I've created a content type 'H'. The following rule (using the Rules module) has been built to prevent the node from being saved/updated if the field 'a' is larger than 5.
Event: Before saving content of type 'H'.
Condition: Data Comparison - Node field 'a' is larger than 5.
Action: Show a message on the site - Message type: Error.
Action: Go back to the front page.
It will show the error message if the condition is TRUE. However, the node is still being saved/updated.
Is it possible to control this without programming?