Logic Apps: Catching errors

Posted: April 19, 2021  |  Categories: Azure Logic Apps

As part of development guides, it’s always a good idea to have a fallback plan and handle errors.

You can be 99.999% confident that your code won’t fail, but that 0.001% chance happens. “Anything that can go wrong will go wrong” – Murphy’s Law

And so, we resort to our very dear friend, Try-Catch.

In Logic Apps, it’s not exactly an out-of-the-box functionality, but it’s actually quite simple to achieve this and with a few steps. Also, there are multiple ways to catch your errors.

In this post, we will try two approaches:

  • Using a For-Each loop
  • and a Filter Array action.

Since I’ve started developing LAs, I’ve used the For-each loop approach but it had some flaws. It involved using a Parse Json to catch only the error message, but not all actions have the same schema.

So, the idea of the Filter array came to play. It’s actually quite easy as well and easy to maintain. You’ll find the same issue with the schemas, but it’s a faster approach.

Let’s dig in. I started to build a simple Logic App, just creating a couple of variables and an HTTP call that I know that will fail. I mocked the results to ensure the outcome is what I needed.

I’ve also built a scope, creating the Try block and a second scope to handle the Catch block. You’ll have to set the “Run after” properties to only trigger on errors, skips or timeouts, if not, it will run on success.

It will always relate to the previous scope.

The For-Each loop approach

Now we start to build our Error handler. I’ve chosen the For Each loop first because it was faster to create since I’m used to it and even have some templates for it as well.

The For Each action requires an array action to iterate on, which means we need to find one. The Scope isn’t a loop, so what will we relate it to?

Well, the scope might not be, but there are N actions inside it, so if you search in the expressions box or the documentation, you’ll find the “result” expression, which records every result of the contained actions within a given scope.

Now, remember, this will need to point to the action you want, but you will not have it in the Dynamic Content, you need to write it using the _ for spaces, because this expression handles the JSON node name like if you’re working in the back code.

Once you have this set, you just need to create a condition to check if the status of the action was “Failed“. Pretty simple.

If you test the execution, you’ll see that the loop is working and iterating the actions batch that the “result” expression returns.

I’m just returning the action outputs in the error string, which will contain the StatusCode, Headers and Body. It should help to diagnose a possible error.

Let’s try the Filter array now.

The Filter Array action approach

Similar to the For Each, we need to iterate through an action that contains child actions. We use the same “result” expression pointing to the same scope as the “From” property and choose “item()?[‘status’]” as the node to search for. Also, we only want the failed actions, so the node should be equal to “Failed“.

As for the error message, it’s a bit different from the For Each type. We’re still picking up the Outputs but we need to get the first action from the Filter array action.

The end result should be the same, as we’re picking up the same info as with the For Each loop.

Usually, an action will return a JSON record as the result of its execution. There are some fields that will always be present, like “Status” and “Tracking ID”. There’s no easy way to find this info, so you have to deconstruct one or more actions to find it. With the information you have now, you can get it from anywhere, you just have to use the “Result” expression.

Here you can see some fields in the Set var action I created and how the status is recorded. For tracking purposes, the execution engine records the begining and end timestamps as well as other useful data.

Now that you know how, it’s time to get working and make your Logic Apps sturdier and with a proper error handling.

Happy coding!

Author: Pedro Almeida

Pedro Almeida is a Senior Integration Developer at Devscope, working with Logic Apps, BizTalk, and other related products. Although he started his career as a Dynamics CRM Consultant, Integrations quickly caught his eyes and has made it his primary area of interest and work. Since then, Pedro has worked with customers from very different areas, from Retail to Banking to Governmental Services and others.

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