Docker Compose: Mock external services

Question

I have the following situation:

My application consists of a single web service that calls an external API (say, some SaaS service, ElasticSearch or so). For non-unit-testing purposes we want to control the external service and later also inject faults. The application and the "mocked" API are dockerized and now I want to use docker-compose to spin all containers up.

Because the application has several addresses hardcoded (e.g. the hostname of external services) I cannot change them and need to work around.

The service container makes a call to http://external-service.com/getsomestuff.

My idea was to use some features that are provided by docker to reroute all outgoing traffic to the external http://external-service.com/getsomestuff to the mock container without changing the URL.

My docker-compose.yaml looks like:

version: '2'
services:
  service:
    build: ./service
    container_name: my-service1
    ports:
      - "5000:5000"
    command: /bin/sh -c "python3 app.py"

  api:
    build: ./api-mock
    container_name: my-api-mock
    ports:
      - "5001:5000"
    command: /bin/sh -c "python3 app.py"

Finally, I have a driver that just does the following:

curl -XGET localhost:5000/
curl -XPUT localhost:5001/configure?delay=10
curl -XGET localhost:5000/

where the second curl just sets the delay in the mock to 10 seconds.

There are several options I have considered:

• Using iptables-fu (would require modifying Dockerfiles to install it)
• Using docker networks (this is really unclear to me)

Is there any simple option to achieve what I want?

Edit:

For clarity, here is the relevant part of the service code:

import requests

@app.route('/')
def do_stuff():
    r = requests.get('http://external-service.com/getsomestuff')
    return process_api_response(r.text())

Answer 1

Docker runs an internal DNS server for user defined networks. Any unknown host lookups are forwarded to you normal DNS servers.

Version 2+ compose files will automatically create a network for compose to use so there's a number of ways to control the hostnames it resolves.

The simplest way is to name your container with the hostname:

version: "2"
services:
  external-service.com:
    image: busybox
    command: sleep 100
  ping:
    image: busybox
    command: ping external-service.com
    depends_on:
      - external-service.com

If you want to keep container names you can use links

version: "2"
services:
  api:
    image: busybox
    command: sleep 100
  ping:
    image: busybox
    links:
      - api:external-service.com
    command: ping external-service.com
    depends_on:
      - api

Or network aliases

version: "2"
services:
  api:
    image: busybox
    command: sleep 100
    networks:
      pingnet:
        aliases:
        - external-service.com
  ping:
    image: busybox
    command: ping external-service.com
    depends_on:
      - api
    networks:
      - pingnet
networks:
  pingnet: