Pessimistic lock with flush

antoska · **Posted:** Wed Jun 07, 2006 6:32 am

Hi,

in the online documentation, inside the paragraph Optimistic concurrency control I have found:

The only approach that is consistent with high concurrency and high scalability is optimistic concurrency control with versioning. NHibernate provides for three possible approaches to writing application code that uses optimistic concurrency.

The first possible approach is the following:

Code:

transaction = session.BeginTransaction();
foo.Property = "bar";
session.Flush();
transaction.Commit();
session.Disconnect();

Applying this approch I note that between the commands
session.Flush() and transaction.Commit()
the system applies a pessimistic lock.

In my application for saving a collection (I need to save only single items under certain conditions) I have multiples

Code:

_session.SaveOrUpdate(entity);
_session.Flush();

and only one transaction.Commit(), at the end.
So my application is under pessimistic lock between the first session.Flush() and the final transaction.Commit().
The time is proportional to the collection lenght.

Someone can suggest me a way to avoid this problem?

Thanks
Antonella

gert · **Joined:** Thu Jan 19, 2006 4:29 pm **Posts:** 348

antoska wrote:

Hi,

Code:

_session.SaveOrUpdate(entity);
_session.Flush();

and only one transaction.Commit(), at the end.
So my application is under pessimistic lock between the first session.Flush() and the final transaction.Commit().
The time is proportional to the collection lenght.

Someone can suggest me a way to avoid this problem?

The obvious answer is: Do not call _session.Flush() after each _session.SaveOrUpdate, but only once before tarnsaction.Commit();

The session.Flush is intended to execute update/insert/delete SQL statements and I see no way to prevent SQL engine to apply pessimistic locks for manipulated data.

Gert

antoska · **Posted:** Wed Jun 07, 2006 8:39 am

Yes,

but Flush() in that position solved me 2 kind of problems (I'm working in a romoting context).

Moving Flush() before transaction commit:

1)

Code:

LoadCollection()

collItem[i].description = "xxx";

int rowVersionBefore = collItem[i].RowVersion;

collection.SaveOrUpdate(); --> this method contains transaction management and saveOrUpdate is called in a iteration on each changed collection item

int rowVersionAfter = collItem[i].RowVersion;

rowVersionBefore and rowVersionAfter have the same value

2)

Code:

LoadCollection()

collItem[i].description = "xxx";

int rowVersionBefore = collItem[i].RowVersion;

collection.SaveOrUpdate(); --> this method contains transaction management and saveOrUpdate is called in a iteration on each changed collection item

int rowVersionAfter = collItem[i].RowVersion;

collItem[i].description = "yyy";
collection.SaveOrUpdate();

the second SaveOrUpdate() throws an exception of
Row was updated or deleted by another transaction because the single item is not refreshed

Antonella

gert · **Joined:** Thu Jan 19, 2006 4:29 pm **Posts:** 348

antoska wrote:

Code:

LoadCollection()

collItem[i].description = "xxx";

int rowVersionBefore = collItem[i].RowVersion;

collection.SaveOrUpdate(); --> this method contains transaction management and saveOrUpdate is called in a iteration on each changed collection item

int rowVersionAfter = collItem[i].RowVersion;

rowVersionBefore and rowVersionAfter have the same value

This is as expected. As the changes are not sent to database, RowVersion is updated yet.

antoska wrote:

Code:

LoadCollection()

collItem[i].description = "xxx";

int rowVersionBefore = collItem[i].RowVersion;

collection.SaveOrUpdate(); --> this method contains transaction management and saveOrUpdate is called in a iteration on each changed collection item

int rowVersionAfter = collItem[i].RowVersion;

collItem[i].description = "yyy";
collection.SaveOrUpdate();

the second SaveOrUpdate() throws an exception of
Row was updated or deleted by another transaction because the single item is not refreshed

Let me guess: the items are MarshalByValue objects? What about the collection? Also MarshalByValue? Have You verified where is the transaction handling code (start/commit) executing, inside client or inside server process? If on client, then it does not sound to be very good.. Anyway, make sure You understand which calls are going to be remoted and which ones not. Otherwise You are not able to design an effective system.

All my remoting is done trough Web Service methods, or similar service interfaces. Service interface gives very strong feeling what happens on server and what on server side.. Anyway, as result, I have no experience with "chatty" remoting programming, and I can't give much advice here. (Expect to switch over to chunky remoting ;) - it performs and scales better)

Gert

antoska · **Posted:** Thu Jun 08, 2006 4:21 am

Hi,

thanks for your response.

gert wrote:

This is as expected. As the changes are not sent to database, RowVersion is updated yet.

It isn't exact. The changes are already sent to the database, because inside the method collection.SaveOrUpdate() I start and commit my transaction.

gert wrote:

Let me guess: the items are MarshalByValue objects? What about the collection? Also MarshalByValue?

Yes, both objects are MarshalByValue.

gert wrote:

Have You verified where is the transaction handling code (start/commit) executing, inside client or inside server process? If on client, then it does not sound to be very good..

The method SaveOrUpdate calls a proxy object that executes every operation related to the db on the server.

Thanks,

Antonella