If the directions are not clear, I apologize, but it is good practice
to always call (make-instance 'persistent-class) from within a
transaction to ensure ACID properties for object creation. This is
not terribly important for interactive, non-critical uses of the DB.
But in multi-threaded environments it becomes important to ensure you
don't corrupt the database in odd ways.
During instance creation the following subtasks are guaranteed to be
in a transaction (i.e. wrapped in an ensure-transaction call):
- initialization of persistent slot values
- class index update for indexed classes
Any other before/after/around methods on initialize-instance or shared-
initialize are not wrapped in a transaction. The bottom line is that
to ensure that these methods plus the two above subtasks are
transactional, be sure that make-instance is called within a
transaction.
If you don't, you will lose Atomicity and Consistency for object
creation of indexed instances or instances with any other persistent
side effects in after/before/around methods.
Why don't we put this in a transaction by default? In part this is
tied up in the recent discussions about initialization vs.
reinitialization during serialization, part is tied up in not creating
a false sense of security leading to problems when you engage in
natural uses of the MOP, for instance (defun initialize-
instance :after ((obj my-class) ...)), and part not having been fully
thought through yet.
Ian
On Jan 21, 2008, at 10:29 AM, Joubert Nel wrote:
On Mon, 2008-01-21 at 09:54 -0500, Ian Eslick wrote:
In general, instance initialization should always happen inside a
transaction. This will catch any updates you do to indices or other
variables.
The instance initialization code in Elephant only forces transactions
for subsets of initialization that use cursors, and which under BDB
would lead to deadlocks or other errors. This allows developers
flexibility in breaking up large initialization transactions but the
consequence is that the set of side effects of a default call to
make-
instance are not atomic unless wrapped in a transaction.
OK, so if I add an initialize-instance :after method, for example,
then
a call to (make-instance 'persistent-class) must now be called from
within a transaction to ensure atomicity?
Joubert
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