Use Traefik for multi-apps with a same domain

I was working with a front end app which sends request to multiple back ends, and I wanted to use a same domain for all of them. For example, I have a front end app which sends requests to api1/messages, api2/persons, api3/shops... and I want to use a same domain api.example.com for all of them with different paths. I found that Traefik is a good solution for this case, which can be used as a reverse proxy and load balancer for multiple apps.

Usecase

Having 2 flask apps:

• app-1 with a route /hello
• app-2 with a route world

I want to use a same domain api.localhost for both of them, with different paths as api.localhost/app-1/hello and api.localhost/app-2/world. The app-1 and app-2 are running in different containers in a same network. With Traefik, I don't need to care the ports of each app, I just need to care about the paths.

Setup

An example flask app

# app.py for app-1

from flask import Flask

app = Flask(__name__)


@app.route("/hello", methods=["GET"])
def hello():
    return "Hello from app_1"


if __name__ == "__main__":
    app.run(host='0.0.0.0',debug=True, port=5001)

This flask app has a route /hello which sends back a string Hello from app_1. For the port, I set it to 5001, but it can be any port. Here's a simple Dockerfile for this app:

FROM python:3.11

WORKDIR /app

COPY requirements.txt .
RUN pip install -r requirements.txt

COPY app.py .

EXPOSE 5001

CMD ["python", "app.py"]

The app-2 is similar to app-1 with a route /world, which sends back a string World from app_2. I set 5001 port for both apps, but it can be any port whether it's the same or different.

Traefik

I need to set the traefik as a reverse proxy for both apps. I use docker-compose to run both apps and traefik in a same network. Here's the docker-compose.yml:

version: '3'

services:
  reverse-proxy:
    # The official v2 Traefik docker image
    image: traefik:v2.10
    # Enables the web UI and tells Traefik to listen to docker
    command: --api.insecure=true --providers.docker
    ports:
      # The HTTP port
      - '80:80'
      # The Web UI (enabled by --api.insecure=true)
      - '8080:8080'
    volumes:
      # So that Traefik can listen to the Docker events
      - /var/run/docker.sock:/var/run/docker.sock

  app-1:
    build:
      context: ./app_1
      dockerfile: Dockerfile
    container_name: app-1
    labels:
      - 'traefik.enable=true'
      - traefik.http.routers.app-1.rule=Host(`api.localhost`) && PathPrefix(`/app-1`)
      - 'traefik.http.services.app-1.loadbalancer.server.port=5001'
      - 'traefik.http.middlewares.app-1-stripprefix.stripprefix.forceslash=true'
      - 'traefik.http.middlewares.app-1-stripprefix.stripprefix.prefixes=/app-1'
      - 'traefik.http.routers.app-1.middlewares=app-1-stripprefix'

  app-2:
    build:
      context: ./app_2
      dockerfile: Dockerfile
    container_name: app-2
    labels:
      - 'traefik.enable=true'
      - traefik.http.routers.app-2.rule=Host(`api.localhost`) && PathPrefix(`/app-2`)
      - 'traefik.http.services.app-2.loadbalancer.server.port=5001'
      - 'traefik.http.middlewares.app-2-stripprefix.stripprefix.forceslash=true'
      - 'traefik.http.middlewares.app-2-stripprefix.stripprefix.prefixes=/app-2'
      - 'traefik.http.routers.app-2.middlewares=app-2-stripprefix'

Let's go through the docker-compose.yml:

• the first part reverse-proxy: the traefik service. I expose the port 8080 for the traefik dashboard.
• the second part app-1: the app-1 service. I set the traefik as a reverse proxy for this service with the labels:
  • traefik.enable=true: enable the traefik for this service
  • traefik.http.routers.app-1.rule=Host('api.localhost') && PathPrefix('/app-1'): set the rule for the router. The router will listen to the request with the host api.localhost and the path prefix /app-1. To support different paths that point to each app we can use the same rule but add a PathPrefix like this label, which tells Traefik to match a request if it has the same host and the /app-1 prefix.
  • traefik.http.services.app-1.loadbalancer.server.port=5001: set the port of the app-1 service. The port can be any port, but it should be the same with the port of the app-1 service which it is listening to.
  • traefik.http.middlewares.app-1-stripprefix.stripprefix.forceslash=true: create a middleware with name "app-1-stripprefix". The middleware will strip the prefix from the request before sending it to the app-1 service(because our flask app doesn't have a route like /app-1/*). The forceSlash option ensures the resulting stripped path is not the empty string, by replacing it with / when necessary.

    Removing Prefixes From the Path Before Forwarding the Request, see the docs.

  • traefik.http.middlewares.app-1-stripprefix.stripprefix.prefixes=/app-1: set the prefix to be stripped from the request for the middleware. The prefix can be any prefix, but it should be the same with the path prefix of the router.
  • traefik.http.routers.app-1.middlewares=app-1-stripprefix: set the middleware "app-1-stripprefix" for the router.

The app-2 service is similar to the app-1 service, but only with different names for service and its middleware to cut the prefix.

Run

Launch the docker-compose command, you can see the traefik dashboard at localhost:8080:

We can access the app-1 with the path api.localhost/app-1/hello which returns the string Hello from app_1. Same with the app-2 with the path api.localhost/app-2/world which returns the string World from app_2.