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From: Ananth Gundabattula <agundabattula@gmail.com>
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Subject: Question on consistent ordering of scanner rows
Message-Id: <4BAD9D34-F5D4-4031-971B-2CF5B67DF92A@gmail.com>
Date: Sun, 13 Aug 2017 12:36:27 +1000
To: user@kudu.apache.org
archived-at: Sun, 13 Aug 2017 02:36:45 -0000

Hello All,


I was wondering if there is any guarantee from the kudu scanner that the =
rows returned from a single tablet scan are always in the same order =
basing on the following assumptions :=20

- There was no change in the underlying kudu tablet for the given scan =
range when the reads are being performed multiple times for the same =
scan token
- I am using Java client
- I am using Kudu version 1.4.0
- The client code is using the KuduScanTokenBuilder API to plan the set =
of scans that can be performed for a given query.
- The client is using the nextRows() followed using hasNext() and next() =
methods in the corresponding iterators.
- There seems to be a variable called orderMode in the asyncScanner =
during a debug session but it looks like this property is not exposed =
yet as a public API. The default value seems to be that it is unordered.=20=


Perhaps the answer is no per the last point above but would like =
confirmation from the community.=20

I am integrating Apache Apex with Apache kudu and am using the scan =
token builder API mechanism to plan the scans in a distributed way. =
While doing so, I would like to provide the end users of Apache Apex a =
mechanism to get a consistent scan ordering as a configurable approach. =
Given it is almost impossible to achieve this ordering in a true =
distributed fashion for downstream compute nodes, the aim is to provide =
consistent ordering within a single Apex partition. Apache apex with =
Kudu integration would be providing configurations to map one tablet to =
one or multiple apex partitions. While scanning in either of these =
mapping styles, I would like to provide further ordering guarantees. =
However I am not sure if Apache Kudu would provide a consistent ordering =
for the same scan provided the above assumptions hold good. =20

Could you please advise regarding the ordering of scan rows for a single =
tablet across multiple launches of the same scan token ?

Regards,
Ananth=