Hi,
you raise a very important point for a distributed MK implementation.
I agree with your suggestions for 1 and 3.
Re 2 I would prefer to specify the MUST NOTs (which we would probably have to
do anyway if we specify the MUSTs IMO)
Michael
On Dec 12, 2012, at 4:46 PM, Michael Dürig wrote:
> Hi,
>
> Currently the Microkernel contract does not specify a merge policy but
> is free to try to merge conflicting changes or throw an exception. I
> think this is problematic in various ways:
>
> 1) Automatic merging may violate the principal of least surprise. It can
> be arbitrary complex and still be incorrect wrt. different use cases
> which need different merge strategies for the same conflict.
>
> 2) Furthermore merges should be correctly mirrored in the journal.
> According to the Microkernel API: "deleting a node is allowed if the
> node existed in the given revision, even if it was deleted in the
> meantime." So the following should currently not fail (it does though,
> see OAK-507):
>
> String base = mk.getHeadRevision();
> String r1 = mk.commit("-a", base)
> String r2 = mk.commit("-a", base)
>
> At this point retrieving the journal up to revision r2 should only
> contain a single -a operation. I'm quite sure this is currently not the
> case and the journal will contain two -a operations. One for revision r1
> and another for revision r2.
>
> 3) Throwing an unspecific MicrokernelException leaves the API consumer
> with no clue on what caused a commit to fail. Retrying a commit after
> some client side conflict resolution becomes a hit and miss. See OAK-442.
>
>
> To address 1) I suggest we define a set of clear cut cases where any
> Microkernel implementations MUST merge. For the other cases I'm not sure
> whether we should make them MUST NOT, SHOULD NOT or MAY merge.
>
> To address 2) My preferred solution would be to drop getJournal entirely
> from the Microkernel API. However, this means rebasing a branch would
> need to go into the Microkernel (OAK-464). Otherwise every merge defined
> for 1) would need to take care the journal is adjusted accordingly.
> Another possibility here is to leave the journal unadjusted. However
> then we need to specify MUST NOT for other merges in 1). Because only
> then can clients of the journal know how to interpret the journal
> (receptively the conflicts contained therein).
>
> To address 3) I'd simply derive a more specific exception from
> MicroKernelException and throw that in the case of a conflict. See OAK-496.
>
> Michael
