Using Caché Objects - InterSystems Documentation

Version Checking
3. Process A saves the object using %Save and receives a success status.
Examining the data on disk shows that the object was not written to disk.
This is an example of concurrent operations that occurred without adequate concurrency
control. In this example, Process B should not have been allowed to delete the object if Process
A really intended to write the object back to disk. Since Process A did not modify the object,
no physical write occurred and unexpected results were observed. Had Process A modified
the object, then %Save would have succeeded as before but the object would have been
written to disk. These inconsistent results are just one example of allowing concurrent access
without adequate controls in place at the application level. (The inconsistent behavior
exhibited by a successful call to %Save and the object not appearing on disk is not a bug in
%Save. It is a concurrency failure which, by definition, will leave the database in an inconsistent
state.)
The solution to this problem is to have Process A open the object with concurrency 3 or 4;
in this case, Process B would then either have been denied access (failed with a concurrency
violation) or would have had to wait until Process A released the object.
A.2 Version Checking
Caché implements version checking using a class parameter called VERSIONPROPERTY.
All persistent classes have this parameter. When defining a persistent class, the procedure
for enabling version checking is:
1. Create a property of type %Integer that holds the updatable version number for each
instance of the class.
2. Set the value of the property's InitialExpression keyword to 0.
3. Set the value of VERSIONPROPERTY to the name of that property. The value of
VERSIONPROPERTY cannot be changed to a different property by a subclass.
This incorporates version checking into updates to instances of the class.
When version checking is implemented, the property specified by VERSIONPROPERTY is
automatically incremented each time an instance of the class is updated (either by objects or
SQL). Prior to incrementing the property, Caché compares its in-memory value to its stored
value. If they are different, then a concurrency conflict is indicated and an error is returned;
if they are the same, then the property is incremented and saved.
Using Caché Objects 203