Spring cloud eureka client to multiple eureka servers - spring-cloud

Question

I am able to get the eureka server to operate in a peer to peer mode. But one thing I am curious about is how do I get a service discovery client to register to multiple eureka servers.
My use case is this:
Say I have a service registering to one of the eureka servers (e.g. server A) and that registration is replicated to its peer. The service is actually pointing at server A. If server A goes down, and the client expects to renew with server A, how do the renewal work if server A is no longer present.
Do I need to register with both and if not then how does the renewal happen if the client cannot communicate with server A. Does it have some knowledge of server B (from its initial and/or subsequent comms with A) and fail over to do its registration renewal there? That is not clear in any of the docs and I need to verify
So based on the answer, I added the following to my application.yml
eureka:
# these are settings for the client that gets services
client:
# enable these two settings if you want discovery to work
registerWithEureka: true
fetchRegistry: true
serviceUrl:
defaultZone: http://localhost:8762/eureka/, http://localhost:8761/eureka/
It only registers to the first in the comma separated list. If I switch them around the registration flips between eureka servers.
I can see that it does separate these based on comma but my guess is that Eureka does not use this underneath (from EurekaClientConfigBean.java)
#Override
public List<String> getEurekaServerServiceUrls(String myZone) {
String serviceUrls = this.serviceUrl.get(myZone);
if (serviceUrls == null || serviceUrls.isEmpty()) {
serviceUrls = this.serviceUrl.get(DEFAULT_ZONE);
}
if (serviceUrls != null) {
return Arrays.asList(serviceUrls.split(","));
}
return new ArrayList<>();
}

Answer 1

I just reviewed the source code for Eureka 1.1.147. It works differently that i expected but at least I know now.
You can put multiple service urls in the set
serviceUrl:
defaultZone: http://localhost:8762/eureka/, http://localhost:8761/eureka/
But the register action only uses the first one to register. There remaining are used only if attempting to contact the first fails.
from (DiscoveryClient.java)
/**
* Register with the eureka service by making the appropriate REST call.
*/
void register() {
logger.info(PREFIX + appPathIdentifier + ": registering service...");
ClientResponse response = null;
try {
response = makeRemoteCall(Action.Register);
isRegisteredWithDiscovery = true;
logger.info(PREFIX + appPathIdentifier + " - registration status: "
+ (response != null ? response.getStatus() : "not sent"));
} catch (Throwable e) {
logger.error(PREFIX + appPathIdentifier + " - registration failed"
+ e.getMessage(), e);
} finally {
if (response != null) {
response.close();
}
}
}
which calls
private ClientResponse makeRemoteCall(Action action) throws Throwable {
return makeRemoteCall(action, 0);
}
It only calls the backup when an exception is thrown in the above makeRemoteCall(action, 0) call
} catch (Throwable t) {
closeResponse(response);
String msg = "Can't get a response from " + serviceUrl + urlPath;
if (eurekaServiceUrls.get().size() > (++serviceUrlIndex)) {
logger.warn(msg, t);
logger.warn("Trying backup: " + eurekaServiceUrls.get().get(serviceUrlIndex));
SERVER_RETRY_COUNTER.increment();
return makeRemoteCall(action, serviceUrlIndex);
} else {
ALL_SERVER_FAILURE_COUNT.increment();
logger.error(
msg
+ "\nCan't contact any eureka nodes - possibly a security group issue?",
t);
throw t;
}
So you can't really register to two eureka servers simultaneously from this code. Unless I missed something.

Answer 2

Your client application should be provided a list of Eureka URLs. The URLs are comma separated.

Answer 3

Yes, as per the documentation, the flow is:
client registers to the first available eureka server
registrant information is replicated between eureka server nodes.
So multiple registration is not only not needed but should be avioded.

Answer 4

Please add below property in application.property or application.yml file
eureka.client.service-url.defaultZone = http://localhost:8761/eureka,http://localhost:8762/eureka
Services will be registered with both eureka server.If one eureka server is down then request will be served from other eureka server.

Answer 5

If you want to a workaround to register on multiple eureka servers.
please review my answer on similar question: https://stackoverflow.com/a/60714917/7982168