1. Service Discovery: Eureka Clients
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 difficult to do and can be brittle. Eureka is the Netflix Service Discovery Server and Client. The server can be configured and deployed to be highly available, with each server replicating state about the registered services to the others.
1.1. How to Include Eureka Client
To include the Eureka Client in your project, use the starter with a group ID of org.springframework.cloud and an artifact ID of spring-cloud-starter-netflix-eureka-client.
See the Spring Cloud Project page for details on setting up your build system with the current Spring Cloud Release Train.
1.2. Registering with Eureka
When a client registers with Eureka, it provides meta-data about itself — such as host, port, health indicator URL, home page, and other details. Eureka receives heartbeat messages from each instance belonging to a service. If the heartbeat fails over a configurable timetable, the instance is normally removed from the registry.
The following example shows a minimal Eureka client application:
@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);
}
}
Note that the preceding example shows a normal Spring Boot application.
By having spring-cloud-starter-netflix-eureka-client on the classpath, your application automatically registers with the Eureka Server. Configuration is required to locate the Eureka server, as shown in the following example:
eureka:
client:
serviceUrl:
defaultZone: http://localhost:8761/eureka/
In the preceding example, defaultZone is a magic string fallback value that provides the service URL for any client that does not express a preference (in other words, it is a useful default).
The defaultZone property is case sensitive and requires camel case because the serviceUrl property is a Map<String, String>. Therefore, the defaultZone property does not follow the normal Spring Boot snake-case convention of default-zone.
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The default application name (that is, the service ID), virtual host, and non-secure port (taken from the Environment) are ${spring.application.name}, ${spring.application.name} and ${server.port}, respectively.
Having spring-cloud-starter-netflix-eureka-client on the classpath makes the app into both a Eureka “instance” (that is, it registers itself) and a “client” (it can query the registry to locate other services).
The instance behaviour is driven by eureka.instance.* configuration keys, but the defaults are fine if you ensure that your application has a value for spring.application.name (this is the default for the Eureka service ID or VIP).
See EurekaInstanceConfigBean and EurekaClientConfigBean for more details on the configurable options.
To disable the Eureka Discovery Client, you can set eureka.client.enabled to false. Eureka Discovery Client will also be disabled when spring.cloud.discovery.enabled is set to false.
Specifying the version of the Spring Cloud Netflix Eureka server as a path parameter is not currently supported. This means you cannot set the version in the context path (eurekaServerURLContext). Instead, you can include the version in the server URL (for example, you can set defaultZone: localhost:8761/eureka/v2).
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1.3. Authenticating with the Eureka Server
HTTP basic authentication is automatically added to your eureka client if one of the eureka.client.serviceUrl.defaultZone URLs has credentials embedded in it (curl style, as follows: user:password@localhost:8761/eureka).
For more complex needs, you can create a @Bean of type DiscoveryClientOptionalArgs and inject ClientFilter instances into it, all of which is applied to the calls from the client to the server.
When Eureka server requires client side certificate for authentication, the client side certificate and trust store can be configured via properties, as shown in following example:
eureka:
client:
tls:
enabled: true
key-store: <path-of-key-store>
key-store-type: PKCS12
key-store-password: <key-store-password>
key-password: <key-password>
trust-store: <path-of-trust-store>
trust-store-type: PKCS12
trust-store-password: <trust-store-password>The eureka.client.tls.enabled needs to be true to enable Eureka client side TLS. When eureka.client.tls.trust-store is omitted, a JVM default trust store is used. The default value for eureka.client.tls.key-store-type and eureka.client.tls.trust-store-type is PKCS12. When password properties are omitted, empty password is assumed.
| Because of a limitation in Eureka, it is not possible to support per-server basic auth credentials, so only the first set that are found is used. |
If you want to customize the RestTemplate used by the Eureka HTTP Client you may want to create a bean of EurekaClientHttpRequestFactorySupplier and provide your own logic for generating a ClientHttpRequestFactory instance.
All default timeout-related properties for RestTemplate used by the Eureka HTTP Client are set to infinite. Therefore, to specify the timeout values, you must specify the value directly with the properties in eureka.client.rest-template-timeout. (All timeout properties are in milliseconds.)
eureka:
client:
rest-template-timeout:
connect-timeout: 5000
connect-request-timeout: 8000
socket-timeout: 100001.4. Status Page and Health Indicator
The status page and health indicators for a Eureka instance default to /info and /health respectively, which are the default locations of useful endpoints in a Spring Boot Actuator application.
