[[Property:title|Error handling]]
[[Property:weight|0]]
[[Property:uuid|5f3afec1-939d-b1f5-3715-8bbd2f44ce79]]
As ABEL is dealing with I/O and databases, a runtime error may happen at any time.
ABEL will inform you of an error by setting the <e>has_error</e> attribute to True in <e>PS_QUERY</e> or <e>PS_TUPLE_QUERY</e> and, 
if available, in <e>PS_TRANSACTION</e>.
The attribute should always be checked in the following cases:

* Before invoking a library command.
* After a transaction commit.
* After iterating over the result of a read-only query.

ABEL maps database specific error messages to its own representation for errors, which is a hierarchy of classes rooted at <e>PS_ERROR</e>.
In case of an error, you can find an ABEL error description in the <e>error</e> attribute in all classes suppoorting the <e>has_error</e> attribute.
The following list shows all error classes that are currently defined with some examples (the <e>PS_</e> prefix is omitted for brevity):

* CONNECTION_SETUP_ERROR: No internet link, or a deleted serialization file.
* AUTHORIZATION_ERROR: Usually a wrong password.
* BACKEND_ERROR: An unrecoverable error in the storage backend, e.g. a disk failure.
* INTERNAL_ERROR: Any error happening inside ABEL.
* PS_OPERATION_ERROR: For invalid operations, e.g. no access rights to a table.
* TRANSACTION_ABORTED_ERROR: A conflict between two transactions.
* MESSAGE_NOT_UNDERSTOOD_ERROR: Malformed SQL or JSON statements.
* INTEGRITY_CONSTRAINT_VIOLATION_ERROR: The operation violates an integrity constraint in the database.
* EXTERNAL_ROUTINE_ERROR: An SQL routine or triggered action has failed.
* VERSION_MISMATCH: The stored version of an object isn't compatible any more to the current type.

For your convenience, there is a visitor pattern for all ABEL error types. 
You can just implement the appropriate functions and use it for your error handling code.

<code>
class
	MY_PRIVATE_VISITOR

inherit
	PS_DEFAULT_ERROR_VISITOR
		redefine
			visit_transaction_aborted_error,
			visit_connection_setup_error
		end

feature -- Visitor features

	visit_transaction_aborted_error (transaction_aborted_error: PS_TRANSACTION_ABORTED_ERROR)
		-- Visit a transaction aborted error
		do
			print ("Transaction aborted")
		end

	visit_connection_setup_error (connection_setup_error: PS_CONNECTION_SETUP_ERROR)
		-- Visit a connection setup error
		do
			print ("Wrong login")
		end

end
</code>