
```
$ kubectl logs -f ${POD_NAME}
The size of routing config: 1
```
Note: ${POD_NAME} must be the name of the pod where the host microservice is running. You can check it using the kubectl get pod command.

# xDS Route Configuration Service API

To obtain the xDS Route Configuration Service:

XdsRouteService xdsRouteService = ServiceManager.getService(XdsCoreService.class).getXdsRouteService();

The xDS Route Configuration Service has two API methods: one for getting the routing information of a Service, and the other for checking whether a Cluster has enabled same AZ routing.

List<XdsRoute> getServiceRoute(String serviceName);

boolean isLocalityRoute(String clusterName);

# Get Service Routing Configuration

Retrieve the routing configuration information of a Kubernetes Service. The return type is List<XdsRoute>, containing all the routing configuration for that Service:
```
xdsRouteService.getServiceRoute("service-test");
```

The routing configuration entity XdsRoute (opens new window) includes the following fields:

Field Name	Field Type	Description
name	String	The name of the route configuration
routeMatch	XdsRouteMatch	The matching rule for the route
routeAction	XdsRouteAction	The routing destination for the match

# Cluster AZ Routing

Check whether the Cluster of a Service has enabled same AZ routing. The return type is boolean:
```
xdsRouteService.isLocalityRoute("outbound|8080||service-test.default.svc.cluster.local");
```

# Load Balancing Configuration Service Based on xDS Protocol

# Feature Introduction

The Load Balancing Configuration Service Based on xDS Protocol allows Sermant to connect with Istio's control plane to retrieve the load balancing rules of Kubernetes Services.

Note: To use Sermant's xDS load balancing configuration service, the service must be deployed in a Kubernetes container environment and running Istio.

# Development Example

This development example is based on the project created in the Create Your First Plugin document, demonstrating how a plugin can retrieve the load balancing rules for services using the xDS load balancing configuration service provided by the Sermant framework:

In the io.sermant.template.TemplateDeclarer class under template/template-plugin in the project, add the variable loadBalanceService to obtain the xDS load balancing configuration service provided by the Sermant framework, which is used to get the load balancing rules of the services:
```
XdsLoadBalanceService loadBalanceService = 
ServiceManager.getService(XdsCoreService.class).getLoadBalanceService();
```
After obtaining the load balancing configuration service, you can call the API provided by loadBalanceServiceto get the load balancing strategy of a service:
```
XdsLbPolicy serviceLbPolicy = loadBalanceService.getBaseLbPolicyOfService("spring-test");
System.out.println("Service lb policy: " + serviceLbPolicy);
```
Note: service-test must be the name of a Kubernetes Service. The load balancing configuration is delivered through Istio's DestinationRule. For the specific supported fields and load balancing rules in the configuration, and configuration templates, please refer to the section Load balancing based on xDS Service.

After development, you can follow the Packaging and Building steps from the first plugin creation to generate the build artifact by executing mvn package in the project root directory.
Enable the xDS service and configure agent/config/config.properties to enable the xDS service. The configuration example is as follows:
```
# xDS service switch
agent.service.xds.service.enable=true
```
After completion, package the Sermant image and the host microservice image. Start the host microservice in Kubernetes and mount Sermant. For guidance on packaging Sermant and host images and mounting Sermant to start host microservices in Kubernetes, please refer to the Sermant Injector User Guide. Use the DestinationRule provided by Sermant in the Istio Load Balancing Configuration Template.
After the host microservice is successfully started with Sermant mounted, you can execute the following command to fetch the logs of the host microservice and check the load balancing strategy retrieved by the xDS load balancing configuration service:
```
$ kubectl logs -f ${POD_NAME}
Service lb policy: ROUND_ROBIN
```
Note: ${POD_NAME} must be the name of the pod where the host microservice is running. You can check it using the kubectl get pod command.

# xDS Load Balancing Configuration Service API

To obtain the xDS Load Balancing Configuration Service:

XdsLoadBalanceService loadBalanceService = 
ServiceManager.getService(XdsCoreService.class).getLoadBalanceService();

The xDS Load Balancing Configuration Service provides two API methods: one for getting the load balancing rules of a Service, and another for the load balancing rules of a Cluster:

XdsLbPolicy getLbPolicyOfCluster(String clusterName);

XdsLbPolicy getBaseLbPolicyOfService(String serviceName);

# Get Load Balancing Rules of a Service

Retrieve the load balancing rules for a Service. The return type is XdsLbPolicy:
```
loadBalanceService.getBaseLbPolicyOf("spring-test");
```

The load balancing rule entity class XdsLbPolicy (opens new window) is an enumeration type.

Load Balancing Rule	Description
XdsLbPolicy.RANDOM	Random load balancing rule
XdsLbPolicy.ROUND_ROBIN	Round-robin load balancing rule
XdsLbPolicy.LEAST_REQUEST	Least-request load balancing rule
XdsLbPolicy.UNRECOGNIZED	Unrecognized load balancing rule

# Get Load Balancing Rules of the Cluster

Retrieve the load balancing rules of a Service Cluster. The return type is XdsLbPolicy.

loadBalanceService.getLbPolicyOfCluster("outbound|8080||service-test.default.svc.cluster.local");

# 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

← Dynamic Configuration Function HTTP Server Service →

# xDS Service

# Service Discovery Based on xDS Protocol

# Feature Introduction

# Development Example

# xDS Service Discovery API

# Directly Obtain Service Instance

# Subscribe to Obtain Service Instances

# Get Service Instances through Cluster

# Route Configuration Service Based on xDS Protocol

# Feature Introduction

# Development Example

# xDS Route Configuration Service API

# Get Service Routing Configuration

# Cluster AZ Routing

# Load Balancing Configuration Service Based on xDS Protocol

# Feature Introduction

# Development Example

# xDS Load Balancing Configuration Service API

# Get Load Balancing Rules of a Service

# Get Load Balancing Rules of the Cluster

# xDS Service Configuration