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From Christopher <ctubb...@apache.org>
Subject Re: Accumulo Working
Date Tue, 22 Nov 2016 17:56:58 GMT
That's basically how it works, yes.

1. The data from tserver1 and tserver2 necessarily comes from at least two
different tablets. This is because tables are divided into discrete,
non-overlapping tablets, and each tablet is hosted only on a single
tserver. So, it is not normally necessary to merge the data from these two
sources. Your application may do a join between the two tablets on the
client side, but that is outside the scope of Accumulo.

2. Custom iterators can be applied to minc, majc, and scan scopes. I
suggest starting here:
https://accumulo.apache.org/1.8/accumulo_user_manual.html#_iterators


On Tue, Nov 22, 2016 at 12:05 PM Yamini Joshi <yamini.1691@gmail.com> wrote:

> Hello all
>
> I am trying to understand Accumulo scan workflow. I've checked the
> official docs but I couldn't understand the workflow properly. Could anyone
> please tell me if I'm on the right track? For example if I want to scan
> rows in the range e-g in a table mytable which is sharded across 3 nodes in
> the cluster:
>
> Step1: Client connects to the Zookeeper and gets the location of the root
> tablet.
> Step2: Client connects to tserver with the root tablet and gets the
> location of mytable.
> the row distribution is as follows:
> tserver1             tserver2                   tserver3
> a-g                       h-k                            l-z
>
> Step3: Client connects to tserver1 and tserver2.
> Step4: tservers merge and sort data from in-memory maps, minc files and
> majc files, apply versioning iterator, seek the requested range and send
> data back to the client.
>
> Is this how a scan works? Also, I have some doubts:
> 1. Where is the data from tserver1 and tserver2 merged?
> 2. when and how are custom iterators applied?
>
>
> Also, if there is any resource explaining this, please point me to it.
> I've found some slides but no detailed explanation.
>
>
> Best regards,
> Yamini Joshi
>

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