NonStop Systems Introduction for H-Series RVUs

Transaction Management
NonStop Systems Introduction for H-Series RVUs—540083-001
5-7
Transaction Commitment
transaction with the assurance that no other transaction could have altered the same
records while this transaction was in progress.
Transaction Commitment
Three things happen while a transaction is in progress:
•
TMF causes the operating system to lock all records affected by the transaction.
•
The transaction performs updates by changing the values of fields in existing
records, deleting existing records, or adding new records.
•
TMF generates audit-trail information consisting of before-images and after-images
of each updated record.
But what happens at the conclusion of a successful transaction? What occurs inside
the database? Figure 5-4 on page 5-8 shows the order-entry transaction getting to its
last step: the END TRANSACTION statement.
The END TRANSACTION statement is executed only after all the preceding database
operations within the transaction have updated records. In Figure 5-4, you can see that
end-transaction processing involves a number of steps:
1. TMF sends a message to each participating node telling the node to move the
audit-trail data that has been building up in main memory buffers to the audit-trail
files on disk.
2. After each node has written the audit-trail data to disk, TMF writes a transaction
commitment record in the audit trail on the home node (the node where the
transaction originated). The audit trails are stored safely on mirrored disk volumes.
If a system failure or a failure of a database disk occurs, the entire transaction can
be reconstructed from the audit-trail file.
3. After writing the transaction commitment record, TMF sends a message to each
participating node telling it to release the locks held for the transaction.
4. Each node releases its locks.