How to connect k8s, redis sentinel, flask_caching? - flask

Question

I deployed a k8s redis sentinel using the Helm chart: https://github.com/bitnami/charts/tree/master/bitnami/redis
I did change only these values ( https://github.com/bitnami/charts/blob/master/bitnami/redis/values.yaml ) :
auth:
enabled: false
sentinel: false
sentinel:
enabled: true
masterSet: mymaster
After the deployment, I got this message:
Redis™ can be accessed via port 6379 on the following DNS name from within your cluster:
redis.default.svc.cluster.local for read only operations
For read/write operations, first access the Redis™ Sentinel cluster, which is available in port 26379 using the same domain name above.
To connect to your Redis™ server:
1. Run a Redis™ pod that you can use as a client:
kubectl run --namespace default redis-client --restart='Never' --image docker.io/bitnami/redis:6.2.6-debian-10-r103 --command -- sleep infinity
Use the following command to attach to the pod:
kubectl exec --tty -i redis-client \
--namespace default -- bash
2. Connect using the Redis™ CLI:
redis-cli -h redis -p 6379 # Read only operations
redis-cli -h redis -p 26379 # Sentinel access
To connect to your database from outside the cluster execute the following commands:
kubectl port-forward --namespace default svc/redis 6379:6379 &
redis-cli -h 127.0.0.1 -p 6379
This is working nicely:
kubectl get pods
NAME READY STATUS RESTARTS AGE
redis-node-0 2/2 Running 0 2m23s
redis-node-1 2/2 Running 0 71s
redis-node-2 2/2 Running 0 43s
But regarding access - to summarize - I have two options to access redis:
read-only access at redis.default.svc.cluster.local:6379
read-write access at redis.default.svc.cluster.local:26379 (some kind of sentinel access, in the docs:
Master-Replicas with Sentinel
When installing the chart with architecture=replication and sentinel.enabled=true, it will deploy a Redis™ master StatefulSet (only one master allowed) and a Redis™ replicas StatefulSet. In this case, the pods will contain an extra container with Redis™ Sentinel. This container will form a cluster of Redis™ Sentinel nodes, which will promote a new master in case the actual one fails. In addition to this, only one service is exposed:
Redis™ service: Exposes port 6379 for Redis™ read-only operations and port 26379 for accessing Redis™ Sentinel.
For read-only operations, access the service using port 6379. For write operations, it's necessary to access the Redis™ Sentinel cluster and query the current master using the command below (using redis-cli or similar):
SENTINEL get-master-addr-by-name <name of your MasterSet. e.g: mymaster>
This command will return the address of the current master, which can be accessed from inside the cluster.
In case the current master crashes, the Sentinel containers will elect a new master node.
Now I want to connect my Flask caching module to it: https://flask-caching.readthedocs.io/en/latest/
As you can see, there is an option to connect to redis sentinel, however I have no idea how. This is the code I have:
from flask_caching import Cache
cache = Cache(app, config={
'CACHE_TYPE': 'RedisSentinelCache',
'CACHE_REDIS_SENTINELS': ['redis.default.svc.cluster.local'],
'CACHE_REDIS_SENTINEL_MASTER': 'mymaster'}
)
My questions are:
What should be in param CACHE_REDIS_SENTINELS? Should I somehow get IP addresses of each node and get those there?
What should be in param CACHE_REDIS_SENTINEL_MASTER? Is it "mymaster" (sentinel -> masterSet?)
Should I connect always to the read-write server (in this case, will other replicas be used)? Or do I need to adjust my app in this way: if I write I always use the sentinel access at port 26379, and in the case that I read I connect always to the read-only 6379 port? Do I need to maintain 2 connections?
Thank you
EDIT: I was digging into the code of flask_caching and it seems this works OK (but I am not sure if replicas are used):
import time
from flask import Flask
from flask_caching import Cache
config = {
"DEBUG": True, # some Flask specific configs
'CACHE_TYPE': 'RedisSentinelCache',
'CACHE_REDIS_SENTINELS': [
['redis.default.svc.cluster.local', 26379]
],
'CACHE_REDIS_SENTINEL_MASTER': 'mymaster'
}
app = Flask(__name__)
# tell Flask to use the above defined config
app.config.from_mapping(config)
cache = Cache(app)
#app.route("/")
#cache.cached(timeout=5)
def index():
return "%d\n" % time.time()
app.run()
EDIT2:
Indeed, a bit digging into flask_caching and it uses replicas as well:
in file flask_caching/backends/rediscache.py
The code is getting hosts for write and read access:
self._write_client = sentinel.master_for(master)
self._read_clients = sentinel.slave_for(master)
Cheers!
EDIT3:
Example with redis driver:
from redis.sentinel import Sentinel
sentinel = Sentinel([('redis.default.svc.cluster.local', 26379)])
redis_conn = sentinel.master_for('mymaster')
redis_conn_read = sentinel.slave_for('mymaster')
redis_conn.set('test', 'Hola!')
print(redis_conn_read.get('test'))

Answer 1

TLDR:
import time
from flask import Flask
from flask_caching import Cache
config = {
"DEBUG": True, # some Flask specific configs
'CACHE_TYPE': 'RedisSentinelCache',
'CACHE_REDIS_SENTINELS': [
['redis.default.svc.cluster.local', 26379]
],
'CACHE_REDIS_SENTINEL_MASTER': 'mymaster'
}
app = Flask(__name__)
# tell Flask to use the above defined config
app.config.from_mapping(config)
cache = Cache(app)
#app.route("/")
#cache.cached(timeout=5)
def index():
return "%d\n" % time.time()
app.run()
For more details see my original question (EDIT and EDIT2).