TS/MP Pathsend and Server Programming Manual (H06.05+, J06.03+)

However, if the transaction work was only partially completed, as in the third case, the server
needs to ensure that the transaction is not committed so that the incomplete work can be backed
out. To ensure transaction backout, the server should call the ABORTTRANSACTION procedure
after reading the server’s request and before sending its reply. A call to ABORTTRANSACTION
by the server does not end the transaction—only the requester can end it—but such a call imposes
the requirement that the requester also call ABORTTRANSACTION, rather than ENDTRANSACTION,
after the requester’s reply.
Writing a Server to Use the TMF Subsystem
If, for some reason, your application does not follow the structure described in “Recommended
Structure for Applications” (page 65), earlier in this section, you should consult the checklist that
follows.
Writing a Pathway server to use the TMF subsystem requires that you:
1. Decide which files in the database should be audited.
2. Determine any modifications that are necessary to convert the application to follow the TMF
subsystem locking rules.
3. Decide how to group sequences of the application operations into TMF transactions (that is,
units of recovery).
4. Ensure that any fault-tolerant servers respond correctly to file-system error 75: REQUESTING
PROCESS HAS NO CURRENT-TRANSACTION IDENTIFIER. A backup server that takes over
in mid-transaction does not have a currenttransaction identifier to send to the disk process;
therefore, the disk process returns error 75 to the server, which passes the error to the requester.
If the requester aborts and retries the transaction, the new request has a currenttransaction
identifier. The preferred solution is to change the servers so they are not fault-tolerant servers.
5. Determine whether any new transaction deadlock situations are introduced as a result of TMF
implicit record locking and modify the application to avoid the deadlock. (Transaction deadlock
is a situation in which two transactions cannot proceed because they are each waiting for the
other to release a lock.) One way to cope with deadlock is to use timeout.
In addition, your requester program must use the necessary transaction-control procedure calls or
statements to begin and end the transaction and to abort or restart the transaction if necessary.
NOTE: Whenever your server begins work on a new queued message, it must call the
ACTIVATERECEIVETRANSID procedure to change the current transaction identifier, as described
in the HP NonStop Transaction Management Facility (TMF) Application Programmer’s Guide.
Using Audited and Nonaudited Files
TMF recovery strategy involves backing out the aborted transaction changes; backing out those
changes enables the transaction to be reexecuted from the beginning (with a new transaction
identifier). This strategy means that if you decide to have a mixture of audited and nonaudited files
in the database, you must be careful: only changes to audited files are backed out. If a transaction
works on a mix of audited and nonaudited files, the operations on the nonaudited files must be
retryable.
A retryable operation is an operation that can be interrupted and repeated an indefinite number
of times without affecting the consistency of the database; for example, all reading operations are
retryable. Whether or not a writing operation (on a nonaudited file) is retryable depends on your
criteria for consistency of the data in the database. If the transaction changes both audited and
nonaudited files, you should analyze the transaction to determine whether backing it out and
reexecuting it affects consistency.
For example, consider a transaction that extracts records from a database, computes some
aggregates like averages or means, and then uses the aggregates to extract a subset of the extracted
records from the database for summary reporting. This transaction can be implemented by doing
the extraction twice, the first time to compute the aggregates and the second time to extract the
66 Writing Pathway Servers