Advise on using DBMS_XA with multiple branches under one global transaction

Question

Dear all
I need some advise on using DBMS_XA from PL/SQL with tightly coupled multiple branches under one global transaction. Basically, I've successfully written some PL/SQL code that in 3 different sessions attaches to 3 different branches of one global transaction and before ending each branch they can see each others uncommitted data. So far so good.
However, I'm not sure I completely understand how each branch must call xa_end, xa_prepare and xa_commit correctly using two phase commit and my calls result in errors like:
ORA-24767: transaction branch prepare returns read-only (XA error code 3 = Transaction was read-only and has been committed)
ORA-24756: transaction does not exist (XA error code -4 = XID is not valid)
ORA-02051: another session or branch in same transaction failed or finalized
This is the structure of my programs (3 SQL*Plus sessions):
main: Uses xid 123|0 (branch 0 of global transaction 123). This should be the coordinator that commits using two phase commit across the 3 branches
m1.xa_start tmnoflags
m2.DML
m3.Wait for thread A + B to manually be started and run xa_end
m4.xa_end tmsuccess
m5.xa_prepare
m6.xa_commit false
thread A: Uses xid 123|A (branch A of global transaction 123)
a1.xa_start tmnoflags
a2.DML -- thread A can see main and thread B's data
a3.xa_end tmsuccess
a4.xa_prepare -- required?
a5.Should we also call xa_commit false?
thread B: Uses xid 123|B (branch B of global transaction 123)
b1.xa_start tmnoflags
b2.DML -- thread B can see main and thread A's data
b3.xa_end tmsuccess
b4.xa_prepare -- required?
b5.Should we also call xa_commit false?
The failing steps are:
m5
m6
a4
a5
b4
b5
Before starting calling xa_end I see 3 rows in V$GLOBAL_TRANSACTION, eg (hex 7B = decimal 123):
FORMATID GLOBALID BRANCHID BRANCHES REFCOUNT PREPARECOUNT STATE FLAGS COUPLING
203348753 0000007B 00000000000000000000000000000000 3 3 0 ACTIVE 0 TIGHTLY COUPLED
203348753 0000007B 0000000000000000000000000000000A 3 3 0 ACTIVE 0 TIGHTLY COUPLED
203348753 0000007B 0000000000000000000000000000000B 3 3 0 ACTIVE 0 TIGHTLY COUPLED
Thanks a lot in advance.
Cheers
Finn

Answer 1

OK, I've figured it out. This is poorly documented as it's not well explained how to handle the various return codes. Turns out that all but the last xa_prepare calls return dbms_xa.xa_rdonly (tightly coupled branches are combined -- "read only" optimization), the last one returns dbms_xa.xa_ok and this is when you should call xa_commit.
Now my next problem is that DBMS_XA doesn't work from within jobs (DBMS_JOB and DBMS_SCHEDULER), which makes it very difficult to use DBMS_XA. My purpose of using DBMS_XA is to coordinate work across multiple sessions in one transaction but if I can't easily create the multiple sessions, I'm stuck.
When called from a job, xa_start throws:
ORA-24789: start not allowed in recursive call
on Oracle 11.2. In Oracle 11.1 it works, but xa_end fails with
ORA-25352: no current transaction
so I guess in fact the xa_start call didn't really work either, even though it returned tm_ok.
I'm now trying to find a workaround on how to use DBMS_XA from within jobs, please comment if you have any suggestions. Or if you have any suggestions on other means of establishing the concurrent sessions (I wouldn't like to resort to external programs that need username/password to connect as password management would be a security issue).
Thanks in advance.
Cheers
Finn