Re: JDBC Transactions - Mailing list pgsql-general

On 01/11/10 19:12, Andy Colson wrote:
> On 11/1/2010 2:01 PM, Jonathan Tripathy wrote:
>>
>>>> I'll give you the exact case where I'm worried:
>>>>
>>>> We have a table of customers, and each customer can have multiple
>>>> memberships (which are stored in the memberships table). We want our
>>>> deleteMembership(int membershipID) method to remove the membership,
>>>> then
>>>> check to see if there are no more memberships left for the
>>>> corresponding
>>>> customer, and if there are none, delete the corresponding customer as
>>>> well.
>>>>
>>>
>>> Hum.. yeah, I can see a race condition there. but even with table
>>> locking I can see it. Not sure how your stuff works, but I'm thinking
>>> website:
>>>
>>> user1 goes to customer page, clicks on "add membership" and starts
>>> filling out info.
>>>
>>> user2 goes to customer page, clicks on "delete membership" of the last
>>> member ship, which blows away the membership, then the customer.
>>>
>>> user1 clicks save.
>>>
>>> Wouldnt matter for user2 if you locked the table or not, right?
>>>
>>> -Andy
>>
>> In the case described above, our code would throw an exception saying
>> "Customer no longer exists", prompting the user to create a fresh
>> customer - So I'm not worried about this (Although it may be
>> inconvenient for the user, I don't think much can be done in this case).
>> Please let me know if I've missed something here.
>>
>> I'm more worried about the following situation (Where a bad interleaving
>> sequence happens):
>>
>> user1 goes to customer page, clicks on "delete membership" of the last
>> member ship, which blows away the membership,
>> user2 goes to customer page, clicks on "add membership" and starts
>> filling out info.
>> user1 then blows away the customer.
>>
>> However I guess that if the relations are set up properly in the
>> database, an exception could be thrown to say that there are
>> corresponding memberships still exist...
>>
>
> yep, that sequence could be a problem too.  It'll be a problem
> whenever more than one person gets to the customer page.  Another user
> could cause that customer to go away at any time.  with or without
> table locks:
>
> user1 and 2 go to customer page.
> user1 deletes last membership, and customer
> user2 does anything... cuz customer has gone away.
>
> Do you really need to delete the customer?  Is leaving it around a
> problem?
>
> -Andy
>
Yeah, unfortunately leaving the customer round is a problem due to Data
Protection Policies in the EU.

However, I'm not worried about the above situation, as if the user tries
to do anything with a customer that doesn't exist, an exception is
thrown which is, I believe, handled properly (i.e. the program doesn't
crash, but will simply tell the user to start again and create a new
customer).

Do you think table relations are enough to solve the situation that I
gave above? I.e:

user1 goes to customer page, clicks on "delete membership" of the last
membership, which blows away the membership,
user2 goes to customer page, clicks on "add membership" and starts
filling out info.
user1 then blows away the customer.

Would my above problem be solved if the database refused to remove a
customer if it had remaining memberships?

Another potential solution could be to leave the customer behind, but
run a script on a Saturday night or something to delete all customers
with no memberships...

What do you think would be best?

Thanks