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From Michael Dürig <mdue...@apache.org>
Subject Detecting move operations in node state diffs
Date Thu, 17 Oct 2013 12:16:43 GMT

Hi,

Currently we can't detect a move operation through diffing node states. 
Those operation are currently seen as separate remove and add operations 
that can't be easily associated with each other. This impacts permission 
evaluation (OAK-710, OAK-783) and observation (OAK-144, OAK-1090), which 
both don't have the same support for moves as had Jackrabbit 2.

As discussed several times before it is not possible to regain move 
operation from simply diffing node states. We need additional 
information. One option is to annotate nodes (*) as they are moved with 
their source path. With that we could detect whether an added node was 
the target of a move operation and if so where the source of that 
operation was. However, this comes with a performance penalty since such 
a diff operation could not be done in a single pass any more. In order 
to decide whether a deleted node has been moved, the corresponding add 
needs to be found first. In essence this requires the diff operation to 
do two passes: the first one for detecting move operations and the 
second one for the other operations.

To avoid the second pass, we could also remember the paths of the moved 
nodes in a global place (*). This would allow us to look up whether a 
deleted node was moved (opposed to deleted) as we go and detect moved 
nodes as soon as we come across an added node that has a source path 
annotation. As an added benefit this approach allows us to detect 
whether there was a move at all simply by checking whether there are 
entries in this global place. If this is not the case, we could fall 
back to a simpler diff mechanism.

(*) All such annotations would happen as hidden items in transient space 
and would have to be removed again by some hook before persisting.

WDYT, is this worth the trouble?

Michael

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