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From "Mullen, Robert" <robert.mul...@pearson.com>
Subject Re: Clarification on how multi-DC replication works
Date Tue, 11 Feb 2014 18:33:18 GMT
>
> The picture shows a sample request, which is why the coordinator points to
> two specific nodes.  What I was trying to convey that the coordinator node
> would ensure that 2 of the 3 nodes were written to before reporting success
> to the client.
>
This is my point. ANY 2 of 3. Your picture shows specific 2 of 3.

True that, I know that and I'm not debating that.  I am showing a single
request sequence in that picture, and during a single request it will
actually be a specific 2 of the 3 nodes.



> I found the article here, it says that the non-blocking writes to the 2nd
> data center are asynchronous.  Is this blog post incorrect as well?
>
> http://www.datastax.com/dev/blog/deploying-cassandra-across-multiple-data-centers
>

Why is it incorrect? Everything is asynchronous, both local and remote. The
coordinator simply waits for response from local nodes. But it doesn't make
it synchronous, because it waits for response from ANY 2 nodes.

I wasn't saying it was incorrect, I was just looking for clarification if
you thought that that blog post was misleading as well, as I've been
sending people to that page for info on multi dc replication.  It it was
erroneous then I would have stopped sending them there.   I was thinking
that the response was synchronous more from the client's point of view,
meaning that the app can't proceed until those specific operations were
completed and a response was returned from cassandra.

Thanks for the help in clarifying all of this, it is very much appreciated.
Regards,
Rob


On Tue, Feb 11, 2014 at 11:25 AM, Andrey Ilinykh <ailinykh@gmail.com> wrote:

>
>
>
> On Tue, Feb 11, 2014 at 10:14 AM, Mullen, Robert <
> robert.mullen@pearson.com> wrote:
>
>> Thanks for the feedback.
>>
>> The picture shows a sample request, which is why the coordinator points
>> to two specific nodes.  What I was trying to convey that the coordinator
>> node would ensure that 2 of the 3 nodes were written to before reporting
>> success to the client.
>>
> This is my point. ANY 2 of 3. Your picture shows specific 2 of 3.
>
>
>
>>
>> I found the article here, it says that the non-blocking writes to the 2nd
>> data center are asynchronous.  Is this blog post incorrect as well?
>>
>> http://www.datastax.com/dev/blog/deploying-cassandra-across-multiple-data-centers
>>
>
> Why is it incorrect? Everything is asynchronous, both local and remote.
> The coordinator simply waits for response from local nodes. But it doesn't
> make it synchronous, because it waits for response from ANY 2 nodes.
>

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