4.09 - Service Discovery¶

Objectives¶

Kubernetes provides a declarative API for deploying apps, services, ingress and load balancer resources
Services, ingress, load balancer resources provide methods to route external IP network traffic into a cluster and to an application.
IP addresses can change as workloads are created and destroyed
Service discovery solves the problem of the above by allowing applications to be registered in a service registry
This can be queried by applications and end users needing access to a particular application.
Additionally, this external registry should be regularly updated whenever these resources are changed.

A service discovery solution that:
Uses DNS as a service registry
Uses FQDNs defined in Ingress, HTTPproxy and Load Balancer resources
Creates DNS entries for applications in the configured DNS server
Allows users and applications outside a cluster to use DNS to access workloads inside a cluster

External DNS:
Watches for hostnames in Kubernetes API resources
- Ingress and HTTPproxy resources are watched for host field values in each rule defined
- Load balancer resources are watched for [external-dns.alpha.kubernetes.io/hostname](http://external-dns.alpha.kubernetes.io/hostname) annotations
Creates a DNS entry in the configured DNS server that points to the ingress or Load Balancer IP
Deletes the DNS entry when the ingress / HTPProxy / LoadBalancer resources are deleted

Example configuration files are of the format external-dns-data-values-<type>.yaml
The config files determines how load balancers are watched for External DNS annotations
To have External DNS watch for Contour Ingress and HTTPproxy resources, the file used will be external-dns-data-values-rfc2136-with-contour.yaml
The following parameters are typically defined when working with a BIND server.