Celery + Flask + Docker, consumer: Cannot connect to amqp://admin:**@rabbit:5672/myhost: failed to resolve broker hostname

Background

I am building a web application that uses Flask for the backend framework. The application uses Celery to handle all the time-consuming tasks as background tasks as to not block the backend thread. I use RabbitMQ as the message broker for Celery workers. I bundled each service using docker-compose.

Problem

The app has been working well until the past few days, and all of sudden, Celery workers keep failing to connect to the message broker with the error message [ERROR/MainProcess] consumer: Cannot connect to amqp://admin:**@rabbit:5672/myhost: failed to resolve broker hostname.

Directory structure and code

I put together files and directories for minimally reproducible example.

debug/
├── code
│   ├── dev.Dockerfile
│   ├── my_app
│   │   ├── celery_app.py
│   │   ├── config.py
│   │   ├── extensions.py
│   │   ├── __init__.py
│   │   ├── my_tasks.py
│   │   └── test_app.py
│   └── requirements.txt
└── docker-compose_dev.yml

docker-compose_dev.yml

version: "3.7"
services:
  rabbit:
    image: rabbitmq:3.8.5-management
    ports:
      - '15673:15672' # in case user has rabbitMQ installed on host
    expose:
      - "5672"
    environment:
      - RABBITMQ_DEFAULT_USER=admin
      - RABBITMQ_DEFAULT_PASS=mypass
      - RABBITMQ_DEFAULT_VHOST=myhost

  non_working_worker:
    build:
      context: ./code
      dockerfile: dev.Dockerfile
    command: "celery worker -A my_app.celery_app:app -l info"
    volumes:
      - ./code:/code
    links:
      - rabbit

  working_worker:
    build:
      context: ./code
      dockerfile: dev.Dockerfile
    command: "celery worker -A my_app.my_tasks:app -l info"
    volumes:
      - ./code:/code
    links:
      - rabbit

dev.Dockerfile

FROM continuumio/miniconda3

# Make /backend working directory; flask code lives here
WORKDIR /code

# Install from requirements.txt using pip
COPY requirements.txt requirements.txt
RUN pip install -r requirements.txt
RUN rm requirements.txt

requirements.txt

luigi==2.8.11
plotnine==0.7.0
celery==4.4.6
flask==1.1.2
flask-cors
flask-socketio
Flask-Mail
eventlet

test_app.py

import eventlet
eventlet.monkey_patch()

from flask import Flask
from my_app.extensions import celery

def create_app():
    """
    Application factory. Create application here.
    """
    app = Flask(__name__)
    app.config.from_object("my_app.config")

    return app

def init_celery(app=None):
    """
    Initialize Celery App
    """
    app = app or create_app()
    app.config.from_object("my_app.config")

    # Set celery worker configuration
    # Use this to load config information from flask config file
    celery.conf.broker_url = app.config["CELERY_BROKER_URL"]
    celery.conf.result_backend = app.config["CELERY_RESULT_BACKEND"]

    class ContextTask(celery.Task):
        """Make celery tasks work with Flask app context"""

        def __call__(self, *args, **kwargs):
            with app.app_context():
                return self.run(*args, **kwargs)

    celery.Task = ContextTask

    return celery

config.py

# RabbitMQ
CELERY_BROKER_URL='pyamqp://admin:mypass@rabbit/myhost'
CELERY_RESULT_BACKEND='rpc://'

extensions.py

from celery import Celery

celery = Celery()

celery_app.py

from my_app.test_app import init_celery

app = init_celery()

my_tasks.py

from celery import Celery


app = Celery()

app.conf.broker_url = 'pyamqp://admin:mypass@rabbit/myhost'
app.conf.result_backend = 'rpc://'

What I've tried

Followings are the things I've tried, but didn't work.

1. RabbitMQ isn't launching properly?
   • a. It launches properly with given username, password, and vhost. (can check using the management plugin @ localhost:15673)
2. RabbitMQ launches after the Celery workers start, so the workers can't find the broker?
   • a. Celery has retry feature, so it will keep on retrying until message broker is up running.
3. Network issue?
   • a. I've tried with/without links to specify service name alias, but still didn't work.
   • b. Note I've already specified broker name as rabbit as specified in the config.py file instead of localhost
   • c. I've tried using both the default network docker-compose creates, and custom network, but both failed.
4. Interestingly, Celery app instance in my_tasks.py works (it's named as working_worker in the docker-compose file), but Celery app instance in the Flask factory pattern does not work (it'a named as non_working_worker in the compose file)
   • a. Again, it shows that RabbitMQ is working fine, but something funky is going on with the Flask factory pattern style Celery app instantiation.

I spent past few days trying to fix this issue and searching for similar problems on internet, but had no luck doing so.

I know it's a fairly long post, but any help/suggestions would greatly be appreciated.

docker-compose version

docker-compose version 1.25.3, build d4d1b42b
docker-py version: 4.1.0
CPython version: 3.7.5
OpenSSL version: OpenSSL 1.1.0l  10 Sep 2019

docker version

Client: Docker Engine - Community
 Version:           19.03.12
 API version:       1.40
 Go version:        go1.13.10
 Git commit:        48a66213fe
 Built:             Mon Jun 22 15:45:36 2020
 OS/Arch:           linux/amd64
 Experimental:      false

Server: Docker Engine - Community
 Engine:
  Version:          19.03.12
  API version:      1.40 (minimum version 1.12)
  Go version:       go1.13.10
  Git commit:       48a66213fe
  Built:            Mon Jun 22 15:44:07 2020
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.2.13
  GitCommit:        7ad184331fa3e55e52b890ea95e65ba581ae3429
 runc:
  Version:          1.0.0-rc10
  GitCommit:        dc9208a3303feef5b3839f4323d9beb36df0a9dd
 docker-init:
  Version:          0.18.0
  GitCommit:        fec3683