2-phase commit

Hello,

Is there any open source implementation of 2-phase commit out there using
Postgres ?
The basic logic says :

MASTER SLAVE
------ -----
begin atomic action
send request 1-n
send "prepare to commit message"
if action_can_be_performed
lock data
store initial state on disk
store request on disk
send "ok" message
else
send "failure" message
if all slaves said "ok"
send "commit" message
else
send "rollback" message
Wait for acknowledgements
if master said commit
do work
unlock data

Send "acknowledgement" message

How can I check "action_can_be_performed" (i.e. what conditions
satisfy an "ok" vote ?)

thanks for your time.
-richard

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