Re: Concurrency in an RDB

paul c wrote:

> Bob Badour wrote:
>

>> Christopher Browne wrote:
>>
>>> In an attempt to throw the authorities off his trail, Bob Badour 
>>> <bbadour_at_pei.sympatico.ca> transmitted:
>>>
>>>> Transactions may acquire and release many locks over the duration of
>>>> the transaction which may be orders of magnitude longer than the
>>>> duration of any lock.
>>>
>>> Are you sure they ought to release them?
>>>
>>> I seem to recall there being some anomalies where, if locks get
>>> released, it gets easier for deadlocks to occur...
>>
>> Well, I suppose never releasing any locks might reduce deadlock, but I 
>> think it would greatly increase the starvation problem.

That then gets to the issue of what locks are we talking about. It is not reasonable for a transaction to hold a mutex on an interprocess message queue for instance.

> When I have heard such complaints, the lack of thought given to the
> transaction design was blatant, not to mention the lack of attention
> paid to the real business requirements. Often the design was a cookbook
> one that somebody had found useful elsewhere. The starvation problems I
> used to see often coincided with EAV designs where a dbms was taking
> locks left right and centre an nobody had any idea exactly what rows and
> tables were being locked without resorting to ponderous traces.
>
> Note that 2PL refers to logical locks, for example predicate locks, not
> the physical locks which might be taken by an engine for internal
> reasonbs and which might be very short-lived, as small as 100
> microseconds or maybe even less.

Yeah, like those locks! Received on Fri Dec 15 2006 - 01:03:31 CET