Re: New to Prevayler: "Baptism problem" basics?

On Fri, Sep 9, 2011 at 4:39 AM, Naveen Chawla <[email protected]> wrote:
> > I'm lost maybe because the code examples on codehaus are gone:
> http://docs.codehaus.org/display/PREVAYLER/The+Baptism+Problem . And also
> because I couldn't understand the phrase "transactions shouldn't carry
> direct
> references to the objects". Does this only mean as instance variables of the
> Transaction, or also within local variables inside "executeOn()"?

I think that the phrase "transactions shouldn't carry direct
references to the objects" means that the arguments to the transaction
should not be direct references to the objects, but should be names of
the objects.  Further, when the transaction gets serialized, it should
not store a direct reference to the object but only the name of the
object.  When the transaction executes, it will use the name to get a
direct reference.

This probably means that direct references are not stored in instance
variables, though if the instance variable was treated as a transient
and not serialized then it might be OK.   It is OK for local variables
to hold references, unless you are referring to the local variables in
a closure and then referring to the closure from an instance variable.
 But that is just another way of saying "don't carry direct references
to the objects", and only possible if your language has closures.

I think the reason for the word "carry" is that transactions get
serialized, i.e. they are permanent.  When you come back to a file
that contains a bunch of transactions, and re-execute them, the
transactions cannot refer to the objects directly, because the objects
are in the database, or don't exist yet.  Business objects are not
stored in the transaction log.  But since the transactions are
serialized, we can move them from one computer to another and so
migrate the database.  The transactions should not "carry" references
to the objects, but only refer to their names.

Names, of course, can be anything you want.  Strings, numbers, arrays
of strings and numbers.  Anything that can be saved and restored
easily.

-Ralph Johnson

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