4. Service Discovery with Consul
Service Discovery is one of the key tenets of a microservice based architecture. Trying to hand configure each client or some form of convention can be very difficult to do and can be very brittle. Consul provides Service Discovery services via an HTTP API and DNS. Spring Cloud Consul leverages the HTTP API for service registration and discovery. This does not prevent non-Spring Cloud applications from leveraging the DNS interface. Consul Agents servers are run in a cluster that communicates via a gossip protocol and uses the Raft consensus protocol.
4.1. How to activate
To activate Consul Service Discovery use the starter with group org.springframework.cloud and artifact id spring-cloud-starter-consul-discovery. See the Spring Cloud Project page for details on setting up your build system with the current Spring Cloud Release Train.
4.2. Registering with Consul
When a client registers with Consul, it provides meta-data about itself such as host and port, id, name and tags. An HTTP Check is created by default that Consul hits the /actuator/health endpoint every 10 seconds. If the health check fails, the service instance is marked as critical.
Example Consul client:
@SpringBootApplication
@RestController
public class Application {
@RequestMapping("/")
public String home() {
return "Hello world";
}
public static void main(String[] args) {
new SpringApplicationBuilder(Application.class).web(true).run(args);
}
}(i.e. utterly normal Spring Boot app). If the Consul client is located somewhere other than localhost:8500, the configuration is required to locate the client. Example:
spring:
cloud:
consul:
host: localhost
port: 8500
If you use Spring Cloud Consul Config, and you have set spring.cloud.bootstrap.enabled=true or spring.config.use-legacy-processing=true or use spring-cloud-starter-bootstrap, then the above values will need to be placed in bootstrap.yml instead of application.yml.
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The default service name, instance id and port, taken from the Environment, are ${spring.application.name}, the Spring Context ID and ${server.port} respectively.
To disable the Consul Discovery Client you can set spring.cloud.consul.discovery.enabled to false. Consul Discovery Client will also be disabled when spring.cloud.discovery.enabled is set to false.
To disable the service registration you can set spring.cloud.consul.discovery.register to false.
4.2.1. Registering Management as a Separate Service
When management server port is set to something different than the application port, by setting management.server.port property, management service will be registered as a separate service than the application service. For example:
spring:
application:
name: myApp
management:
server:
port: 4452
Above configuration will register following 2 services:
-
Application Service:
ID: myApp Name: myApp
-
Management Service:
ID: myApp-management Name: myApp-management
Management service will inherit its instanceId and serviceName from the application service. For example:
spring:
application:
name: myApp
management:
server:
port: 4452
spring:
cloud:
consul:
discovery:
instance-id: custom-service-id
serviceName: myprefix-${spring.application.name}
Above configuration will register following 2 services:
-
Application Service:
ID: custom-service-id Name: myprefix-myApp
-
Management Service:
ID: custom-service-id-management Name: myprefix-myApp-management
Further customization is possible via following properties:
/** Port to register the management service under (defaults to management port) */ spring.cloud.consul.discovery.management-port /** Suffix to use when registering management service (defaults to "management" */ spring.cloud.consul.discovery.management-suffix /** Tags to use when registering management service (defaults to "management" */ spring.cloud.consul.discovery.management-tags
4.2.2. HTTP Health Check
The health check for a Consul instance defaults to "/actuator/health", which is the default location of the health endpoint in a Spring Boot Actuator application. You need to change this, even for an Actuator application, if you use a non-default context path or servlet path (e.g. server.servletPath=/foo) or management endpoint path (e.g. management.server.servlet.context-path=/admin).
The interval that Consul uses to check the health endpoint may also be configured. "10s" and "1m" represent 10 seconds and 1 minute respectively.
This example illustrates the above (see the spring.cloud.consul.discovery.health-check-* properties in the appendix page for more options).
spring:
cloud:
consul:
discovery:
healthCheckPath: ${management.server.servlet.context-path}/actuator/health
healthCheckInterval: 15s
You can disable the HTTP health check entirely by setting spring.cloud.consul.discovery.register-health-check=false.
Applying Headers
Headers can be applied to health check requests. For example, if you’re trying to register a Spring Cloud Config server that uses Vault Backend:
spring:
cloud:
consul:
discovery:
health-check-headers:
X-Config-Token: 6442e58b-d1ea-182e-cfa5-cf9cddef0722
According to the HTTP standard, each header can have more than one values, in which case, an array can be supplied:
spring:
cloud:
consul:
discovery:
health-check-headers:
X-Config-Token:
- "6442e58b-d1ea-182e-cfa5-cf9cddef0722"
- "Some other value"
4.2.3. Actuator Health Indicator(s)
If the service instance is a Spring Boot Actuator application, it may be provided the following Actuator health indicators.