You need to change these, even for an Actuator application if you use a non-default context path or servlet path (such as server.servletPath=/custom). The following example shows the default values for the two settings:
eureka:
instance:
statusPageUrlPath: ${server.servletPath}/info
healthCheckUrlPath: ${server.servletPath}/health
These links show up in the metadata that is consumed by clients and are used in some scenarios to decide whether to send requests to your application, so it is helpful if they are accurate.
| In Dalston it was also required to set the status and health check URLs when changing that management context path. This requirement was removed beginning in Edgware. |
1.5. Registering a Secure Application
If your app wants to be contacted over HTTPS, you can set two flags in the EurekaInstanceConfigBean:
-
eureka.instance.[nonSecurePortEnabled]=[false] -
eureka.instance.[securePortEnabled]=[true]
Doing so makes Eureka publish instance information that shows an explicit preference for secure communication.
The Spring Cloud DiscoveryClient always returns a URI starting with https for a service configured this way.
Similarly, when a service is configured this way, the Eureka (native) instance information has a secure health check URL.
Because of the way Eureka works internally, it still publishes a non-secure URL for the status and home pages unless you also override those explicitly. You can use placeholders to configure the eureka instance URLs, as shown in the following example:
eureka:
instance:
statusPageUrl: https://${eureka.hostname}/info
healthCheckUrl: https://${eureka.hostname}/health
homePageUrl: https://${eureka.hostname}/
(Note that ${eureka.hostname} is a native placeholder only available
in later versions of Eureka. You could achieve the same thing with
Spring placeholders as well — for example, by using ${eureka.instance.hostName}.)
| If your application runs behind a proxy, and the SSL termination is in the proxy (for example, if you run in Cloud Foundry or other platforms as a service), then you need to ensure that the proxy “forwarded” headers are intercepted and handled by the application. If the Tomcat container embedded in a Spring Boot application has explicit configuration for the 'X-Forwarded-\*` headers, this happens automatically. The links rendered by your app to itself being wrong (the wrong host, port, or protocol) is a sign that you got this configuration wrong. |
1.6. Eureka’s Health Checks
By default, Eureka uses the client heartbeat to determine if a client is up. Unless specified otherwise, the Discovery Client does not propagate the current health check status of the application, per the Spring Boot Actuator. Consequently, after successful registration, Eureka always announces that the application is in 'UP' state. This behavior can be altered by enabling Eureka health checks, which results in propagating application status to Eureka. As a consequence, every other application does not send traffic to applications in states other then 'UP'. The following example shows how to enable health checks for the client:
eureka:
client:
healthcheck:
enabled: true
eureka.client.healthcheck.enabled=true should only be set in application.yml. Setting the value in bootstrap.yml causes undesirable side effects, such as registering in Eureka with an UNKNOWN status.
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If you require more control over the health checks, consider implementing your own com.netflix.appinfo.HealthCheckHandler.
1.7. Eureka Metadata for Instances and Clients
It is worth spending a bit of time understanding how the Eureka metadata works, so you can use it in a way that makes sense in your platform.
There is standard metadata for information such as hostname, IP address, port numbers, the status page, and health check.
These are published in the service registry and used by clients to contact the services in a straightforward way.
Additional metadata can be added to the instance registration in the eureka.instance.metadataMap, and this metadata is accessible in the remote clients.
In general, additional metadata does not change the behavior of the client, unless the client is made aware of the meaning of the metadata.
There are a couple of special cases, described later in this document, where Spring Cloud already assigns meaning to the metadata map.
1.7.1. Using Eureka on Cloud Foundry
Cloud Foundry has a global router so that all instances of the same app have the same hostname (other PaaS solutions with a similar architecture have the same arrangement).
This is not necessarily a barrier to using Eureka.
However, if you use the router (recommended or even mandatory, depending on the way your platform was set up), you need to explicitly set the hostname and port numbers (secure or non-secure) so that they use the router.
You might also want to use instance metadata so that you can distinguish between the instances on the client (for example, in a custom load balancer).
By default, the eureka.instance.instanceId is vcap.application.instance_id, as shown in the following example:
eureka:
instance:
hostname: ${vcap.application.uris[0]}
nonSecurePort: 80
Depending on the way the security rules are set up in your Cloud Foundry instance, you might be able to register and use the IP address of the host VM for direct service-to-service calls. This feature is not yet available on Pivotal Web Services (PWS).
1.7.2. Using Eureka on AWS
If the application is planned to be deployed to an AWS cloud, the Eureka instance must be configured to be AWS-aware. You can do so by customizing the EurekaInstanceConfigBean as follows:
@Bean
@Profile("!default")
public EurekaInstanceConfigBean eurekaInstanceConfig(InetUtils inetUtils) {
EurekaInstanceConfigBean bean = new EurekaInstanceConfigBean(inetUtils);
AmazonInfo info = AmazonInfo.Builder.newBuilder().autoBuild("eureka");
bean.setDataCenterInfo(info);
return bean;
}
1.7.3. Changing the Eureka Instance ID
A vanilla Netflix Eureka instance is registered with an ID that is equal to its host name (that is, there is only one service per host). Spring Cloud Eureka provides a sensible default, which is defined as follows:
${spring.cloud.client.hostname}:${spring.application.name}:${spring.application.instance_id:${server.port}}
An example is myhost:myappname:8080.
