NonStop Systems Introduction

Transaction Management
NonStop Systems Introduction—527825-001
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 on page 5-8,
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 should occur, 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.