The base iterator on the server side implements a seek fence, so you can't seek outside of the underlying source anyway. So, it's safe to seek ahead as much as you want until you exhaust the source (reach null top key).

A BatchScanner with a single range will also break things up internally into multiple smaller ranges if they are spread across different tablets. You only really need to compute your own separate ranges in this case if you have large known gaps you don't want to bother with. On the other hand, you may not care about going through these unnecessary ranges if they typically seek straight to the end, because the next computed key is outside the scope of that tablet.

It is hard to optimize this problem. There is no easy answer. I would suggest experimentation, based on your data, to determine the optimal case. It's basically a trade-off between seeks and pre-computing ranges to run in parallel.

Another optimization you can try: instead of always seeking to the computed next, you can advance internally inside your iterator by calling its source's next method a few times. If you don't reach the next element that you would have seek'd to in some reasonable number of iterations, you can then seek. This also is a strategy that is hard to optimize: Do I need to advance, on average 3 or 20 or 10000000  keys? How many before it would have been more efficient to just seek? There's no easy answer. Experimentation helps.


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Christopher L Tubbs II
http://gravatar.com/ctubbsii

On Fri, Jan 9, 2015 at 6:54 PM, Eugene Cheipesh <echeipesh@gmail.com> wrote:
That’s would work well enough and is my next choice.

 The thought was, rows are stored in increasing order, so as long as I know when I walked off the edge, and flag the iterator as empty it’d be good.  I’m just chasing the optimal in this case, but if it doesn’t exist, oh well.

Thank you for the reference link, it’s very helpful. 

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Eugene Cheipesh

From: Russ Weeks <rweeks@newbrightidea.com>
Reply: user@accumulo.apache.org <user@accumulo.apache.org>>
Date: January 9, 2015 at 6:48:47 PM
To: user@accumulo.apache.org <user@accumulo.apache.org>>
Subject:  Re: Seeking Iterator

Hi, Eugene,

I think the conventional approach is to decompose your search area (bounding box?) into a set of scan ranges that introduce minimal extraneous curve segments, and then pass all those scan ranges into a BatchScanner. The excellent Accumulo Recipes site has an example[1]. Does this approach not work for you?

In general, your custom iterator should never try to seek to a row id different from the current row id, because that row could be hosted by a different tablet server.

-Russ


On Fri, Jan 9, 2015 at 2:37 PM, Eugene Cheipesh <echeipesh@gmail.com> wrote:
Hello,

I am attempting to write an Iterator based on a Z-curve index to search through multi-dimensional data. Essentially, given a record that I have encountered that is in the index range not in the multi-demensional query range I have a way to generate the next candidate record, potentially far ahead of the current point.

Ideally I would be able to refine my search range with subsequent calls to seek(). It appears that Accumulo will create an iterator for every RFile (or some split other split point). The beginning of the range argument to seek will be the record at beginning of this split (which is good), however all instances of the iterator have the same, global range end (which is bad).

I need to avoid the case where I seek past the range boundary of each individual iterator instance and throw a NullPointerException. Is there any way to get enough information to achieve this?

Thank you,

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Eugene Cheipesh