Use Redis Sentinel for Caching/Async Queries

[maltesander]

Description

The PR #724 added celery worker support using Redis following the standard Superset example: https://superset.apache.org/admin-docs/configuration/async-queries-celery/

• This only works for RedisCache not RedisSentinelCache
• The schema used is always redis:// instead of sentinel://
• The master-name cannot be configured for sentinel nodes

So Redis in HA mode is not available without overrides (not tested).

Possible untested fix

configOverrides:
  superset_config.py:
    FILE_FOOTER: |
      from flask_caching.backends.rediscache import RedisSentinelCache
      _SENTINELS = [("redis-sentinel-0.redis-sentinel-headless", 26379),
                    ("redis-sentinel-1.redis-sentinel-headless", 26379),
                    ("redis-sentinel-2.redis-sentinel-headless", 26379)]
      _MASTER = "mymaster"
      _PW = os.environ["CELERY_RESULTS_BACKEND_DATABASE_PASSWORD"]

      RESULTS_BACKEND = RedisSentinelCache(
          sentinels=_SENTINELS, master=_MASTER, password=_PW, db=0,
          key_prefix="superset_results",
          # sentinel_password=..., only if the sentinels themselves require auth
      )

      _URL = ";".join(f"sentinel://:{_PW}@{h}:{p}" for h, p in _SENTINELS) + "/0"
      class CeleryConfig(object):
          broker_url = _URL
          result_backend = _URL
          broker_transport_options = {"master_name": _MASTER}
          result_backend_transport_options = {"master_name": _MASTER}
          imports = ("superset.sql_lab", "superset.tasks.scheduler")
          worker_prefetch_multiplier = 10
          task_acks_late = True
          task_annotations = {"sql_lab.get_sql_results": {"rate_limit": "100/s"}}
      CELERY_CONFIG = CeleryConfig

This is just a workaround and will need adaptation whenever the redis replicas change.

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