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From Yamini Joshi <>
Subject Re: Accumulo Working
Date Tue, 22 Nov 2016 18:09:11 GMT
So, for a batch scan, the merge is not required but, for a scan, since it
returns sorted data, data from tserver1 and tserver2 is merged at the

I know how to write iterators but I can't vsiualize the workflow. Lets say
in the same example I have 3 custom iterators to be applied on data: it1,
it2, it3 respectively. When are the iterators applied:

1. scan on tserver -> client -> it1 on tserver -> client -> it2 on tserver
-> client -> it3 on tserver -> client
I'm sure this is not the case, it adds a lot of overhead

2. scan on tserver ->  it1 on tserver ->  it2 on tserver  -> it3 on tserver
-> client
The processing is done in batches?
Data is returned to the client when it reaches the max limit for
table.scan.max.memory even if it is in the middle of the pipeline above?

Best regards,
Yamini Joshi

On Tue, Nov 22, 2016 at 11:56 AM, Christopher <> wrote:

> 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 <>
> 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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