Tuesday, July 09, 2019

The Fault Encapsulation Pattern

This posting discusses an integration pattern where you return a fault as a message instead of as a fault, to prevent that the execution of the integration is indicated as having errored.

There are a couple of situations where you may not want a synchronous integration to return a fault to its consumer. Examples are:
  • Some back-end system is raising a fault which is not really a fault but a way to give the consumer a particular outcome. Like a credit limit check that returns OK when the limit is not reached, but otherwise gives a CreditLimitReached fault. 
  • A call to the back-end system may time out, telling the integration that the system is not available, which may be a regular state. For example, the integration calls the system to check if it is still running, and if it is tell it to shut down. When the system is already shut down the call will time out.
The reason you may not want to return a fault to the consumer of your integration, at least not as a fault, might be that this flags the execution of the integration to be errored. For example, integrations in the Oracle Integration Cloud (OIC) will show up in the Dashboard as errored instances. That on its turn should trigger Systems Administration to have a look why it failed, only to find out it did not as that is normal behavior. Before you know it, Systems Administration stops having a look, also when there is something seriously wrong with your integration.

To prevent this from happening you may want to handle the fault as an alternate flow instead of an exception flow. This is what the Fault Encapsulation pattern is about. 

Fault Encapsulation Pattern

In simple terms, when applying the Fault Encapsulation pattern, you don't return an error for business faults, but instead encapsulate the error in a "message" element of the response which is an optional part of the normal response.

The following "BPEL-ish" diagram shows how this looks. 

The invoke to the back-end system is a scope with a catch block that catches the error, wraps it in a normal message and then returns the response. In OIC this works in a similar way.

More formally:

A business fault in a synchronous service operation should not stop its processing, to allow returning other information than the fault alone.
A business fault caught by a synchronous service operation that otherwise executed properly, should not flag the operation as failed to prevent false positive error notifications. Instead handling of the fault should be part of normal process execution by the consumer.

The fault in the synchronous service operation is caught using an exception handler that wraps the fault in a message element. The message element is an optional part of the regular response message of the synchronous service operation. System faults in the processing of the synchronous service operation itself are handled as regular faults, in case of SOAP by raising a SOAP Fault, or in case of REST by returning a 4xx or 5xx HTTP status code.

The consumer cannot use any regular fault handling mechanisms to handle the business fault. Instead it will have to check for the message element being present in the response and act on that.

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