On 5.3.13 9:27, Marcel Reutegger wrote:
Hi,
I think it's better to discuss on the mailing list instead of an already
closed issue...
Michael wrote:
It should be possible to combine this with the "branch, rebase,
fast forward merge" approach I described above : we just need to
make the fast forward merge a bit more clever such that it could
detect and merge changes in distinct areas of the tree instead of
just giving up when there was concurrent change.
Right, but then why do we branch and rebase in the first place, when
we need the merge (and commit!) do the same (the more clever
bit you mentioned) in a concurrent write scenario?
The only reason I see right now is consistency we gain with the
conflict handling Michael proposed [0] for the MicroKernel API
and Jukka implemented in the SegmentMK with the NodeStore
API.
My idea was to balance off things: rebase on branch is for handling the
heavy duty stuff while the clever merge should only take care of very
simple things like changes in separate sub trees. But see below.
Jukkas implementation of the conflict handling provides
Serializable Snapshot Isolation because it re-runs the hooks on
a rebased branch whenever a concurrent change is detected
before it merges. If we allow it to be more clever, we lose
the Serializable part of the isolation level. Though, I think that's
quite OK. See below.
With the MicroKernel the story is a bit different. E.g. the conflict
handling model allows the commit to internally rebase, but
without re-running the commit hooks. validation and changes
performed by the commit hook are already part of the JSOP.
This means the MicroKernel will exhibit 'write skew' and not
provide Serializable Snapshot Isolation. This was already described
a while ago by Michael [1].
My take on this was: when we rebase internally anyway, why not make this
rebase available externally so branches could rebase themselves and thus
make it easier for the commit later on? AFAICS the internal rebase would
either have to be on something which pretty much resembles a private
branch (optimistic locking) or would need to be synchronised for
serialising concurrent commits. The latter would result in a very fat
lock and is not what we want. FYI the H2 based Microkernel tries to
implement commit without such a fat lock and fails. See OAK-532.
Michael
I think this is OK and we shouldn't require the MicroKernel nor
the NodeStore to provide more strict consistency guarantees
than Snapshot Isolation. I don't see how we can add the Serializable
part without severely impacting throughput in a distributed write
scenario.
If we still need Serializable Snapshot Isolation for some of our
Validators, we can use other techniques [2] to ensure consistency.
Materializing the conflict is one option and sometimes just happens
automatically. E.g. with a unique index on jcr:uuid we can ensure
consistency (every referenceable node has a unique UUID) even
with an implementation that only provides snapshot isolation.
What I propose, is to make this explicit in the JavaDoc of the MicroKernel
and the NodeStore API. Specifically the NodeStore API lacks quite some
details. Maybe we can just reference the relevant parts in the MicroKernel.
Regards
Marcel
[0]
http://wiki.apache.org/jackrabbit/Conflict%20handling%20through%20rebasing%20branches
[1]
http://wiki.apache.org/jackrabbit/Transactional%20model%20of%20the%20Microkernel%20based%20Jackrabbit%20prototype
[2]
http://en.wikipedia.org/wiki/Snapshot_isolation#Serializable_Snapshot_Isolation
