Thread by @iximiuz

How to Expose Multiple Containers On the Same Port

First off, why you may need it:

- Load Balancing - more containers mean more capacity
- Redundancy - if one container dies, there won't be downtime
- Single Facade - run multiple apps behind one frontend

Interested? Read on!🔽

Docker doesn't support binding multiple containers to the same host port.

Instead, it suggests using an extra container with a reverse proxy like Nginx, HAProxy, or Traefik.

Here are two ways you can trick Docker and avoid adding the reverse proxy:

1. SO_REUSEPORT
2. iptables

Multiple Containers On the Same Port w/o Proxy (I)

1) Use SO_REUSEPORT sockopt for your server sockets
2) Run containers with `--network host` and the same port

SO_REUSEPORT allows binding diff processes to the same port.
--network host puts all containers on one network stack.

Multiple Containers On the Same Port w/o Proxy (II)

`--network host` reduces containers' isolation. Here is an alternative that keeps containers in separate network namespaces.

1) Pick an ip:port on the host
2) Replace (ip:port) in packets with (cont_ip:port) using iptables NAT

Check out this article for working examples of both techniques.

Continue with the thread to learn how to expose multiple containers on the same port with a reverse proxy. 🔽

iximiuz.com/en/posts/multi…

SO_REUSEPORT and iptables pros:

- no overhead
- you feel like a hacker

SO_REUSEPORT and iptables cons:

- almost no control on load balancing - only weights
- you have to be a hacker...

For more flexible load balancing and extra peace of mind, you can use a Reverse Proxy 🔽

Multiple Containers On the Same Port with Reverse Proxy

You can use

- Nginx
- Apache
- HAProxy
- Envoy Proxy
- Traefik (my favorite)
- etc.

A proxy can work on L4 (TCP, UDP) or L7 (HTTP, or others).

HTTP proxy allows routing different requests to different upstream containers

Multiple Containers On the Same Port - Dynamic Service Discovery

If the set of upstream containers is volatile, you'll need to update the proxy config on the fly.

For Nginx, you can use github.com/nginx-proxy/ng….

But Traefik does it right out of the box!

iximiuz.com/en/posts/traef…