By using Spring Cloud, you can override this value by providing a unique identifier in eureka.instance.instanceId, as shown in the following example:
eureka:
instance:
instanceId: ${spring.application.name}:${vcap.application.instance_id:${spring.application.instance_id:${random.value}}}
With the metadata shown in the preceding example and multiple service instances deployed on localhost, the random value is inserted there to make the instance unique.
In Cloud Foundry, the vcap.application.instance_id is populated automatically in a Spring Boot application, so the random value is not needed.
1.8. Using the EurekaClient
Once you have an application that is a discovery client, you can use it to discover service instances from the Eureka Server.
One way to do so is to use the native com.netflix.discovery.EurekaClient (as opposed to the Spring Cloud DiscoveryClient), as shown in the following example:
@Autowired
private EurekaClient discoveryClient;
public String serviceUrl() {
InstanceInfo instance = discoveryClient.getNextServerFromEureka("STORES", false);
return instance.getHomePageUrl();
}
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Do not use the |
1.8.1. EurekaClient with Jersey
By default, EurekaClient uses Spring’s RestTemplate for HTTP communication.
If you wish to use Jersey instead, you need to add the Jersey dependencies to your classpath.
The following example shows the dependencies you need to add:
<dependency>
<groupId>com.sun.jersey</groupId>
<artifactId>jersey-client</artifactId>
</dependency>
<dependency>
<groupId>com.sun.jersey</groupId>
<artifactId>jersey-core</artifactId>
</dependency>
<dependency>
<groupId>com.sun.jersey.contribs</groupId>
<artifactId>jersey-apache-client4</artifactId>
</dependency>
1.9. Alternatives to the Native Netflix EurekaClient
You need not use the raw Netflix EurekaClient.
Also, it is usually more convenient to use it behind a wrapper of some sort.
Spring Cloud has support for Feign (a REST client builder) and Spring Cloud LoadBalancer through the logical Eureka service identifiers (VIPs) instead of physical URLs.
You can also use the org.springframework.cloud.client.discovery.DiscoveryClient, which provides a simple API (not specific to Netflix) for discovery clients, as shown in the following example:
@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;
}
1.10. Why Is It so Slow to Register a Service?
Being an instance also involves a periodic heartbeat to the registry
(through the client’s serviceUrl) with a default duration of 30 seconds.
A service is not available for discovery by clients until the instance, the server, and the client all have the same metadata in their local
cache (so it could take 3 heartbeats).
You can change the period by setting eureka.instance.leaseRenewalIntervalInSeconds.
Setting it to a value of less than 30 speeds up the process of getting clients connected to other services.
In production, it is probably better to stick with the default, because of internal computations in the server that make assumptions about the lease renewal period.
1.11. Zones
If you have deployed Eureka clients to multiple zones, you may prefer that those clients use services within the same zone before trying services in another zone. To set that up, you need to configure your Eureka clients correctly.
First, you need to make sure you have Eureka servers deployed to each zone and that they are peers of each other. See the section on zones and regions for more information.
Next, you need to tell Eureka which zone your service is in.
You can do so by using the metadataMap property.
For example, if service 1 is deployed to both zone 1 and zone 2, you need to set the following Eureka properties in service 1:
Service 1 in Zone 1
eureka.instance.metadataMap.zone = zone1
eureka.client.preferSameZoneEureka = trueService 1 in Zone 2
eureka.instance.metadataMap.zone = zone2
eureka.client.preferSameZoneEureka = true1.12. Refreshing Eureka Clients
By default, the EurekaClient bean is refreshable, meaning the Eureka client properties can be changed and refreshed.
When a refresh occurs clients will be unregistered from the Eureka server and there might be a brief moment of time
where all instance of a given service are not available. One way to eliminate this from happening is to disable
the ability to refresh Eureka clients. To do this set eureka.client.refresh.enable=false.
1.13. Using Eureka with Spring Cloud LoadBalancer
We offer support for the Spring Cloud LoadBalancer ZonePreferenceServiceInstanceListSupplier.
The zone value from the Eureka instance metadata (eureka.instance.metadataMap.zone) is used for setting the
value of spring-cloud-loadbalancer-zone property that is used to filter service instances by zone.
If that is missing and if the spring.cloud.loadbalancer.eureka.approximateZoneFromHostname flag is set to true,
it can use the domain name from the server hostname as a proxy for the zone.
If there is no other source of zone data, then a guess is made, based on the client configuration (as opposed to the instance configuration).
We take eureka.client.availabilityZones, which is a map from region name to a list of zones, and pull out the first zone for the instance’s own region (that is, the eureka.client.region, which defaults to "us-east-1", for compatibility with native Netflix).