Cheers Ralf and Werner,
That tallies with my understanding.
Does anyone have real-world examples they can share about complexity of
objects graphs that they have in their domain? One of the applications I
deal with locks a lot of objects (maybe ten objects) when a root object
is loaded.
Some of this (I believe) is mitigated since some of the domain objects
can be marked in the mapping file as read-only, and so I would hope that
Castor never gets any lock on that; e.g. if one of the objects
references a Country object that is marked as read-only in the mapping
file, any attempt to load something referencing a Country object will
always return that Country object as read-only and thus no transaction
should experience race-conditions on attempting to gain a lock on a
Country object, no matter what AccessMode is being used by the client
transaction.
But my main concern is whether having such a complex object graph is
damaging to concurrency, and whether I should look at de-normalisation
to flatten it a bit.
Cheers,
James
Werner Guttmann wrote:
In addition to what Ralf has said, please note that loading objects as
read-only mode (in one way or the other) will help you improving
performance as Castor will perform less checks at the end of the
transaction.
In other words, make sure that you know your domain model, especially
when it comes down to categorizations such as 'stability',
'cacheability', etc.
Regards
Werner
Ralf Joachim wrote:
Hi James,
Castor locks the whole object graph from the time when it gets loaded
until the end of the transaction which is _db.commit(). If Castor would
not lock the whole object graph that could lead to inconsistenties. With
the default access mode 'Shared' these locks are handled by Castor
internally.
As the transaction at your first example is open during 'some long
operation' the whole graph gets locked until the end of this transaction.
At the second example you have 2 short transaction. The first one loads
the object and thereafter releases any locks at first _db.commit().
According to this no object is locked during the 'some long operation'.
After that operation you start the second transaction which again loads
and locks the object including all related ones. Then you do some
changes to the objects and commit them. At this commit the changes are
written to database and the locks get released again.
As the objects are not locked during 'some lock operation' at the second
example, you need to be aware that your object may have changed in the
meantime.
Hope this helps
Ralf
James Abley schrieb:
Hi,
I'm trying to understand Castor's locking mechanism for object graphs.
Say I have the following classes:
class Book {
int id;
String author;
...
}
class Customer {
int id;
// Assume this will be a collection of Books that
// has been browsed by each customer.
Collection booksBrowsed;
// Assume that this is the transaction history of
// each customer, so a collection of Transaction
// objects
Collection transactions;
Date lastLoginTime;
...
}
class Transaction {
int id;
Date purchaseTime;
Book item;
...
}
If I have client code that does something like the following:
Database db = getDatabase();
db.begin();
db.load(Customer.class, 3, Database.Shared);
// do some long operation here
customer.setLastLoginTime(Calendar.getInstance().getTime());
db.commit();
db.close();
Is there a lock held on all of the objects in the collections in the
Customer object? Or is a lock only obtained on the Customer object when
the transaction is ending and Castor can examine each object, see if
it's dirty and thus needs to get a lock to update the object?
Second related part. I'm very familiar with
http://www.castor.org/jdo-best-practice.html, but I was curious about
another aspect. Assume that I re-write the client to try to minimize the
transaction:
Database db = getDatabase();
db.begin();
db.load(Customer.class, 3, Database.ReadOnly);
db.commit();
db.close();
// do some long operation here
db.begin();
db.load(Customer.class, 3, Database.Shared);
customer.setLastLoginTime(Calendar.getInstance().getTime());
db.commit();
db.close();
Again, what gets locked? Is it the entire object graph, or just the
dirty objects?
Presumably the latter code sample is the recommended way to go,
irrespective of how the object graph gets locked?
Cheers,
James
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