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From Nimbus Lin <jiaxin...@live.com>
Subject Re: How can I check cassandra cluster has a real working function of high availability?
Date Sat, 29 Jun 2019 03:55:54 GMT
To Sir Oleksandr :




Thank you!

Sincerely
Nimbuslin(Lin JiaXin)
Mobile: 0086 180 5986 1565
Mail: jiaxinlin@live.com


________________________________________
From: Oleksandr Shulgin <oleksandr.shulgin@zalando.de>
Sent: Monday, June 17, 2019 7:19 AM
To: User
Subject: Re: How can I check cassandra cluster has a real working function of high availability?

On Sat, Jun 15, 2019 at 4:31 PM Nimbus Lin <jiaxinlin@live.com<mailto:jiaxinlin@live.com>>
wrote:
Dear cassandra's pioneers:
    I am a 5 years' newbie,  it is until now that I have time to use cassandra. but I cann't
check cassandra's high availabily when I stop a seed node or none seed DN as CGE or Greenplum.
    Would someone can tell me how to check the cassandra's high availability? even I change
the consistency level from one to local_one, the cqlsh's select is always return an error
of NoHostAvailable.

 By the way, would you like to answer me other two questions:
2nd question: although cassandrfa's consistency is a per-operation setting, isn't there a
whole all operations' consistency setting method?
3rd question: how can I can cassandra cluster's running variables as mysql's show global variables?
such as hidden variable of  auto_bootstrap?

Hi,

For the purpose of serving client requests, all nodes are equal -- seed or not.  So it shouldn't
matter which node you are stopping (or making it unavailable for the rest of the cluster using
other means).

In order to test it with cqlsh you should ensure that the replication factors of the keyspace
you're testing with is sufficient.  Given the NoHostAvailable exception that you are experiencing
at consistency level ONE (or LOCAL_ONE), I can guess that you are testing with a keyspace
with replication factor 1 and the node which is unavailable happen to be responsible for the
particular partition.

For your second question: it depends on a client (or "client driver") you are using.  In cqlsh
you can set consistency level that will be applied for all subsequent queries using the "CONSISTENCY
..." command.  I think that the Java driver does have an option to set the default consistency
level, as well as has an option to set consistency level per query.  Most likely this is also
true for Python and other drivers.

And for the third question: I'm not aware of a CQL or nodetool command that would fulfill
the need.  Most likely it is possible to learn (and update) most of the configuration parameters
using JMX, e.g. with JConsole: https://docs.datastax.com/en/archived/cassandra/3.0/cassandra/operations/opsMonitoring.html#opsMonitoringJconsole

Cheers,
--
Alex


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