I think servlet database access synchronization issues is a perfect subject
for this list.
I think you are going to run into update contention with storing access
class instances as local variables; I think synchronizing on a static update
variable will not cause you appreciable performance problems unless you are
running a massive query. Remember to release the lock with notify and
release the connection in your catch (or preferably, your finally) clause.
Cheers!
Mark
----- Original Message -----
From: "Paul Foxton" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Tuesday, July 17, 2001 6:33 AM
Subject: Re: concurrent access to variables with class or block scope
> Thanks Mark,
>
> this brings up an interesting point I think
> (assuming the database does not handle concurrency - I _am_ using MySQL
and
> tomcat so no EJB):
>
> If the servlet was accessing a class used to handle database connections,
> there would be
> 2 possibilities to handle the concurrency issue:
>
> 1. Instantiate the class in a local variable in a method in the servlet.
> This would mean a new instance of the class for every request to the
> database.
> Some kind of row locking for the update would then still need to be
used
> to handle concurrent access to the same row in the databse.
> As far as I can gather from the MySQL manual only table-level locking
is
> supported.
>
> 2. Store the class in an instance variable in the servlet.
> The methods used to connect to the database would then need to be
> synchrionized.
>
> Presumably solution 1 would mean a greater load on the server resources,
> while solution 2
> would mean queueing requests to the class, perhaps with the possibility of
> deadlock, or at the least
> meaning that users may potentially have to wait longer for their request
to
> be processed.
>
> Both could create problems on a high traffic site, am I right?
>
> In this case which would be the better solution?
>
> just let me know if I'm getting too far off topic with this :)
>
> thanks,
>
> Paul
>
> > -----Original Message-----
> > From: Mark Galbreath [mailto:[EMAIL PROTECTED]]
> > Sent: 16 July 2001 23:36
> > To: [EMAIL PROTECTED]
> > Subject: Re: concurrent access to variables with class or block scope
> >
> >
> > In a servlet world, the database handles concurrency (at
> > least, any database
> > that supports transactions - mySQL does not). If the
> > database does not
> > support transactions, you must either synchronize the database calling
> > methods or the instance variables holding the update values.
> > If you use
> > entity CMP EJBs, the EJB container will take care of
> > transactions for you,
> > or if you use BMP EJBs (either entity or stateful session),
> > the database
> > must handle transactions.
> >
> > Cheers!
> > Mark
> >
> > ----- Original Message -----
> > From: "Paul Foxton" <[EMAIL PROTECTED]>
> > To: <[EMAIL PROTECTED]>
> > Sent: Monday, July 16, 2001 10:21 AM
> > Subject: concurrent access to variables with class or block scope
> >
> >
> > > Hi all,
> > >
> > > I've been wondering about the fact that concurrent requests
> > to a servlet
> > > will result in a new thread being created to access the
> > same instance of
> > the
> > > servlet as long as singleThreadModel is not implemented
> > with reference to
> > > the following:
> > >
> > > If you have a servlet which accesses a database, and
> > request parameters
> > from
> > > an HTML form are used to build the SQL where clause, if
> > these request
> > > parameters are stored in class instance variables (i.e.
> > declared at the
> > > beginning of the class and outside of any methods, so class
> > scope), would
> > > this mean that there is a possibility of data being
> > overwritten in the
> > > middle of 2 or more concurrent requests and therefore
> > compromising the
> > > integrity of the data retrieved from the database?
> > >
> > > If so, I would have thought that the way around this is to store the
> > values
> > > from the request parameters in variables declared inside
> > methods, which
> > > would then have block scope within that method.
> > >
> > > But surely, if two requests (threads) run through this
> > block at the same
> > > time there is still a possibility of a problem with data integrity?
> > >
> > > This doesn't seem quite right to me tho, because then you
> > would have to
> > > synchronize access to any method or variable which was used
> > to build an
> > sql
> > > query or update.
> > >
> > > Can anyone clarify?
> > >
> > > Sorry if this is an obvious question. I've read through the
> > documentation
> > on
> > > threads in servlets at Suns java site, but am still a
> > little confused.
> > >
> > > Thanks,
> > >
> > > Paul
> > >
> > >
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