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From "Mullen, Robert" <>
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?

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


On Tue, Feb 11, 2014 at 11:02 AM, Andrey Ilinykh <> 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 <
> > 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 <>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 <
>>>> 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 <
>>>>> 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 =
>>>>> 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
>>>>> 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
>>>>> DC2?

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