I recently came across a new approach to persistence called object
prevalence {1}, implemented in the open source Prevayler project [2]. It is
basically a journalling object system in the same vein as a ReiserFS, etc
From [1]:
"In a prevalent system, everything is kept in RAM, as though you were just
using a programming language. Serializable command objects manage all the
necessary changes required for persistence. Queries are run against pure
Java language objects, giving developers all the flexibility of the
Collections API and other APIs, such as the Jakarta Commons Collections and
Jutil.org. Before changes are applied to business objects, each command is
serialized and written to a log file. Then, each command is executed
immediately. Optionally, in a low-use period, the system can take a
snapshot of the business objects, aggregating all the commands applied into
one large file to save some reloading time.
During system recovery after a shutdown or system crash, the system
retrieves its last saved state from the snapshot file (if one is available)
and then reads the commands from the log files created since the snapshot
was taken. These commands are applied to the business objects exactly as if
they had just come from the system's clients, including the system clock.
Thus, the system returns to the state it was in just before the shutdown
and is ready to run.
For a prevalent system to work correctly, its business objects must follow
two very simple rules. The business objects must be:
� Serializable - At any point in time, the system might want to persist an
object to disk or other non-volatile media.
� Deterministic - Given some input, the business object's methods must
always return the same output. "
Now, I was thinking of using it in Cocoon and instead of being business
object focused it would be XML-focused or perhaps more accurately
DOM-focused, in that it would be persisting DOM's rather than business
objects. My initial thoughts are to implement a variation of the
SourceWritingTransformer using Prevayler instead of the file system as the
persistence mechanism by coding the insertFragment() function as a Command
object (or better still 2 Command objects - insert and replace).
My knowledge of the internals of Cocoon at this point in time is minimal,
so I would appreciate some feedback on whether this is the right place to
start experimenting with a Cocoon/Prevayler read/write tie in. All comments
gratefully accepted!
I'd encourage everyone to read these links as it appears to be a very
exciting development in making persistence a transparent, simple, yet
extremely fast and robust process.
Robert
[1] http://www-106.ibm.com/developerworks/web/library/wa-objprev/index.html
[2] http://www.prevayler.org/wiki.jsp?topic=StartingPoints
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