Re: computational model of transactions

"Erwin" <e.smout_at_myonline.be> wrote in message news:1154677918.807478.221990_at_i42g2000cwa.googlegroups.com...
>> Let's say that you're account starts out with a balance of $550.
>>
>> At time T1 a transaction is started to post a check for $500.
>> At time T2 a transaction is started to post a check for $100.
>> At time T3 a transaction is started to post a check for $75.
>> At time T4 the $100 transaction completes, recording a balance of $450.
>> At time T5 the $75 transaction completes, recording a balance of $375.
>> At time T6 the $500 transaction completes, recording a balance of $-125.
>
> The error is that these transactions are "allowed to run concurrently".
>
> Since each of them wants to update the very same resource (the same
> attribute of the same tuple of the same relvar), these transactions
> should be serialized anyway.
>

I disagree. It is not always the case that if more than one actor is updating the same resource, that those updates must be serialized. To illustrate this, ignore the business rules in the above example. The semantics of the update involve modification, not replacement, the operation involved, addition, is communitive and associative, and all of the updates in question have the same semantics; therefore, it is only important that the modifications be either disjoint or aggregated, but not necessarily serialized: the order in which the modifications occur is not important, since the end result is the same. In addition, only the operation on the shared resource need be disjoint, other operations involving other resources within each transaction can occur simultaneously, so serializing each entire transaction would be overkill.

> No two things can be in the same place at the same time.
>
Received on Fri Aug 04 2006 - 11:17:23 CEST