[Tutor] glibc error while Python script runs - Solved

[Kent Johnson]

Bernard Lebel wrote:
> On 1/19/06, Kent Johnson <kent37 at tds.net> wrote:
> 
>>Hi Bernard,
>>
>>I'm glad you got it working but kind of surprised at what you had to do.
>>You shouldn't have to have a single thread to access the database. In
>>your original desing were you sharing a connection between threads? That
>>could cause trouble. But if each connection has its own thread and you
>>are using transactions and isolation levels appropriately, they
>>shouldn't stomp on each other.
> 
> 
> I'm not sure what you mean by transaction and isolation levels. I get
> some sort of an idea, but could you elaborate on the idea?

Transactions are helpful in two ways. One is to make a sequence of 
operations atomic - they either all succeed or all fail. A classic 
example is a banking transaction where you have to debit one account and 
credit another. You wouldn't want one request to succeed and the other 
fail - that would be disastrous. By wrapping the two operations in a 
transaction, the database ensures that they succeed or fail together.

The other thing is to isolate the intermediate steps from other database 
clients. You might not want a second client to see the database in the 
intermediate step. By setting the proper isolation level on the 
transaction the database will not make the changes visible to other 
clients until the transaction commits.

http://en.wikipedia.org/wiki/Database_transaction
http://en.wikipedia.org/wiki/ACID

> 
> I have not considered having one connection per thread, and have not
> tested the idea. The way it works right now is that there is only one
> connection, as a class instance attribute. All transaction go through
> that connection.

So you *were* sharing one connection between several threads before? 
ouch. I'm not surprised that that broke badly...

Kent