Dynamic Ports on Kubernetes

This tutorial will walk you through running pods using dynamic ports. This is a specialized use case and should only be used in very rare cases. If you gotta ask, then you probably don't need this.

Overview

• Each pod joins the host network

# deployments/dynamic-port-server.yaml 
spec:
  hostNetwork: true
  containers:
    ...

• Each container allocates a dynamic port by binding to port 0:

# dynamic-port-server/main.go
net.Listen("tcp4", "0.0.0.0:0")

• Each container is responsible for registering its endpoints with a service registry. Kubernetes built-in service discovery does not work with this use case.

Usage

The tutorial will leverage the following components:

• service-registry - an example registration service with basic support for health checks.
• dynamic-port-server - A simple HTTP service that binds to a dynamic port at startup time.

service-registry

The service registry provides a centralized registration service where applications can post their endpoint information. Each service record includes the following information:

• Unique service instance name (dynamic-port-server-507527175-5bphp)
• Service address (10.240.0.2:52238)

Create the service-registry deployment:

kubectl create -f deployments/service-registry.yaml

Expose the service-registry service:

kubectl create -f services/

At this point applications can register their endpoints.

dynamic-port-server

The dynamic-port-server starts a basic web server on a dynamic port and registers with the service-registry.

Create the dynamic-port-server deployment:

kubectl create -f deployments/dynamic-port-server.yaml

Review the dynamic-port-server logs:

kubectl logs $(kubectl get pods -l app=dynamic-port-server \
    -o jsonpath='{.items[0].metadata.name}')

2017/02/03 20:37:34 Starting dynamic port server...
2017/02/03 20:37:34 Listening on 0.0.0.0:43800
2017/02/03 20:37:34 Registering advertised endpoint [35.185.194.181:43800]
35.185.194.181:34724 - - [Fri, 03 Feb 2017 20:37:38 UTC] "GET /healthz HTTP/1.1" Go-http-client/1.1
...

At this point the dynamic-port-server is registered with the service-registry. You can now hit the dynamic-port-server on the advertised address.

The Service Registry UI

The service-registry exposes as basic web UI listing the registered backends. Connect to the service-registry UI:

kubectl port-forward \
  $(kubectl get pods -l app=service-registry \
    -o jsonpath='{.items[0].metadata.name}') \
  8888:80

Visit http://127.0.0.1:8888

Add more dynamic-port-server backends:

kubectl scale deployments dynamic-port-server --replicas=3

Refresh the service-registry UI: