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From James Hughes <jn...@virginia.edu>
Subject Re: Iterators that alter key-values
Date Sat, 16 May 2015 13:55:53 GMT
Hi Dave,

I can speak to the first question a little bit.  The one time I saw this, I
traced the code and saw that after emitting a certain number of bytes, the
iterator stack was recreated.  In that case, no further keys would have
been filtered since the current key-value pair being emitted would trigger
the reset and that key would be used for the re-seek.  I'll apply all
caveats to that explanation: it was Accumulo 1.4 and didn't learn about why
the stack was stopped and recreated or other times that may happen.

On the other hand, one could imagine a tablet server dying in the middle of
returning entries.  I have no idea of the details of how Accumulo handles
that.  Worst case, you may be right about some reprocessing, but all this
sounds like a corner case.

For the documentation, writing about implementation details directly may
not be the best way.  I'd hope that the documentation would make it clear
that all iterators (even presumed 'top' or 'final' iterators) should
conform to the 'sorted key' part of the contract.

Thanks,

Jim


On Sat, May 16, 2015 at 3:27 AM, Dave Hardcastle <hardcastle.dave@gmail.com>
wrote:

> A couple of follow-up questions...
>
> So, is it true to say that a filtering iterator that is filtering out a
> high percentage of the key-values in a range, might have to redo a lot of
> work if a reseek happens? (It's reseeked to the last emitted key, but a lot
> of key-values past that may already have been rejected by the filter.)
>
> Would it be worth making the fact the the reseek happens to the last
> emitted key explicit in the documentation? It seems natural to me to assume
> that the reseek happens to one key past the last read key. I don't think
> the javadoc for the seek() method in SortedKeyValueIterator makes it quite
> clear enough.
>
> Thanks,
>
> Dave.
>
> On 15 May 2015 at 19:32, Eric Newton <eric.newton@gmail.com> wrote:
>
>> is it the same instance of the iterator object
>>
>>
>> No, it is not.
>>
>> On Fri, May 15, 2015 at 2:16 PM, Dave Hardcastle <
>> hardcastle.dave@gmail.com> wrote:
>>
>>> Jim,
>>>
>>> That explains a lot - I knew that the iterator stack could be resumed in
>>> the middle of a range, but didn't realise that it used the last emitted key
>>> to decide where to resume.
>>>
>>> Just so I'm clear, when iterators get stopped and later resumed, is it
>>> the same instance of the iterator object that's restarted (so that I could
>>> store state in there and use that to help the reseek) or is it a new
>>> instance of the iterator that has to be able to resume purely on the basis
>>> of the last emitted key?
>>>
>>> As you say though, it's probably best to stick to modifying values only.
>>>
>>> Thanks very much,
>>>
>>> Dave.
>>>
>>> On 15 May 2015 at 18:55, James Hughes <jnh5y@virginia.edu> wrote:
>>>
>>>> Hi Dave,
>>>>
>>>> The big thing to note is that your iterator stack may get stopped and
>>>> torn down for various reasons.  As Accumulo recreates the stack, it will
>>>> call 'seek' with the last emitted key in order to resume.
>>>>
>>>> If you are returning keys out of order in an iterator, the 'seek'
>>>> method needs to be able to undo the transformation and call 'seek'
>>>> appropriately.  That's not impossible, but it isn't trivial.
>>>>
>>>> In GeoMesa, we did something like that at one point (without having a
>>>> smart 'seek').  I enjoyed two days of debugging trying to figure out why
>>>> medium sized requests would hang.  (There was an infinite loop....)  From
>>>> that experience, I'd suggest only modifying values.
>>>>
>>>> Cheers,
>>>>
>>>> Jim
>>>>
>>>>
>>>> On Fri, May 15, 2015 at 1:26 PM, Dave Hardcastle <
>>>> hardcastle.dave@gmail.com> wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>> I've always assumed that the last iterator in the stack can make
>>>>> arbitrary changes to keys and values, including not returning the keys
in
>>>>> sorted order. I know that SortedKeyValueIterator says that "anything
>>>>> implementing this interface should return keys in sorted order" - but
I
>>>>> don't see a good reason that has to be true for the final iterator. This
>>>>> assumption seems to be backed up by the manual which says that "the only
>>>>> safe way to generate additional data in an iterator is to alter the current
>>>>> key-value pair" - it doesn't say that making arbitrary modifications
to the
>>>>> rowkey or key is forbidden.
>>>>>
>>>>> I have a situation where I am making a transformation of the rowkey
>>>>> that may not preserve the ordering of the keys. When I scan for individual
>>>>> ranges I get the correct results. When I scan for two ranges using a
>>>>> BatchScanner, I get lots of data back which is not in the ranges I queried
>>>>> for. I am not explicitly checking that I have not gone beyond the range,
>>>>> but that should not be necessary as I am not doing any seeking, only
>>>>> consuming the key-values I receive.
>>>>>
>>>>> So, my main question is whether the last iterator is allowed to not
>>>>> return keys in sorted order?
>>>>>
>>>>> Thanks,
>>>>>
>>>>> Dave.
>>>>>
>>>>
>>>>
>>>
>>
>

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