Access indexing service using a proper web client

[groldan]

org.fao.geonet.searching.controller.EsHttpProxy uses manual http connection handling to query the indexing service at the URL defined by ${gn.index.url}.

In order for the app to be "cloud native", it must do so in a way that supports automatic client-side load balancing, so that the value of ${gn.index.url} can be the name of the indexing service, and the load-balanced http client can discern which service instance to hit (usually it has a cache of service instances retrieved from discovery-service).

The easiest way of achieving so is using spring's WebClient (see https://spring.io/guides/gs/spring-cloud-loadbalancer/)
The recommended way is to create an OpenFeign client for the target service, which under the hood will use WebClient

[fxprunayre]

In order for the app to be "cloud native", it must do so in a way that supports automatic client-side load balancing, so that the value of ${gn.index.url} can be the name of the indexing service, and the load-balanced http client can discern which service instance to hit (usually it has a cache of service instances retrieved from discovery-service).

The search app needs only the Elasticsearch instance. For the time being we have usually only one, but the idea is to have a cluster of 3 at least.

When using Java API, you can select an array of node to connect to
https://www.elastic.co/guide/en/elasticsearch/client/java-rest/7.9/java-rest-high-getting-started-initialization.html. So maybe we should have at some point similar config for the HTTP proxy? or a route which balance across nodes?

So the current situation is

• Search app A > ES node1
• Search app B > ES node1

And we should move to something like

• Gateway /search >
  • Search app A > ES node1
  • Search app B > ES node2
    or even
• Gateway /search >
  • Search app AorB > LoadBalance > ES node1or2

So the actions would be:

• Gateway / Register /search route
• Gateway / Config to balance across services
• ES / Setup cluster
• Search app / Define how to balance on ES nodes