Transaction Processing

Transaction Commands

Caché supports the ANSI SQL operations COMMIT WORK and ROLLBACK WORK (in Caché SQL the keyword WORK is optional). It also supports the Caché SQL extensions SET TRANSACTION, START TRANSACTION, SAVEPOINT, and %INTRANS. In addition, Caché implements some of the transaction commands that are part of the M Type A standard.

These SQL and ObjectScript commands are summarized in the following table.

These ObjectScript and SQL commands are fully compatible and interchangeable, with the following exception:

ObjectScript TSTART and SQL START TRANSACTION both start a transaction if no transaction is current. However, START TRANSACTION does not support nested transactions. Therefore, if you need (or may need) nested transactions, it is preferable to start the transaction with TSTART. If you need compatibility with the SQL standard, use START TRANSACTION.

The Caché MultiValue Basic (MVBasic) transaction commands are also compatible with ObjectScript and SQL transaction commands.

Using LOCK in Transactions

Whenever you access a global which might be accessed by more than one process, you need to protect the integrity of the database by using the LOCK command on that global. You issue a lock corresponding to the global variable, change the value of the global, then unlock the lock. The LOCK command is used to both lock and unlock a specified lock. Other processes wishing to change the value of the global request a lock which waits until the first process releases the lock.

There are three important considerations when using locks in transactions:

• Lock/unlock operations do not roll back.

• Within a transaction, when you unlock a lock held by the process, one of two things may occur:

  • The lock is immediately unlocked. The lock can be immediately acquired by another process.

  • The lock is placed in a delock state. The lock is unlocked, but cannot be acquired by another process until the end of the current transaction.

  If the lock is in a delock state, Caché defers the unlock until the transaction is committed or rolled back. Within the transaction, the lock appears to be unlocked, permitting a subsequent lock of the same value. Outside of the transaction, however, the lock remains locked. For further details, refer to the “Lock Management” chapter of this book.

• Lock operations that time out set $TEST. A value set in $TEST during a transaction does not roll back.

Using $INCREMENT and $SEQUENCE in Transactions

A call to the $INCREMENT or $SEQUENCE function is not considered part of a transaction. It is not rolled back as part of transaction rollback. These functions can be used to get an index value without using the LOCK command. This is advantageous for transactions where you may not want to lock the counter global for the duration of the transaction.

$INCREMENT allocates individual integer values in the order that increment requests are received from one or more processes. $SEQUENCE provides a fast way for multiple processes to obtain unique (non-duplicate) integers for the same global variable by allocating a sequence (range) of integer values to each incrementing process.

Transaction Rollback within an Application

If you encounter an error during a transaction, you can roll it back in three ways:

• Issue the SQL rollback command, ROLLBACK WORK

• Issue the ObjectScript rollback command, TROLLBACK

• Make a call to %ETN

Issue an SQL or ObjectScript Rollback Command

Application developers can use two types of rollback commands to designate the unsuccessful end of a transaction and automatically roll back incomplete transactions:

• Use ##sql(ROLLBACK WORK), in the macro source routine.

• Use the ObjectScript TROLLBACK command, in macro or intermediate source code.

The rollback command must cooperate with an error trap, as in the following example:

ROU        ##sql(START TRANSACTION) set $ZT="ERROR" 
            SET ^ZGLO(1)=100 
            SET ^ZGLO=error 
            SET ^ZGLO(1,1)=200 
            ##sql(COMMIT WORK) Write !,"Transaction Committed" Quit
ERROR        ##sql(ROLLBACK WORK) 
            Write !,"Transaction failed." Quit

In the example code, $ZT is set to run the subroutine ERROR if a program error occurs before the transaction is committed. Line ROU begins the transaction and sets the error trap. Lines ROU+1 and ROU+3 set the nodes of the global ^ZGLO. However, if the variable error is undefined, ROU+2 causes a program error and line ROU+3 does not execute. Program execution goes to the subroutine ERROR and the set of ^ZGLO(1) is undone. If line ROU+2 were deleted, ^ZGLO would have its value set both times, the transaction would be committed, and the message “Transaction committed” would be written.

