Sunday, July 28, 2019

OIC: Making a REST Integration Returning a 404 instead of 500

In this article I describe how to return a HTTP 404 (resource) Not Found with a REST integration that on its turn calls another REST service that returns a 404.
This article is superseded by my article Fault Handling in OIC, which gives you the proper way to do this.

When an integration invokes the GET action on a REST service that returns a 404, the integration will raise an APIInvocationError. As a result, the integration on its turn will respond with a HTTP 500 error, which is typically not what you want.

Embedding the invoke in a Scope gives you the option to add a Fault Handler:

Chosing the APIInvocationError gives you the option to configure how any APIInvocationError should be handled. As you can see below, I have configured it to use a Switch, where the top flow will make it return a 404:

In all honesty this is not watertight, because I filter on all APIInvocationErrors where the type is empty (“”). The reason being that all the elements, type, title, detail and errorCode are empty, so I cannot filter on anything.

As I found out, you will also run into this situation when the URL of the Connection used for the invoke is wrong, and probably a few other situations as well. I rely on the assumption that my integration is properly configured, so that the most likely cause of an APIInvocationError indeed is that it concerns a 404.

To make my integration return a 404, I map this as a hard-coded value to the errorCode:

Except for the errorCode I have hard-coded the other elements as well. Probably not exactly according to the specifications, but good and especially clear enough for me:

In the meantime, in the background there is a SOAP fault with a reason containing the HTTP 404, so at some point I hope this will be exposed so that I can filter in a reliable way:

Saturday, July 27, 2019

OIC: Handling Optional Elements in a REST Integration

This article is a follow-up of the a previous article where I discuss how to handle optional elements in case of XML in the Oracle Integration Cloud (OIC). In the following I discuss how to create an integration that invokes VBCS REST service and works in (almost) the same way as the VBCS REST service itself.

A challenge with mapping is always how to handle optional elements. In the previous posting that I refer to above, I describe a way how to deal with this in case of XML messages. As I found out (the hard way) this is cannot be applied 100% to JSON.

I have made it work for an integration that invokes the REST service on a VBCS business object. As there are challenges especially with numeric fields and references (foreign keys), I have used a simple BO called Detail having a string field, a number field and a reference. With VBCS BO’s the latter implies a number field that references the (number) id field of another BO it refers to, which in this case is called Master.

The Master BO looks as follows (ignore the create/update fields, those are generated by default and for the example not relevant):

The Detail BO looks as follows:

As you can see, name (string), master (reference to, number) and age (number) are all optional.

I created a single REST integration that, using the OIC Pick Action feature has a POST, GET and PATCH action to create, get and update a Detail:

Use if-function for Mapping Input

Except for the PATCH action, all mappings to the requests of the invoke use the if-function to check if the source has a value, and only if it has maps it to the target:

Use string-length() for Mapping Output

The mappings from the invokes to the response of the integration all use the string-length() function to check if the response element of the invoke has a value, and if so maps it to the target.

I make use of the fact that internally JSON is transformed into XML and there is no payload validation, so numbers also can be checked using string-length(). By doing it like this the element will be left out completely, instead of being returned as an empty string “” or failing in case of a number. This is not 100% as the VBCS service works (which will return null instead of leaving the field out), but for me that is not an issue when using the integration.

Special Case: PATCH

In case of a PATCH I need my integration working so that left-out elements are not updated (i.e. stay untouched) and that I can nullify them by passing null as a value. For the invoke to the VBCS REST service this will fail for number elements (with JSON a number is a primitive type that cannot be null). I therefore apply a trick by using a JSON sample payload that treats all elements as string, including the master and age (both number):

{"name": "Huey", "master": "1", "age": "15"}

This, in combination with the if-function when mapping the request to the PATCH invoke, makes it work the same way as the VBCS REST service works. For the response I use the string-length() as described above.

The picture on the left side shows an example of an invoke to the PATCH action with all elements present, and on the right side where all elements are nullified:

As you can see, nullifying the master results in a reference to a row with = 2, which happens to be the only Master row with no name. The VBCS REST service works the same way, so apparently some ‘intelligence’ is applied here. When adding an extra Master with no name so that now I have two of those, VBCS can no longer decide which one to take and nullifies the reference to the master altogether:

When I leave out any element in the request, the field in the BO is not touched. It’s a bit boring to see, so I will spare you the screenshots. You will have to trust me on that.

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.