The ATM instances involved in a transaction can be running with
TMSYNCMGR=RRMS or
TMSYNCMGR=NONE.
When a transaction is under the control of a client-side transaction
coordinator such as the CICS Syncpoint Manager, each ATM transaction manager is
aware of this and does not involve RRMS, even if its own parameters indicate
that its RRMS interface is to be activated
(TMSYNCMGR=RRMS).
The transaction executing under the CICS/RMI can change Adabas and DB2
databases, for example, and these changes are coordinated by the CICS Sycnpoint
Manager. For such a transaction, the ATM manager behaves as if it were running
with TMSYNCMGR=NONE.
In order for the transactions of a job or IMS TM system to be
coordinated by RRMS, you must set the client runtime control
HostSystemTransactionManager
to YES, and the ATM manager must run with parameter
TMSYNCMGR=RRMS.
In certain situations, it may be necessary for a resource manager to make a heuristic decision about completing a transaction that it has already prepared.
For example, suppose a resource manager is asked by its local transaction manager to prepare its part of a global transaction initiated by a remote client, and then the network fails. The resource manager would normally lock any resources held by the transaction and wait to receive an instruction from the transaction manager to commit or roll back.
The transaction resources could remain locked for a long time. While the locks remain, the resource manager is unable to perform certain functions, such as a database save.
Note:
A prepared transaction is not subject to the normal timeout
rules.
An Adabas nucleus will terminate its part of a global transaction which it has successfully prepared, if all of the following are true:
an internal ET syncpoint occurs;
the TT time limit currently in effect for the
user has expired.
An internal ET syncpoint occurs when:
an ADAEND command is issued;
an online save or delta save syncpoint occurs; or
a SYNCC command is issued.
An Adabas nucleus will also terminate its part of a global transaction
if a HALT command is received, or if its WORK 4
dataset overflows.
In most of the above cases, the Adabas nucleus decides to commit the
local transaction. However, if HALT is issued or
Work 4 overflows, the nucleus backs out the local transaction. In all cases,
Adabas writes information about the terminated transaction to its DDPRINT
dataset for audit purposes.
Caution:
It is strongly recommended not to use the
HALT command for an Adabas nucleus running with the
parameter setting DTP=RM.
The risk of heuristic transaction terminations would increase if an
operator command were allowed to reduce the TT value (in
preparation for an ADAEND command or an online save,
for example). For this reason, an Adabas nucleus running with
DTP=RM normally ignores a command to reduce its
TT value if it has any prepared transactions that have
not yet completed. However, if the nucleus has a pending
ET syncpoint (at the end of an online save, for
example), the new TT value will take effect.
If a prepared transaction remains at the end of an online save
operation, and has not been completed within a period of 60 seconds plus the
value of the TTSYN parameter, measured from the
beginning of the transaction, the Adabas nucleus will heuristically back out
the transaction, to allow the pending ET syncpoint
to complete.
When an Adabas nucleus running with DTP=RM
receives an ADAEND command, it prevents new
transactions from being prepared or started and asks its local ATM manager to
quiesce any global transactions in which it is involved. If this process is
successful, any prepared transactions are completed (committed or backed out)
and the ADAEND command is then processed. If
prepared transactions remain incomplete, they are heuristically terminated when
their TT time limits expire, and then the
ADAEND command is processed.
An internal synchronized checkpoint takes place at the end of an online
save operation. This requires all clients who are using the nucleus to be at
ET status simultaneously. No client can begin a new
transaction until the checkpoint has completed. The
TTSYN parameter of ADASAV can be used to make a
temporary reduction in the nucleus’s TT value; this
causes incomplete transactions to be backed out after the specified time. This
logic applies only to unprepared transactions. If ATM is in use, there is a
possibility for prepared transactions to remain open after the period specified
by TTSYN. If this is the case, an additional period of
approximately 60 seconds is allowed for prepared transactions to complete. Once
this extra period has elapsed, the nucleus will heuristically back out any
transactions remaining in prepared status. An ADAN89 message will be written to
the console and DDPRINT for each such heuristic termination.
Caution:
Heuristic termination can result in the loss of transaction
integrity. When part of a global transaction is terminated, the global
transaction as a whole may have mixed completion, which means that parts of it
are committed while other parts are backed out. Mixed completion means a loss
of global transaction integrity. However, since unilateral local transaction
completion applies only to prepared transactions, the likelihood of mixed
completion is small.
