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

[Matthew B. <mat...@ou...>]

Jon Maber wrote:
> 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?

At the moment I don't have a quota table but use two metadata entries as 
most of the time we don't need to know the quota so performance isn't 
crucial. This could change in the future.

> 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.

Even if I don't have a Quota class I could do the synchronization on the 
resource object to which the quota is attached. This only works in a 
single JVM although there are some projects attempting to weave extra 
bits in to allow locking primatives to work across JVMs.

http://www.terracottatech.com/terracotta_Spring.shtml

-- 
  -- Matthew Buckett, VLE Developer
  -- Learning Technologies Group, Oxford University Computing Services
  -- Tel: +44 (0)1865 283660 http://www.oucs.ox.ac.uk/ltg/