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From "" <>
Subject Combiner design clarification
Date Wed, 15 Jul 2015 09:15:43 GMT
Hi all,

I'm implementing a range of combiners and my assumption is that at scan time we will only
have one single combined value returned (regardless of whether the versioning iterator is
applied), however the documentation in the iterator_design.txt document isn't clear whether
that assumption is correct:

A second consideration is that a Combiner is not guaranteed to see every Key-Value pair

which differ only by timestamp every time it is invoked. For example, if there are 5 Key-Value

pairs in a table which only differ by the timestamps 1, 2, 3, 4, and 5, it is not guaranteed

every invocation of the Combiner will see 5 timestamps. One invocation might see the Values

Keys with timestamp 1 and 4, while another invocation might see the Values for Keys with the

timestamps 1, 2, 4 and 5.

Is the above an implementation detail that we need to consider when writing combiners, but
not for the client implementation? Will the client only have one value returned to it?
Equally does the same statement above stay true for other forms of iterators that are not

Many thanks,


Michael Griffiths
BAE Systems Applied Intelligence


BAE Systems Applied Intelligence, Surrey Research Park, Guildford, Surrey, GU2 7RQ.<>

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