Re: computational model of transactions


"J M Davitt" <jdavitt@xxxxxxxxxx> wrote in message
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Brian Selzer wrote:
"Brian Selzer" <brian@xxxxxxxxxxxxxxxxxxx> wrote in message
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"Erwin" <e.smout@xxxxxxxxxxx> wrote in message
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It is not always the case that if more than one actor is
updating the same resource, that those updates must be serialized.

I leave that one on your account.


To illustrate this, ignore the business rules in the above example.

Ignoring business rules is not my idea of a good example. And
especially not this kind of example, since I've been a bank programmer
for 15 years. I can assure you no one in the bank business would
accept the kind of risky manipulations with account balances that you
propose.


The semantics of the update involve modification, not replacement

You obviously see a difference between modification and replacement. I
don't. So please explain.


If Rummy says that he's going to increase the troop levels in Baghdad,
that doesn't mean that he's going to replace all of the troops there,
just that additional troops will be sent.

It all comes down to whether the result of the operation depends on the
current value and whether the operation is atomic.

X += 5 is different from X = 10 even if X == 5.

X += 5 translates to something like:
ADD [EBP + 24], 5
which modifies the memory location in place in a single instruction;
whereas X = 10 translates to:
MOV [EBP + 24], 10
which just replaces the value at that memory location.


the operation
involved, addition, is communitive and associative.

You mean "commuTAtive", of course, and furthermore that's completely
irrelevant. As for associativity : it is important to observe that
each transaction in this example does exactly one addition with exactly
two arguments. So unless you can think of a way for the system to
detect that multiple independent transactions are doing such a thing
(performing an associative operation), and then replace those multiple
independent operations with one single, transaction-surpassing,
operation that has the same result, associativity is also irrelevant.
If you cannot think of such a way for the system to detect this (I'm in
that case), you're stuck with doing multiple additions one-at-a-time,
and you're stuck with the fact that for the additions that are executed
second and third, one of those arguments should be the result of the
former. Therefore it is necessary that the transactions be serialized.
Otherwise it would mean a transaction is allowed to see uncommitted
results from another one.


I really want to address this, but I'm going to be late for work, so I'll
have to address it in a later post.

I'm back. I agree that the updates need to be isolated, but I disagree
with the idea that the entire transaction needs to be isolated or
serialized. It is only necessary to obtain an exclusive lock on the
affected row at the time that the update to the shared resource occurs,
so it's possible to have several other intervening transactions commit
between the time that the transaction starts and the time that the update
starts. My point is that it is not necessary to isolate the entire
transaction, only that portion from the start of the update until the
commit.

Are we to understand that "it's possible to have several other
intervening transactions commit between the time that the
transaction starts and the time that the update starts" means
that you believe that at "the time the update starts" the value
of whatever attribute is being changed isn't the same as it was
when the transaction started?


Yes. The nature of the update makes this possible. An update that simply
decreases inventory by 5 need not know the state of the inventory at the
time that the transaction started. If you issue,

UPDATE Inventory
SET ON_HAND = ON_HAND - 5
WHERE PartNo = '1322314'
AND Warehouse = 'Cleveland'
AND Bin = 'F2A31S3B6'

then it doesn't matter what the value was at the start of the transaction,
since the current value is read out during the update; whereas, if you
issue:


UPDATE Inventory
SET ON_HAND = 25
WHERE PartNo = '1322314'
AND Warehouse = 'Cleveland'
AND Bin = 'F2A31S3B6'

then it does matter because the value, 25, represents 5 less then the value
that existed at the start of the transaction. Also in this case intervening
transactions cannot have occurred, otherwise the values set by the
intervening transactions will be overwritten. Hence with replacement, the
row to be updated must be reserved for exclusive use (at least for writing)
throughout the transaction.

(And it would make a lot of sense
to defer this until the end of the transaction whenever possible.) Nor
is it necessary to serialize, since the update modifies the value that is
current at the time that the update commences, not at the time that the
transaction commences. So there is no danger of seeing uncommitted data
from other transactions. If the effect of an update is a net increase or
a net decrease, then the order in which transactions start is not
important: it is the order in which they complete that determines the
state of the attribute for each corresponding database state. On the
other hand, if the effect of an update is a replacement, then the
transactions must be serialized in their entirety, otherwise you run the
risk of losing information.



.

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