Maximum clients reached on Heroku and Redistogo Nano - heroku

Question

I am using celerybeat on Heroku with RedisToGo Nano addon
There is one web dyno and one worker dyno
The celerybeat worker is set to perform a task every minute.
The problem is: Whenever I deploy a new commit, dynos restart, and I get this error
2014-02-27T13:19:31.552352+00:00 app[worker.1]: Traceback (most recent call last):
2014-02-27T13:19:31.552352+00:00 app[worker.1]: File "/app/.heroku/python/lib/python2.7/site-packages/celery/worker/consumer.py", line 389, in start
2014-02-27T13:19:31.552352+00:00 app[worker.1]: self.reset_connection()
2014-02-27T13:19:31.552352+00:00 app[worker.1]: File "/app/.heroku/python/lib/python2.7/site-packages/celery/worker/consumer.py", line 727, in reset_connection
2014-02-27T13:19:31.552352+00:00 app[worker.1]: self.connection = self._open_connection()
2014-02-27T13:19:31.552352+00:00 app[worker.1]: File "/app/.heroku/python/lib/python2.7/site-packages/celery/worker/consumer.py", line 792, in _open_connection
2014-02-27T13:19:31.552352+00:00 app[worker.1]: callback=self.maybe_shutdown)
2014-02-27T13:18:23.864287+00:00 app[worker.1]: self.on_connect()
2014-02-27T13:18:23.864287+00:00 app[worker.1]: File "/app/.heroku/python/lib/python2.7/site-packages/redis/connection.py", line 263, in on_connect
2014-02-27T13:18:23.864287+00:00 app[worker.1]: if nativestr(self.read_response()) != 'OK':
2014-02-27T13:18:23.864287+00:00 app[worker.1]: File "/app/.heroku/python/lib/python2.7/site-packages/redis/connection.py", line 314, in read_response
2014-02-27T13:18:23.864287+00:00 app[worker.1]: raise response
2014-02-27T13:18:23.864287+00:00 app[worker.1]: ResponseError: max number of clients reached
2014-02-27T13:19:31.552352+00:00 app[worker.1]: File "/app/.heroku/python/lib/python2.7/site-packages/kombu/connection.py", line 272, in ensure_connection
2014-02-27T13:19:31.552352+00:00 app[worker.1]: interval_start, interval_step, interval_max, callback)
2014-02-27T13:19:31.552591+00:00 app[worker.1]: File "/app/.heroku/python/lib/python2.7/site-packages/kombu/utils/__init__.py", line 218, in retry_over_time
2014-02-27T13:19:31.552591+00:00 app[worker.1]: return fun(*args, **kwargs)
2014-02-27T13:19:31.552591+00:00 app[worker.1]: File "/app/.heroku/python/lib/python2.7/site-packages/kombu/connection.py", line 162, in connect
2014-02-27T13:19:31.552591+00:00 app[worker.1]: return self.connection
2014-02-27T13:19:31.552591+00:00 app[worker.1]: File "/app/.heroku/python/lib/python2.7/site-packages/kombu/connection.py", line 617, in connection
2014-02-27T13:18:23.870811+00:00 app[worker.1]: [2014-02-27 13:18:23,870: ERROR/MainProcess] consumer: Connection to broker lost. Trying to re-establish the connection...
and those logs go on endlessly. till I stop both dynos and restart them.
It has become a problem because it happens almost every time a new commit is deployed.
Any ideas why this is happening and how to solve this?

Answer 1

The nano redistogo plan caps concurrent redis connections at 10.
The number of redis connects used will vary based on your front-end and celery worker settings. It sounds like you are using >= 5 redis connections for your production stack.
When you deploy new code, Heroku spins up an entirely new stack. This means you are using >= 10 redis connections at the time of deploy.
There are two ways to fix this:
Increase the maximum number of redistogo connections allowed, by upgrading to a larger plan ($$$)
Decrease the number of used connections for your stack (decrease celery concurrency or redis connections used by your web worker)
This is a simple matter of resource exhaustion. I would just pay for a larger RedisToGo plan.