[m-users.] Impurity needed?

[Zoltan Somogyi]

2023-02-14 02:13 GMT+11:00 "Volker Wysk" <post at volker-wysk.de>:
>> It would be trivial to change the signature as you ask. It would also be
>> a very bad idea to do so. The point of making an operation a transaction
>> is to ensure that only two things can happen:
>> 
>> - the transaction succeeds, and ALL of its effects happen, or
>> - the transaction fails, and NONE of its effects happen.
> 
> In my understanding, this applies only to the effects on the database...

Consider this sequence modelling an automatic teller machine:

1. Start a transaction.
2. User specifies the account and the amount to withdraw.
3. Teller machine program commands cash dispenser
    to give out the requested cash.
4. The program subtracts the amount from the account,
    and discovers that the result is negative.
5. Because of this, the program aborts the transaction.

Disregard the fact that the test should have been done before
giving out the requested cash. The point I am making is that
giving out the cash should happen IF AND ONLY IF the same
amount could be deducted from the account. Any violation
of that rule either creates or destroys money. Creating money
is illegal in most countries unless you are the central bank,
and if you are anyone other than the central bank, you probably
don't want to pointlessly destroy money either.

>> If you allowed I/O in the middle of a transaction, and then later
>> it turned out that the transaction cannot succeed, then neither
>> of those end states would be reachable.
> 
> So someone using a vanilla imperative language is forbidden to do IO when
> being inside a transaction, too -?

In imperative language interfaces to DBMSs, there is no practical way to forbid
the program to do I/O during a transaction, and, as your use case demonstrates,
there are *some* kinds of I/O that you do want to permit. However, saying that
database programmers should feel free to do ANY kind of I/O inside transactions
is like saying that those programmers should feel free to point ANY device at the
users of the database and pull the trigger, whether the device is a water pistol
or a flame thrower.

Zoltan.