Understanding Spring Cloud Eureka Server self preservation and renew threshold

I got the same question as @codependent met, I googled a lot and did some experiment, here I come to contribute some knowledge about how Eureka server and instance work.

Every instance needs to renew its lease to Eureka Server with frequency of one time per 30 seconds, which can be define in eureka.instance.leaseRenewalIntervalInSeconds.

Renews (last min): represents how many renews received from Eureka instance in last minute

Renews threshold: the renews that Eureka server expects received from Eureka instance per minute.

For example, if registerWithEureka is set to false, eureka.instance.leaseRenewalIntervalInSeconds is set to 30 and run 2 Eureka instance. Two Eureka instance will send 4 renews to Eureka server per minutes, Eureka server minimal threshold is 1 (written in code), so the threshold is 5 (this number will be multiply a factor eureka.server.renewalPercentThreshold which will be discussed later).

SELF PRESERVATION MODE: if Renews (last min) is less than Renews threshold, self preservation mode will be activated.

So in upper example, the SELF PRESERVATION MODE is activated, because threshold is 5, but Eureka server can only receive 4 renews/min.

1. Question 1:

The SELF PRESERVATION MODE is design to avoid poor network connectivity failure. Connectivity between Eureka instance A and B is good, but B is failed to renew its lease to Eureka server in a short period due to connectivity hiccups, at this time Eureka server can't simply just kick out instance B. If it does, instance A will not get available registered service from Eureka server despite B is available. So this is the purpose of SELF PRESERVATION MODE, and it's better to turn it on.

2. Question 2:

The minimal threshold 1 is written in the code. registerWithEureka is set to false so there will be no Eureka instance registers, the threshold will be 1.

In production environment, generally we deploy two Eureka server and registerWithEureka will be set to true. So the threshold will be 2, and Eureka server will renew lease to itself twice/minute, so RENEWALS ARE LESSER THAN THRESHOLD won't be a problem.

3. Question 3:

Yes, you are right. eureka.instance.leaseRenewalIntervalInSeconds defines how many renews sent to server per minute, but it will multiply a factor eureka.server.renewalPercentThreshold mentioned above, the default value is 0.85.

4. Question 4:

Yes, it's normal, because the threshold initial value is set to 1. So if registerWithEureka is set to false, renews is always below threshold.

I have two suggestions for this:

1. Deploy two Eureka server and enable registerWithEureka.
2. If you just want to deploy in demo/dev environment, you can set eureka.server.renewalPercentThreshold to 0.49, so when you start up a Eureka server alone, threshold will be 0.

I've created a blog post with the details of Eureka here, that fills in some missing detail from Spring doc or Netflix blog. It is the result of several days of debugging and digging through source code. I understand it's preferable to copy-paste rather than linking to an external URL, but the content is too big for an SO answer.