Containers launched with Docker-Compose cannot connect to each other - docker-compose

Question

I have a beginner question with Docker Compose. I am trying to extend the docker-compose-slim.yml example file from Zipkin GitHub repository.
I need to change it so that it can include a simple FastAPI app that I have written. Unfortunately, I cannot make them connect to each other. FastAPI gets rejected when it attempts to send a POST request to the Zipkin container, even though they are both connected to the same network with explicit links and port mapping defined in the YAML file. However, I am able to connect to both of them from the host, however.
Could you please tell me what I have done wrong?
Here is the error message:
Error emitting zipkin trace. ConnectionError(MaxRetryError("HTTPConnectionPool(host='127.0.0.1', port=9411): Max retries exceeded with url: /api/v2/spans (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7fce354711c0>: Failed to es
tablish a new connection: [Errno 111] **Connection refused**'))"))
Here is the Docker Compose YAML file:
version: '2.4'
services:
zipkin:
image: openzipkin/zipkin-slim
container_name: zipkin
environment:
- STORAGE_TYPE=mem
ports:
# Port used for the Zipkin UI and HTTP Api
- 9411:9411
depends_on:
- storage
storage:
image: busybox:1.31.0
container_name: fake_storage
myfastapi:
build: .
ports:
- 8000:8000
links:
- zipkin
depends_on:
- zipkin
dependencies:
image: busybox:1.31.0
container_name: fake_dependencies
networks:
default:
name: foo_network
Here is the Dockerfile:
FROM python:3.8.5
ADD . /app
WORKDIR /app
RUN pip install -r requirements.txt
EXPOSE 8000
CMD ["uvicorn", "wsgi:app", "--host", "0.0.0.0", "--port", "8000"]

Answer 1

You must tell the containers the network "foo_network". The External flag says that the containers are not accessible from outside. Of course you don't have to bet, but I thought as an example it might be quite good.
And because of the "links" function look here Link
version: '2.4'
services:
zipkin:
image: openzipkin/zipkin-slim
container_name: zipkin
environment:
- STORAGE_TYPE=mem
ports:
# Port used for the Zipkin UI and HTTP Api
- 9411:9411
depends_on:
- storage
networks:
- foo_network
storage:
image: busybox:1.31.0
container_name: fake_storage
networks:
- foo_network
myfastapi:
build: .
ports:
- 8000:8000
links:
- zipkin
depends_on:
- zipkin
networks:
- foo_network
dependencies:
image: busybox:1.31.0
container_name: fake_dependencies
networks:
- foo_network
networks:
foo_network:
external: false