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Readonly_mode.md

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READONLY mode

By default, Redis Cluster always returns MOVE redirection response on accessing slave node. You can overcome this limitation for scaling read with READONLY mode.

redis-py-cluster also implements this mode. You can access slave by passing readonly_mode=True to StrictRedisCluster (or RedisCluster) constructor.

>>> from rediscluster import StrictRedisCluster
>>> startup_nodes = [{"host": "127.0.0.1", "port": "7000"}]
>>> rc = StrictRedisCluster(startup_nodes=startup_nodes, decode_responses=True)
>>> rc.set("foo16706", "bar")
>>> rc.set("foo81", "foo")
True
>>> rc_readonly = StrictRedisCluster(startup_nodes=startup_nodes, decode_responses=True, readonly_mode=True)
>>> rc_readonly.get("foo16706")
u'bar'
>>> rc_readonly.get("foo81")
u'foo'

We can use pipeline via readonly_mode=True object.

>>> with rc_readonly.pipeline() as readonly_pipe:
...     readonly_pipe.get('foo81')
...     readonly_pipe.get('foo16706')
...     readonly_pipe.execute()
...
[u'foo', u'bar']

But this mode has some downside or limitations.

  • It is possible that you cannot get the latest data from READONLY mode enabled object because Redis implements asynchronous replication.
  • You MUST NOT use SET related operation with READONLY mode enabled object, otherwise you can possibly get 'Too many Cluster redirections' error because we choose master and its slave nodes randomly.
  • You should use get related stuff only.
  • Ditto with pipeline, otherwise you can get 'Command # X (XXXX) of pipeline: MOVED' error.
>>> rc_readonly = StrictRedisCluster(startup_nodes=startup_nodes, decode_responses=True, readonly_mode=True)
>>> # NO: This works in almost case, but possibly emits Too many Cluster redirections error...
>>> rc_readonly.set('foo', 'bar')
>>> # OK: You should always use get related stuff...
>>> rc_readonly.get('foo')