Web Services can be fickle - solution here if third party service being used

Web Services can be fickle…

It is very common to get a HTTP 500 internal server error when calling 3rd party web services. One common reason for this is that SPD workflows add lots of stuff to the HTTP request when calling a web service because it assumes it is going to be SharePoint and some authorisation needs to take place. But 3rd party webservices are likely not to care, as they tend to use API keys. In fact, sometimes it can cause you problems if the remote webserver is not expecting this stuff in the header.

Here is an example of a problem that my colleague Hendry Khouw had. After successfully crafting a REST request to a 3rd party service on his workstation, he tried to call the same web service using the Call HTTP Web Service workflow action. But when the webservice was called from the workflow, the HTTP responsecode was 500 (internal server error). Using Andrew Connell’s method of fiddler tracing explained earlier, he captured the request that was returning a HTTP 500 error.

Hendry then pasted the web service URL directly into Fiddler and captured the following trace that returned a successful HTTP 200 response and the JSON data expected.

By comparing the request header between the failed and successful request, it becomes clear that Workflow Manager sends oAuth data, as well as various other things in the header that were not sent when the URL was manually called. Through a little trial and error using Fiddler’s composer function, Hendry isolated the problem down to one particular oAuth header variable,Authorization: Bearer. By using Fiddler composer and removing the Authorization variable (or setting it as a blank value), the request was successful as can be shown on the second and third images below:

  

Now that we have worked out the offensive HTTP header variable that was causing the remote end to spit the dummy, we can craft a custom request header in the workflow to prevent the default value of Bearer being set.

Step 1. Build a dictionary to be included into the RequestHeaders for the web service call. Note the blank value.

Step 2. Set the RequestHeaders to use the dictionary we’ve just created.

Hendry then republished his workflow, the request was successful and he was able to parse the results into a dictionary variable.

Hope this helps others…