Make a Call To %ETN

If you have not handled transaction rollback with a rollback command, the error trap utility %ETN detects incomplete transactions and prompts the user to either commit or rollback the transaction. You should handle rollback within your application, since committing an incomplete transaction usually leads to degradation of logical database integrity.

If you run %ETN after an error when a transaction is in progress, the following rollback prompt is displayed:

You have an open transaction.
Do you want to perform a Commit or Rollback?
Rollback =>

If there is no response within a 10-second timeout period, the system defaults to rollback. In a jobbed job or an application mode job, the transaction is rolled back with no message.

%ETN itself does not do anything to trigger transaction rollback, but it typically ends by halting out of Caché. Transaction rollback occurs when you halt out of ObjectScript and the system runs %HALT to perform Caché process cleanup. There is an entry point into %ETN, called BACK^%ETN, which ends with a quit, rather than a halt. If a routine calls BACK^%ETN, rather than ^%ETN or FORE^%ETN, it will not perform transaction rollback as part of the error handling process.

Examples of Transaction Processing Within Applications

The following example shows how transactions are handled in macro source routines. It performs database modifications with SQL code. The SQL statements transfer funds from one account to another:

Transfer(from,to,amount)   // Transfer funds from one account to another
{
   TSTART
    &SQL(UPDATE A.Account
        SET A.Account.Balance = A.Account.Balance - :amount
        WHERE A.Account.AccountNum = :from)
    If SQLCODE TRollBack  Quit "Cannot withdraw, SQLCODE = "_SQLCODE
    &SQL(UPDATE A.Account
        SET A.Account.Balance = A.Account.Balance + :amount
        WHERE A.Account.AccountNum = :to)
    If SQLCODE TROLLBACK  QUIT "Cannot deposit, SQLCODE = "_SQLCODE
    TCOMMIT
    QUIT "Transfer succeeded"
}

Backups and Journaling with Transaction Processing

Consider the following backup and journaling procedures when you implement transaction processing.

Each instance of Caché keeps a journal. The journal is a set of files that keeps a time-sequenced log of changes that have been made to the database since the last backup. Caché transaction processing works with journaling to maintain the logical integrity of data.

The journal contains SET and KILL operations for globals in transactions regardless of the journal setting of the databases in which the affected globals reside, as well as all SET and KILL operations for globals in databases whose Global Journal State you set to “Yes.”

Backups can be performed during transaction processing; however, the resulting backup file may contain partial transactions. In the event of a disaster that requires restoring from a backup, first restore the backup file, and then apply journal files to the restored copy of the database. Applying journal files restores all journaled updates from the time of the backup, up to the time of the disaster. Applying journals is necessary to restore the transactional integrity of your database by completing partial transactions and rolling back uncommitted transactions, since the databases may have contained partial transactions at the time of the backup. For detailed information, see:

• “Journaling” chapter of the Caché Data Integrity Guide

• Importance of Journals section of the “Backup and Recovery” chapter of the Caché Data Integrity Guide

Asynchronous Error Notifications

You can specify whether a job can be interrupted by asynchronous errors using the AsynchError()Opens in a new tab method of the %SYSTEM.ProcessOpens in a new tab:

• %SYSTEM.Process.AsynchError(1) enables the reception of asynchronous errors.

• %SYSTEM.Process.AsynchError(0) disables the reception of asynchronous errors.

The AsynchErrorOpens in a new tab property of the Config.MiscellaneousOpens in a new tab class sets a system-wide default for new processes for whether processes are willing to be interrupted by asynchronous errors. It defaults to 1, meaning “YES.”

If multiple asynchronous errors are detected for a particular job, the system triggers at least one such error. However, there is no guarantee which error will be triggered.

The asynchronous errors currently implemented include:

• <LOCKLOST> — Some locks once owned by this job have been reset.

• <DATALOST> — Some data modifications performed by this job have received an error from the server.

• <TRANLOST> — A distributed transaction initiated by this job has been asynchronously rolled back by the server.

Even if you disable a job receiving asynchronous errors, the next time the job performs a ZSync command, the asynchronous error is triggered.

At each TStart, TCommit, or LOCK operation, and at each network global reference, Caché checks for pending asynchronous errors. Since SET and KILL operations across the network are asynchronous, an arbitrary number of other instructions may interpose between when the SET is generated and when the asynchronous error is reported.