On 15 June 2012 17:57, Chris Muller ma.chris.m@gmail.com wrote:
In terms of implementation, this is quite easy to do: since graph comes from database in serialized form, creating an initial copy is just about materializing same object twice (or materializing and then copying) instead of once. One will be used in working set, and another will be used to detect the changes upon committing transaction.
Magma essentially does this today. Back in the early 2000's, it made shallow copies of each materialized object and identity-mapped them to the "working copy" of the object. It would compare them upon transaction commit to see if they should be part of the commit package.
Ah.. cool.. so even less work. So, i presume you miss one little primitive which scans the memory subgraph from the roots which you provide and gives you back all objects which inside it. Like that you can very quickly discover what is a new shape of a working set and then iterate over it and see what objects were changed and which were added/removed.
Since then, it was changed to just keep the original buffers that were already part of the materialization (so not making copies when just reading from the db). On transaction commit it now compares the referenced oids of each to determine if different.
BTW: I just realized a BIG problem with the WriteBarrier approach to change-detection: If the method which modifies the inst-variable ALSO happens to perform the commit, then the commit will not notice the changes!
For example, consider a simple setter:
name: aString myDbSession commit: [ name = aString ]
WB will override this method in its generated anonymous subclass to:
name: aString | t1 | t1 := name. returnVal := super name: aString. t1 == name ifFalse: [ self writeBarrier modified ]. ^ returnVal
Do you see the problem?
yes, i do :) A correct implementation should modify the existing methods, but not use subclassing. But that's of course harder to do without decent support from tools (btw Opal Compiler will allow to do that easily).
but i think you can easily fix it by implementing #maValue in block and use aBlock maValue in commit method, instead of #value.
A sketch implementaiton of #maValue is something like:
maValue | rcvr receiverStateBefore result | rcvr := self home receiver. receiverStateBefore :=rcrvr copyState. result := self value. (rcvr isSameAs: receiverStateBefore) ifFalse: [ rcvr writeBarrier modified ]. ^ result
Super call already tried the commit before it even knew it was changed. It emphasizes the limitation of WB: That it can only work via method calls, not direct assignments like a immutability-bit could.
The only solution is to NEVER co-locate the updating of a persistent inst-var in the same method as performing/signaling the commit. That sucks bad enough that I've made Magma sessions now default allowWriteBarrier to false. :-(