I've been using modified RDF Patch for the data exchanged to keep
multiple datasets synchronized.
My primary use case is having multiple copies of the datasets for a high
availability solution. It has to be a general solution for any data.
There are some changes to the format that this work has highlighted.
[RDF Patch - v1]
https://afs.github.io/rdf-patch/
1/ Record changes to prefixes
Just handling quads/triples isn't enough - to keep two datasets in-step,
we also need to record changes to prefixes. While they don't change the
meaning of the data, application developers and users like prefixes.
2/ Remove the in-data prefixes feature.
RDF Patch has the feature to define prefixes in the data and use them
for prefix names later in the data using @prefix.
This seems to have no real advantage, it can slow things down (c.f.
N-Triples parsing is faster than Turtle parsing - prefixes is part of
that), and it generally complicates the data form.
When including "add"/"delete" prefixes on the dataset (1) it also makes
it quite confusing.
Whether the "R" for "repeat" entry from previous row should also be
removed is an open question.
3/ Record transaction boundaries.
(A.3 in RDF Patch v1)
http://afs.github.io/rdf-patch/#transaction-boundaries
Having the transaction boundaries recorded means that they can be
replayed when applying the patch. While often a patch will be one
transaction, patches can be consolidated by concatenation.
There 3 operations:
TB, TC, TA - Transaction Begin, Commit, Abort.
Abort is useful to include because to know whether a transaction in a
patch is going to commit or abort means waiting until the end. That
could be buffering client-side, or buffering server-side (or not writing
the patch to a file) and having a means to discard a patch stream.
Instead, allow a transaction to record an abort, and say that aborted
transactions in patches can be discarded downstream.
4/ Reversibility is a patch feature.
The RDF Patch v1 document includes "canonical patch" (section 9)
http://afs.github.io/rdf-patch/#canonical-patches
Such a patch is reversible (it can undo changes) if the adds and deletes
are recorded only if they lead to a real change. "Add quad" must mean
"there was no quad in the set before". But this only makes sense if the
whole patch has this property.
RDF Patches are in general entries in a "redo log" - you can apply the
patch over and over again and it will end up in the same state (they are
idempotent).
A reversible patch is also an "undo log" entry and if you apply it in
reverse order, it acts to undo the patch played forwards.
Testing whether a triple or quad is already present while performing
updates is not cheap - and in some cases where the patch is being
computed without reference to an existing dataset may not be possible.
What would be useful is to label the patch itself to say whether it is
reversible.
5/ "RDF Git"
A patch should be able to record where it can be applied. If RDF Patch
is being used to keep two datasets in-step, then some checking to know
that the patch can be applied to a copy because it is a patch created
from the previous version
So give each version of the dataset a UUID for a version then record the
old ("parent") UUID and the new UUID in the patch.
If the version checked and enforced, we get a chain of versions and
patches that lead from one state to another without risk of concurrent
changes getting mixed in.
This is like git - a patch can be accepted if the versions align
otherwise it is rejected (more a git repo not accepting a push than a
merge conflict).
Or some system may want to apply any patch and so create a tree of
changes. For the use case of keeping two datasets in-step, that's not
what is wanted but other use cases may be better served by having the
primary version chain sorted out by higher level software; a patch may
be a "proposed change".
6/ Packets of change.
To have 4 (label a patch with reversible) and 5 (the version details),
there needs to be somewhere to put the information. Having it in the
patch itself means that the whole unit can be stored in a file. If it
is in the protocol, like HTTP for E-tags then the information becomes
separated. That is not to say that it can't also be in the protocol but
it needs support in the data format.
7/ Checksum
Another feature to add to the packet is a checksum. A hash (which one?
git uses SHA1) from start of packet header, including the initial
version (UUID), the version on applying the patch (UUID) and the changes
(i.e. start of packet to after the DOT of the last line of change),
makes the packet robust to editting after creating it. Like git; git
uses it as the "object id".
So a patch packet for a single transaction:
PARENT <UUID>
VERSION <UUID>
REVERSIBLE optional
TB
QA ...
QD ...
PA ...
PD ...
TC
H <sha1sum>
where QA and QD are "quad add" "quad delete", and "PA" "PD" are "add
prefix" and "delete prefix"
Andy
[RDF Patch - v1]
https://afs.github.io/rdf-patch/
RDF Patch - updated library
work in progress (does not have "packets").
https://github.com/afs/rdf-delta/tree/master/rdf-patch
