On Mon, May 18, 2015 at 6:31 PM, Keith Turner <[email protected]> wrote:
> > > On Sat, May 16, 2015 at 3:27 AM, Dave Hardcastle < > [email protected]> 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.) >> > > This may happen, it depends on what the tserver is doing. Lets assume a > call to next on the iterator advances to the next top key, and not past > it. If the tserver calls next after the buffer is full, then what you > described could happen. > > So if the tserver is doing something like the following, I think it would > redo work. Need to investigate this. > I investigated. Sorry for the spam, should have done this before sending. The following code services batch scans. Seems like it checks if the buffer is full before calling next. https://github.com/apache/accumulo/blob/1.6.2/server/tserver/src/main/java/org/apache/accumulo/tserver/Tablet.java#L1538 The following code services scans, its seems to also check if the buffer is full before calling next. https://github.com/apache/accumulo/blob/1.6.2/server/tserver/src/main/java/org/apache/accumulo/tserver/Tablet.java#L1684 > > iter = .... > iter.seek(...) > > while(iter.hasTop() && !buffer.ifFull()){ > buffer.add(iter.getTopKey(), iter.getTopValue()) > iter.next() //if this call to next is made even when buffer is full, > it could redo work > } > > return buffer; //will reseek with last key (non-inclusive) in buffer > later. > > > >> >> 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. >> > > When the reseek is done using the last key returned, it makes it > non-inclusive. What are your thoughts on the following paragraph? > > > https://github.com/apache/accumulo/blob/1.6.2/core/src/main/java/org/apache/accumulo/core/iterators/SortedKeyValueIterator.java#L81 > > >> >> Thanks, >> >> Dave. >> >> On 15 May 2015 at 19:32, Eric Newton <[email protected]> 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 < >>> [email protected]> 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 <[email protected]> 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 < >>>>> [email protected]> 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. >>>>>> >>>>> >>>>> >>>> >>> >> >
