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From Jun Wu <wuxiaomi...@hotmail.com>
Subject Re: Questions about the replicas selection and remote coordinator
Date Mon, 01 Feb 2016 21:46:14 GMT
Hi Steve,

    Thank you so much for your kind reply and now it makes more sense. But for the remote
coordinator issue, it’s definitely a interesting topic. If you have any other conclusion
 on this. I’d be pretty happy to learn from you. 

    Thanks again!

Jun
> On Jan 29, 2016, at 13:09, Steve Robenalt <srobenalt@highwire.org> wrote:
> 
> Hi Jun,
> 
> The replicas are chosen according to factors that are generally more easily selected
internally, as is the case with coordinators. Even if the replicas were selected in a completely
round-robin fashion initially, they could end up being re-distributed as a result of node
failures, additions/removals to/from the cluster, etc, particularly when vnodes are used.
As such, the diagrams and the nodes they refer to are hypothetical, but accurate in the sense
that they are non-contiguous, and that different sets of replicas are distributed to various
parts of the cluster.
> 
> As far as the remote coordinator is concerned, I'm not sure what motivated the change
from 1.2 to 2.1 and would be interested in understanding that change myself. I do know that
improved performance was a big part of the 2.1 release, but I'm not sure if the change in
coordinators was part of that effort or not.
> 
> Steve
> 
> 
> On Fri, Jan 29, 2016 at 10:13 AM, Jun Wu <wuxiaomin98@hotmail.com <mailto:wuxiaomin98@hotmail.com>>
wrote:
> Hi Steve,
> 
>    Thank you so much for your reply. 
> 
>    Yes, you're right, I'm using the version of 2.1. So based on this, I think I'm outdated.

> 
>     However, this comes to another interesting question: why we change this part from
version 1 to version 2. As we can see that in version 1, there's connections from node 10
in DC 1 with node 10 in DC 2, then node 10 in DC 2 send 3 copies to 3 nodes in DC 2, which
should be more time-saving than version 2.1, which send data from node 10 in DC 1 to 3 nodes
in DC 2 directly.
> 
>      Also, is there any information on how to choose the replicas. Like here https://docs.datastax.com/en/cassandra/2.1/cassandra/dml/architectureClientRequestsMultiDCWrites_c.html
<https://docs.datastax.com/en/cassandra/2.1/cassandra/dml/architectureClientRequestsMultiDCWrites_c.html>
>     Why we choose node 1, 3, 6 as replicas and 4, 8, 11 as another 3 replicas?
> 
>     Also, is node 11 working as remote coordinator here? Or is the concept of remote
coordinator really existed, as the figure shows, we even don't need the remote coordinator.

> 
>     Thanks!
> 
> Jun
> 
>     
>     
> 
> Date: Fri, 29 Jan 2016 09:55:58 -0800
> Subject: Re: Questions about the replicas selection and remote coordinator
> From: srobenalt@highwire.org <mailto:srobenalt@highwire.org>
> To: user@cassandra.apache.org <mailto:user@cassandra.apache.org>
> 
> 
> Hi Jun,
> 
> The 2 diagrams you are comparing come from versions of Cassandra that are significantly
different - 1.2 in the first case and 2.1 in the second case, so it's not surprising that
there are differences. since you haven't qualified your question with the Cassandra version
you are asking about, I would assume that the 2.1 example is more representative of what you
would be likely to see. In any case, it's best to use a consistent version for your documentation
because Cassandra changes quite rapidly with many of the releases.
> 
> As far as choosing the coordinator node, I don't think there's a way to force it, nor
would it be a good idea to do so. In order to make a reasonable selection of coordinators,
you would need a lot of internal knowledge about load on the nodes in the cluster and you'd
need to also handle certain classes of failures and retries, so you would end up duplicating
what is already being done for you internally.
> 
> Steve
> 
> 
> On Fri, Jan 29, 2016 at 9:11 AM, Jun Wu <wuxiaomin98@hotmail.com <mailto:wuxiaomin98@hotmail.com>>
wrote:
> Hi there,
> 
>     I have some questions about the replicas selection. 
> 
>     Let's say that we have 2 data centers: DC1 and DC2, the figure also be got from link
here: https://docs.datastax.com/en/cassandra/1.2/cassandra/images/write_access_multidc_12.png
<https://docs.datastax.com/en/cassandra/1.2/cassandra/images/write_access_multidc_12.png>.
There're 10 nodes in each data center. We set the replication factor to be 3 and 3 in each
data center, which means there'll be 3 and 3 replicas in each data center.
> 
>     (1) My first question is how to choose which 3 nodes to write data to, in the link
above, the 3 replicas are node 1, 2, 7. But, is there any mechanism to select these 3?
> 
>     (2) Another question is about the remote coordinator, the previous figure shows that
node 10 in DC1 will write data to node 10  in DC 2, then node 10 in DC2 will write 3 copies
to 3 nodes in DC2.
> 
>     But, another figure from datastax shows different method, the figure can be found
here, https://docs.datastax.com/en/cassandra/2.1/cassandra/dml/architectureClientRequestsMultiDCWrites_c.html
<https://docs.datastax.com/en/cassandra/2.1/cassandra/dml/architectureClientRequestsMultiDCWrites_c.html>.
It shows that node 10 in DC 1 will send directly 3 copies to 3 nodes in DC2, without using
remote coordinator.
> 
>     I'm wondering which case is true, because in multiple data center, the time duration
for these two methods varies a lot.
> 
>     Also, is there any mechanism to select which node to be remote coordinator?
> 
>     Thanks!
> 
> Jun
> 
> 
> 
> -- 
> Steve Robenalt 
> Software Architect
> srobenalt@highwire.org <mailto:bzavon@highwire.org> 
> (office/cell): 916-505-1785 <tel:916-505-1785>
> 
> HighWire Press, Inc.
> 425 Broadway St, Redwood City, CA 94063
> www.highwire.org <http://www.highwire.org/>
> 
> Technology for Scholarly Communication
> 
> 
> 
> -- 
> Steve Robenalt 
> Software Architect
> srobenalt@highwire.org <mailto:bzavon@highwire.org> 
> (office/cell): 916-505-1785
> 
> HighWire Press, Inc.
> 425 Broadway St, Redwood City, CA 94063
> www.highwire.org <http://www.highwire.org/>
> 
> Technology for Scholarly Communication


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