For exception handling in orchestration there are 3 shapes available:
- Throw Exception
- Scope
- Compensate
I want go in much details for the Scope and Compensate shapes. If you need more detailed description on how Exception handling can be used with these 2 shapes I would suggest reading this blog from Richard Seroter. And if this still doens't answers all of your questions, let me know... than I can devote another post with a more detailed explanation on some aspects of the exception handling in orchestrations.
Throw Exception
Firstly we have the 'Throw Exception'-shape. this one is the simplest of all shapes for exception handling.
Frankly, it does nothing more than it's name suggests... It just throws an Exception, nothing more fancy. To achieve this you'll need to follow these three simple steps:
- Define a variable with the type set to any Exception class you prefer. In below example I've created an exception variable of type Microsoft.Biztalk.Message.Interop.BTSException
- Instantiate the variable and set any values according to the occured error
- Now use the Throw Exception shape and configure it ro use the Exception object you've created
Scope
Scopes are used for three reasons
- to configure transaction (long running or atomic)
- to handle exceptions
- to trigger compensating logic
A scope shape can have one or more Exception handling blocks and/or one Compensation Block. The Transaction type you've chosen for your scope will define which of these blocks can be added. An atomic scope can only have a compensation block, and a scope configured with the transaction type non can only have an Exception block. While a long running scope can have both blocks.
For an overview of the possibilities, check the below screenshot.
Exception handling block
The Exception handling block is used to catch exceptions that occur inside the scope the block belongs to. The Exception Object Type in the properties of the exception block will define which Exceptions types will be handled by this Exception block.
When an exception occurs, the engine will check for the closest exception block that can handle this type of exception. The consideration of which exception block will be called is determined sequentially, just like in .NET. If no suitable exception handler is found, the default exception handles is triggered, and this means that the compensation blocks for all nested scopes will be called, after which the exception is rethrowed and the orchestration will become suspended.
Compensation block
Compensation is a way to correct or undo logical piece of work that have previously committed. An exception can occur in the orchestration afther succesfully executing all logic in the scope. In this case, the process might be in a state where it is logically incorrect. So the action already performed in a transactional scope need to be compensated... this can be achieved using a compensation block
If no compensation block is added, the default compensation will be performed. this means calling all compensation blocks of the nested scopes, starting with the most recently completed ones, just as with the default exception handler.
Compensate
The compensate shape can only be used inside of an exception handler. The explicit use of this compensate shape, allows us to specify the order of compensation execution ourselfs. So this can be used in cases the default order (starting from te most recently completed ones and working its way back) doesn't fit.
1 compensate shape can only trigger 1 compensation block of a transaction. So the order must be defined using multiple compensate shapes, calling the compensation blocks in the desired order.
Remarks
- Keep in mind that the default Exception handler, if triggered, will initiate the compensation by calling all compensation blocks for the nested scopes. Compensation blocks of the nested scopes will be called starting with the most recently completed scope and work its way back.
But whenever you catch an exception using the Exception handler block, then you must explicitly do the compensation. - If no custom compensation block has been added, the default compensation will be performed. Just like with the exception handler, this will call the compensation blocks of any nested scopes starting with the most recently completed scope.
- A Compensate shape can only be used inside an Exception handling block or another Comensation Block. When calling the compensate of the current scope, the default compensatino handler will be triggered instead of the compensation code defined by the compensation block.
Note