Improvements in v3 for Eureka / ZUUL to reduce the delays between onboarding/instance updates and routing

[Shobhajayanna]

There is an implementation in v2 and v1 to improve the communication of services & Eureka client, which is yet to be implemented in v3.

once a service updated registry in the Discovery service
the Eureka caches could be cleaned
the Gateways could be notified (reverting of GatewayNotifier)
once Gateway is notified, a gateway could call applicationEventPublisher.publishEvent(new RefreshRoutesEvent("List of services changed")); to regenerate routing issue

[balhar-jakub]

The implementation would need to be different.

[pj892031]

This issue was created because of #3617. In the PR was removed document https://github.com/zowe/api-layer/blob/v2.x.x/docs/registry-communication.md. It described how was improved the communication in v2 (cleaning caches, direct notification, and almost immediate change of routing rules). The document describes not just the idea, but also the technology. The document is not valid anymore (GatewayNotifier, ZUUL, and Ribon were removed). Although that is possible to implement a similar approach in v3, just missing pieces should be replaced.

The aim of this issue is to consider benefits and could lead to implementing such a feature...