Re: [Bodington-developers] Locking Issues (Java, Explicit DB, Optimistic DB)

[Jon M. <jo...@te...>]

Matthew Buckett wrote:
> User 1: get current quota 8.5MB of 10MB
> User 2: get current quota 8.5MB of 10MB
> User 2: set quota to 9.5MB and upload 1MB file.
> User 1: set quota to 10.5MB and upload 1MB file.
>
> Now user 1 has exceeded the quota but still has the file uploaded which 
> shouldn't be allowed (or should it?)
>
> The other option is to check after changing the quota with another 
> select that the quota is set to the expected value (although this really 
> is just an expansion of the collection of fields update)
I'll let you and Ian discuss the general case (because id have to study 
a book on the subject to catch up with you).

Did you say if your code is directly calls the SQL or wraps records in 
the quotas table in a Bodington PersistentObject subclass?

If the latter then database reading and writing will operate through a 
single instance of that class so you could add a method like this;

public synchronized boolean attemptUpload( long size )
{
// this class knows the value of current uploaded bytes
// return false if already over limit.
// add to uploaded bytes record.
// call standard save record routine
// return true
}

The important thing is that this routine does the 'over limit' 
calculation and the save record part in the same synchronized block.

Jon