Re: Two-phase commit

On Thu, 7 Oct 2004, Zeugswetter Andreas DAZ SD wrote:

>> Note that this is only really necessary because of Heikki's choice to
>> make the API work in terms of a user-assigned GID.
>
> This was not an arbitrary choice, but is required by most/all TX managers :-(
> I agree that it would be cleaner for the tx manager to keep such a map.
> Maybe the idea was for the gid's to be human readable since human intervention
> is rarely but still required with 2PC, especially in the case where the tx manager
> is unavailable/unrecoverable.

Actually, I think the reason is the following scenario:

1. TM tells DB to prepare for commit
2. DB connection dies after sending the message, but before the TM gets a 
reply.
3. The TM now has no way to know if the transaction got prepared or not.

With a TM assigned transaction id, it works like this:

1. TM tells DB to prepare for commit with transaction id 1234
2. DB connection dies after sending the message, but before the TM gets a 
reply.
3. TM reconnects, and asks the DB if the transaction 1234 is prepared or 
not.

- Heikki