Using Caché Multi-Dimensional Storage - InterSystems ...

2. Acquire a lock (or locks) on the node (or nodes) you wish to modify. This is usually
referred to as a “write” lock.
3. Modify the node (or nodes).
4. Release the lock (or locks). Because we are in a transaction, these locks are not actually
released at this time.
5. Commit the transaction using TCOMMIT. At this point, all the locks released in the
previous step are actually released.
If another process wants to look at the nodes involved in this transaction and does not want
to see uncommitted modifications, then it simply tests for a lock (referred to a “read” lock)
before reading the data from the nodes. Since the write locks are held until the end of the
transaction, the reading process does not see the data until the transaction is complete (committed
or rolled back).
Most database management systems use a similar mechanism to provide transaction isolation.
Caché is unique in that it makes this mechanism available to developers. This makes it possible
to create custom database structure for new application types while still supporting transactions.
Of course, you can simply use Caché Objects or SQL to manage your data and let your
transactions be managed automatically.
3.11.2 Nested Calls to TSTART
Managing Transactions
Caché maintains a special system variable, $TLEVEL, that tracks how many times the
TSTART command has been called. $TLEVEL starts with a value of 0; each call to TSTART
increments the value of $TLEVEL by 1, while each call to TCOMMIT decrements its value
by 1. If a call to TCOMMIT results in setting $TLEVEL back to 0, the transaction ends (with
a commit).
A call to the TROLLBACK command always terminates the current transaction and sets
$TLEVEL back to 0, regardless of the value of $TLEVEL.
This behavior gives applications the ability to wrap transactions around code (such as object
methods) that itself contains a transaction. For example, the %Save method, provided by
persistent objects, always performs it operation as a transaction. By explicitly calling TSTART
and TCOMMIT you can create a larger transaction that encompasses several object save
operations:
Using Caché Multi-Dimensional Storage 31