Redis Sentinel: How to refresh master client object when master changes on failover?

[Kaushal28]

Version: redis-py: 3.5.3 and redis: 6.2

Platform: Mac OS Big Sur

Description:

I have setup a Redis replica with one master and two slaves with Redis Sentinel monitoring each of the nodes. I'm using redis-py library to communicate with master and slaves. Here is the code that I use to connect with master and slaves:

from redis.sentinel import Sentinel

sentinel = Sentinel([("localhost", 5000)])  # connect to one of the sentinels
master = sentinel.master_for("mymaster")  # get master object
slave = sentinel.slave_for("mymaster")  # get slave object

Now, I would use master for write and slave for reads. But the problem is that when failover occurs, the master object needs to be refreshed in order to point to a new master. For that, as mentioned in this answer and the docs, I'm supposed to subscribe to +switch-master channel to receive the address of new master.

But the question is that, using which object am I supposed to subscribe to that channel? I tried this using master object as follows:

    ps = master.pubsub()
    ps.subscribe("+switch-master")

And when the master was down, I tried getting the message as following:

ps.get_message()

But this does not return the expected message because the master itself is down I think.

The sentinel object does not support the pubsub.

    >>> sentinel.pubsub()
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    AttributeError: 'Sentinel' object has no attribute 'pubsub'

As an alternative, in the linked answer it's suggested to use client-reconfig-script. But how am I supposed to modify an object in my application using an external script?

What am I missing here? I am new to Redis, so any reference would be appreciated.

EDIT

However, if I login into one of the sentinels and then subscribe over the "+switch-master" channel, it receives the message successfully. So How do it do this using redis-py? Is there a way to achieve this currently?

Here is the output from one of the sentinels:

127.0.0.1:5001> SUBSCRIBE +switch-master
Reading messages... (press Ctrl-C to quit)
1) "subscribe"
2) "+switch-master"
3) (integer) 1

1) "message"
2) "+switch-master"
3) "mymaster 127.0.0.1 6379 127.0.0.1 6381"

[tsonglew]

My sentinel is on localhost:26379, and I got the +switch-master message in this way:

>>> from redis import Redis
>>> r = Redis(host='localhost', port=26379)
>>> p = r.pubsub()
>>> p.subscribe("+switch-master")
>>> p.get_message()
{'type': 'subscribe', 'pattern': None, 'channel': b'+switch-master', 'data': 1}

# failover the current master here

>>> p.get_message()
{'type': 'message', 'pattern': None, 'channel': b'+switch-master', 'data': b'mymaster 172.22.0.2 6379 172.22.0.3 6379'}

[github-actions]

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