DiscoveryClientHealthIndicator
When Consul Service Discovery is active, a DiscoverClientHealthIndicator is configured and made available to the Actuator health endpoint. See here for configuration options.
ConsulHealthIndicator
An indicator is configured that verifies the health of the ConsulClient.
By default, it retrieves the Consul leader node status and all registered services.
In deployments that have many registered services it may be costly to retrieve all services on every health check.
To skip the service retrieval and only check the leader node status set spring.cloud.consul.health-indicator.include-services-query=false.
To disable the indicator set management.health.consul.enabled=false.
| When the application runs in bootstrap context mode (the default), this indicator is loaded into the bootstrap context and is not made available to the Actuator health endpoint. |
4.2.4. Metadata
Consul supports metadata on services. Spring Cloud’s ServiceInstance has a Map<String, String> metadata field which is populated from a services meta field. To populate the meta field set values on spring.cloud.consul.discovery.metadata or spring.cloud.consul.discovery.management-metadata properties.
spring:
cloud:
consul:
discovery:
metadata:
myfield: myvalue
anotherfield: anothervalue
The above configuration will result in a service who’s meta field contains myfield→myvalue and anotherfield→anothervalue.
Generated Metadata
The Consul Auto Registration will generate a few entries automatically.
| Key | Value |
|---|---|
'group' |
Property |
'secure' |
True if property |
Property |
Property |
Older versions of Spring Cloud Consul populated the ServiceInstance.getMetadata() method from Spring Cloud Commons by parsing the spring.cloud.consul.discovery.tags property. This is no longer supported, please migrate to using the spring.cloud.consul.discovery.metadata map.
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4.2.5. Making the Consul Instance ID Unique
By default a consul instance is registered with an ID that is equal to its Spring Application Context ID. By default, the Spring Application Context ID is ${spring.application.name}:comma,separated,profiles:${server.port}. For most cases, this will allow multiple instances of one service to run on one machine. If further uniqueness is required, Using Spring Cloud you can override this by providing a unique identifier in spring.cloud.consul.discovery.instanceId. For example:
spring:
cloud:
consul:
discovery:
instanceId: ${spring.application.name}:${vcap.application.instance_id:${spring.application.instance_id:${random.value}}}
With this metadata, and multiple service instances deployed on localhost, the random value will kick in there to make the instance unique. In Cloudfoundry the vcap.application.instance_id will be populated automatically in a Spring Boot application, so the random value will not be needed.
4.3. Looking up services
4.3.1. Using Load-balancer
Spring Cloud has support for Feign (a REST client builder) and also Spring RestTemplate
for looking up services using the logical service names/ids instead of physical URLs. Both Feign and the discovery-aware RestTemplate utilize Spring Cloud LoadBalancer for client-side load balancing.
If you want to access service STORES using the RestTemplate simply declare:
@LoadBalanced
@Bean
public RestTemplate loadbalancedRestTemplate() {
return new RestTemplate();
}
and use it like this (notice how we use the STORES service name/id from Consul instead of a fully qualified domainname):
@Autowired
RestTemplate restTemplate;
public String getFirstProduct() {
return this.restTemplate.getForObject("https://STORES/products/1", String.class);
}
If you have Consul clusters in multiple datacenters and you want to access a service in another datacenter a service name/id alone is not enough. In that case
you use property spring.cloud.consul.discovery.datacenters.STORES=dc-west where STORES is the service name/id and dc-west is the datacenter
where the STORES service lives.
| Spring Cloud now also offers support for Spring Cloud LoadBalancer. |
4.3.2. Using the DiscoveryClient
You can also use the org.springframework.cloud.client.discovery.DiscoveryClient which provides a simple API for discovery clients that is not specific to Netflix, e.g.
@Autowired
private DiscoveryClient discoveryClient;
public String serviceUrl() {
List<ServiceInstance> list = discoveryClient.getInstances("STORES");
if (list != null && list.size() > 0 ) {
return list.get(0).getUri();
}
return null;
}
4.4. Consul Catalog Watch
The Consul Catalog Watch takes advantage of the ability of consul to watch services. The Catalog Watch makes a blocking Consul HTTP API call to determine if any services have changed. If there is new service data a Heartbeat Event is published.
To change the frequency of when the Config Watch is called change spring.cloud.consul.config.discovery.catalog-services-watch-delay. The default value is 1000, which is in milliseconds. The delay is the amount of time after the end of the previous invocation and the start of the next.
To disable the Catalog Watch set spring.cloud.consul.discovery.catalogServicesWatch.enabled=false.
The watch uses a Spring TaskScheduler to schedule the call to consul. By default it is a ThreadPoolTaskScheduler with a poolSize of 1. To change the TaskScheduler, create a bean of type TaskScheduler named with the ConsulDiscoveryClientConfiguration.CATALOG_WATCH_TASK_SCHEDULER_NAME constant.