Need help setting BROKER_URL in Airflow's config and Celery Executor - celery

Question

Summary
I'm using Apache-Airflow for the first time. I've gotten the webserver, SequentialExecutor and LocalExecutor to work, but I'm running into issues when using the CeleryExecutor with rabbitmq-server. I currently have two AWS EC2 instances.
Error
To summarize: My worker cannot connect to the rabbitmq-server on my scheduler node. Whenever I run airflow worker on the worker instance, it gives:
- ** ---------- [config]
- ** ---------- .> app: airflow.executors.celery_executor:0x7f53a8dce400
- ** ---------- .> transport: amqp://guest:**#localhost:5672//
- ** ---------- .> results: disabled://
- *** --- * --- .> concurrency: 16 (prefork)
-- ******* ----
--- ***** ----- [queues]
-------------- .> default exchange=default(direct) key=default
[2019-02-15 02:26:23,742: ERROR/MainProcess] consumer: Cannot connect to amqp://guest:**#127.0.0.1:5672//: [Errno 111] Connection refused.
Configuration
I followed all of the directions I could find online. Both instances have the same airflow.cfg file, with
[core]
executor = CeleryExecutor
[celery]
broker_url = pyamqp://username:password#hostname:port/virtual_host
and result_backend pointing at the same MySQL database on RDS that airflow is working off of.
From what I could tell, no matter what, the worker node always tried connecting to a local rabbitmq-server and completely ignored that broker_url in my airflow.cfg file.
What I've Tried
I went spelunking in the source code, and noticed in celery/app/base.py, if I error log out the configurations it gets in _get_config() when it goes to create a connection, there are actually TWO values in the dictionary returned.
BROKER_URL = None
broker_url = pyamqp://username:password#hostname:port/virtual_host
and all of the connection logic seems to point at the BROKER_URL key.
I tried setting BROKER_URL and CELERY_BROKER_URL in airflow.cfg, but it seems to be case insensitive, and ignores the latter. Just to see if it would work, I modified the _get_config() method and hacked in:
s['BROKER_URL'] = s['broker_url']
return s
And, like I expected, everything started working.
Am I doing something wrong? I'd really rather not use this hack, but I can't understand why it's ignoring the configuration values.
Thanks!

Answer 1

From the error message, it seems like the hostname being passed in the URI is wrong:
If rabbitmq-server and worker are in different machines: instead of localhost/127.0.0.1, the hostname should be the IP address of the rabbitmq machine
If rabbitmq-server and worker are in the same machine as part of a Docker Compose application (e.g. if you took inspiration from here): the hostname should be the service name associated to the RabbitMQ image in docker-compose.yml, e.g. amqp://guest:guest#rabbitmq:5672/