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From Jaydeep Chovatia <chovatia.jayd...@gmail.com>
Subject Re: do all nodes actually send the data to the coordinator when doing a read?
Date Fri, 25 Jul 2014 22:32:36 GMT
Yes. Digest includes following: {name, value, timestamp, flags(deleted,
expired, etc.)}


On Fri, Jul 25, 2014 at 2:33 PM, DuyHai Doan <doanduyhai@gmail.com> wrote:

> Thanks Mark for the very detailed explanation.
>
>  However what's about timestamp checking ? You're saying that the
> coordinator checks for the digest of data (cell value) from both nodes but
> if the cell name have different timestamp would it still request a full
> data read to the node having the most recent time ?
>
>
> On Fri, Jul 25, 2014 at 11:25 PM, Mark Reddy <mark.reddy@boxever.com>
> wrote:
>
>> Hi Brian,
>>
>> A read request will be handled in the following manner:
>>
>> Once the coordinator receives a read request it will firstly determine
>> the replicas responsible for the data. From there those replicas are sorted
>> by "proximity" to the coordinator. The closest node as determined by
>> proximity sorting will be sent a command to perform an actual data read
>> i.e. return the data to the coordinator
>>
>> If you have a Replication Factor (RF) of 3 and are reading at CL.QUORUM,
>> one additional node will be sent a digest query. A digest query is like a
>> read query except that instead of the receiving node actually returning the
>> data, it only returns a digest (hash) of the would-be data. The reason for
>> this is to discover whether the two nodes contacted agree on what the
>> current data is, without sending the data over the network. Obviously for
>> large data sets this is an effective bandwidth saver.
>>
>> Back on the coordinator node if the data and the digest match the data is
>> returned to the client. If the data and digest do not match, a full data
>> read is performed against the contacted replicas in order to guarantee that
>> the most recent data is returned.
>>
>> Asynchronously in the background, the third replica is checked for
>> consistency with the first two, and if needed, a read repair is initiated
>> for that node.
>>
>>
>> Mark
>>
>>
>>
>> On Fri, Jul 25, 2014 at 9:12 PM, Brian Tarbox <briantarbox@gmail.com>
>> wrote:
>>
>>> We're considering a C* setup with very large columns and I have a
>>> question about the details of read.
>>>
>>> I understand that a read request gets handled by the coordinator which
>>> sends read requests to <quorum> of the nodes holding replicas of the data,
>>> and once <quorum> nodes have replied with consistent data it is returned
to
>>> the client.
>>>
>>> My understanding is that each of the nodes actually sends the full data
>>> being requested to the coordinator (which in the case of very large columns
>>> would involve lots of network traffic).  Is that right?
>>>
>>> The alternative (which I don't think is the case but I've been asked to
>>> verify) is that the replicas first send meta-data to the coordinator which
>>> then asks one replica to send the actual data.  Again, I don't think this
>>> is the case but was asked to confirm.
>>>
>>> Thanks.
>>>
>>> --
>>> http://about.me/BrianTarbox
>>>
>>
>>
>

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