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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 18:14:10 GMT
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.

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

I'd like to get clarification on how this works and hope to clear up some
of the misinformation about multi-DC replication that is out there.  I like
a lot of the features of cassandra and enjoy working with it, but the
amount of conflicting information out on the web is a little disconcerting
sometimes.

thanks,
Rob

On Tue, Feb 11, 2014 at 11:02 AM, Andrey Ilinykh <ailinykh@gmail.com> wrote:

> 1. reply part is missing.
> 2. It is confusing a little bit. I would not use term "synchronous".
> Everything is asynchronous here. Coordinator writes data to all local nodes
> and waits for  response from ANY two of them (in case of quorum). In your
> picture it looks like the coordinator first makes decision what nodes
> should reply. It is not correct.
>
>
> On Tue, Feb 11, 2014 at 9:36 AM, Mullen, Robert <robert.mullen@pearson.com
> > wrote:
>
>> So is that picture incorrect, or just incomplete missing the piece on how
>> the nodes reply to the coordinator node.
>>
>>
>> On Tue, Feb 11, 2014 at 9:38 AM, sankalp kohli <kohlisankalp@gmail.com>wrote:
>>
>>> @Mullen,
>>> I think your diagram does not answer the question on responses.
>>> @Sameer
>>> All nodes in DC2 will replay back to the co-ordinator in DC1. So if you
>>> have replication of DC1:3,DC2:3. A co-ordinator node will get 6 responses
>>> back if it is not in the replica set.
>>> Hope that answers your question.
>>>
>>>
>>> On Tue, Feb 11, 2014 at 8:16 AM, Mullen, Robert <
>>> robert.mullen@pearson.com> wrote:
>>>
>>>> I had the same question a while back and put together this picture to
>>>> help me understand the flow of data for multi region deployments. Hope that
>>>> it helps.
>>>>
>>>>
>>>> On Mon, Feb 10, 2014 at 7:52 PM, Sameer Farooqui <
>>>> sameer@blueplastic.com> wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>> I was hoping someone could clarify a point about multi-DC replication.
>>>>>
>>>>> Let's say I have 2 data centers configured with replication factor =
3
>>>>> in each DC.
>>>>>
>>>>> My client app is sitting in DC 1 and is able to intelligently pick a
>>>>> coordinator that will also be a replica partner.
>>>>>
>>>>> So the client app sends a write with consistency for DC1 = Q and
>>>>> consistency for DC2 = Q to a coordinator node in DC1.
>>>>>
>>>>> That coordinator in DC1 forwards the write to 2 other nodes in DC1 and
>>>>> a coordinator in DC2.
>>>>>
>>>>> Is it correct that all 3 nodes in DC2 will respond back to the
>>>>> original coordinator in DC1? Or will the DC2 nodes respond back to the
DC2
>>>>> coordinator?
>>>>>
>>>>> Let's say one of the replica nodes in DC2 is down. Who will hold the
>>>>> hint for that node? The original coordinator in DC1 or the coordinator
in
>>>>> DC2?
>>>>>
>>>>
>>>>
>>>
>>
>

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