• Introduction to transactions: What is a transaction, how to use transactions to suit your needs, etc.
• Principle: managing transactions on HFSQL data files and special cases.
• Handling the transactions programmatically.
• Managing the special cases (power outage, restoring the consistency of the database, etc).
• Advanced management: the files created during a transaction, managing the computer identifier.

The management of transactions is available for HFSQL Client/Server databases.

Using the transactions according to your requirements

• Performing transactions in applications installed on reliable computers (surge protector, only one application running, etc.): disk transaction
  In this case, the purpose of transactions is to be able to cancel a set of operations. It is possible to display windows in the middle of a transaction, to perform different lengthy processes, ...
  Each operation performed during the transaction is saved in a transaction file. The record used is locked in write mode until the transaction is validated or canceled.
• Perform trusted write operations in the data files (banking sector, accounting, ...).
  Transactions ensure the security of a set of operations. In this case, you must group all the write operations in your code in order to run them in a transaction. The code executed during the transaction must not display windows, browse data files, ... This code should only include data file modification operations.
  Each operation performed during the transaction is saved in a transaction file. The record used is locked in write mode until the transaction is validated or canceled.

Principle of transactions on HFSQL data files

• The transaction isolation level used is "READ UNCOMMITED".
• The transaction isolation level is defined with HTransactionIsolation. The default isolation level is the "READ UNCOMMITED" mode.

Special cases

• Error in the program, the transaction is automatically abandoned.
• Call to EndProgram, the transaction is automatically abandoned.
• Test canceled from the editor, the transaction is automatically abandoned.
• Power outage or unexpected stop of the application (application stopped by the task manager for example):
  When the application is restarted, the consistency of the database can be restored:
  • By the WDTRANS tool (or WDOptimizer).
  • By the HFSQL Control Center.
  • During the first call to HTransactionStart.
  • By the HTransactionCancel function.

Special cases specific to HFSQL Client/Server:

• If the Client is lost, the transaction is cancelled.
• Restarting the server cancels the current transactions. Please note: the transaction is not rolled back if it involves a data file with a secure password. The transaction will be canceled during the next opening of the data file.

For more details, see Managing special cases.