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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](http://redis.io/topics/cluster-spec#scaling-reads-using-slave-nodes).
redis-py-cluster also implements this mode. You can access slave by passing `readonly_mode=True` to RedisCluster (or RedisCluster) constructor.
.. code-block:: python
>>> from rediscluster import RedisCluster
>>> startup_nodes = [{"host": "127.0.0.1", "port": "7000"}]
>>> rc = RedisCluster(startup_nodes=startup_nodes, decode_responses=True)
>>> rc.set("foo16706", "bar")
>>> rc.set("foo81", "foo")
True
>>> rc_readonly = RedisCluster(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.
.. code-block:: python
>>> 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.
.. code-block:: python
>>> rc_readonly = RedisCluster(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')
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