Logic Apps and the Service Bus Connector - The Case of the Incomplete Message!

The Symptom

The other day I tracked down an issue my team was seeing with an Azure App Service Logic App. The Logic App was being triggered by an Azure Service Bus Connector that was configured to work with a Service Bus Topic. With a message published to the Service Bus Topic, an instance of the Logic App would execute successfully — as expected on the trigger’s configured schedule. But then, unexpectedly, on the next scheduled time period, often a new instance of the Logic App would execute and reprocess the exact same message from the Service Bus Topic. It was as if the Service Bus Topic message was never successfully “completed” and thus removed from the Topic.

What You Need To Understand

In order to understand why the same Service Bus Topic message was being processed twice, this is what you need to know.

  • The Service Bus Connector operates in ReceiveMode.PeekLock, so it is not receiving and immediately deleting the message from the Topic in one atomic call like in ReceiveMode.ReceiveAndDelete. In PeekLock mode, two calls to the Service Bus must be made. The first call is to the Receive method, and the Service Bus puts a lock on the message for a specific lock duration. The default LockDuration for a Queue or Subscription is 1 minute, and currently that cannot be configured in the Service Bus Connector. Then a second call, to the Complete method, is required to mark that message as processed and delete it from the Service Bus.

  • After a call to a Connector’s trigger method returns a result indicating that data is available to be processed and that a Logic App instance should be created and executed, the Logic App trigger mechanism immediately calls the Connector’s trigger method again. This logically is being done in case there is even more data to be processed. This makes sense because if there is more data available to be processed, one typically would want that data being processed as soon as possible, and would not want to wait for the next scheduled trigger time period.

  • Connectors can have their own specified triggerState data passed from one call of their trigger method to subsequent calls of their trigger method.

  • The Service Bus Connector passes the message’s unique LockToken as triggerState from the first successful trigger invocation to its next trigger call that runs immediately after it.

  • When the Service Bus Connector trigger method is executed, one of the first things that it does is to determine if the triggerState data has been provided. If the triggerState data has been provided, the Service Bus Connector calls the Complete method of the message represented by that given triggerState/LockToken. Only after doing that will it attempt to receive the next message from the configured Topic or Queue.

The Cause

Now that we understand how the Complete method is called as part of the Service Bus Connector’s trigger mechanism, it is easy to understand the cause of the problem.

If the second call to the trigger fails (the call that would complete the message), for instance, because of a transient error on the gateway, then the Service Bus message will not be completed — until perhaps the next scheduled trigger.

However, after the default lock duration of 1 minute expires, the lock is lost and Service Bus makes the message available to be received again.

This is exactly what happened — the message was not completed and when the next scheduled trigger executes, the same message is processed again.

Proof — The Smoking Trigger

From the Trigger History of the Logic App, we can see the trigger that failed in the image below.

Logic App Trigger History with Failed Trigger

The call to this trigger returned a 502 “Bad Gateway” error.

1502 - Web server received an invalid response while acting as a gateway or proxy server.

The full JSON of the output message for the failed trigger is:

2    "headers": {
3        "date": "Mon, 07 Sep 2015 23:28:32 GMT",
4        "server": "Microsoft-IIS/8.0"
5    },
7    "body": "<!DOCTYPE html PUBLIC \"-//W3C//DTD XHTML 1.0 Strict//EN\" \"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd\">\r\n<html xmlns=\"http://www.w3.org/1999/xhtml\">\r\n<head>\r\n<meta http-equiv=\"Content-Type\" content=\"text/html; charset=iso-8859-1\"/>\r\n<title>502 - Web server received an invalid response while acting as a gateway or proxy server.</title>\r\n<style type=\"text/css\">\r\n<!--\r\nbody{margin:0;font-size:.7em;font-family:Verdana, Arial, Helvetica, sans-serif;background:#EEEEEE;}\r\nfieldset{padding:0 15px 10px 15px;} \r\nh1{font-size:2.4em;margin:0;color:#FFF;}\r\nh2{font-size:1.7em;margin:0;color:#CC0000;} \r\nh3{font-size:1.2em;margin:10px 0 0 0;color:#000000;} \r\n#header{width:96%;margin:0 0 0 0;padding:6px 2% 6px 2%;font-family:\"trebuchet MS\", Verdana, sans-serif;color:#FFF;\r\nbackground-color:#555555;}\r\n#content{margin:0 0 0 2%;position:relative;}\r\n.content-container{background:#FFF;width:96%;margin-top:8px;padding:10px;position:relative;}\r\n-->\r\n</style>\r\n</head>\r\n<body>\r\n<div id=\"header\"><h1>Server Error</h1></div>\r\n<div id=\"content\">\r\n <div class=\"content-container\"><fieldset>\r\n  <h2>502 - Web server received an invalid response while acting as a gateway or proxy server.</h2>\r\n  <h3>There is a problem with the page you are looking for, and it cannot be displayed. When the Web server (while acting as a gateway or proxy) contacted the upstream content server, it received an invalid response from the content server.</h3>\r\n </fieldset></div>\r\n</div>\r\n</body>\r\n</html>\r\n"

The Remedy

This situation should not happen often, but unfortunately it was occurring multiple times per day, and only ever on the second call to the trigger method. The only real solution is to follow Microsoft’s advice and design your systems to be idempotent, because it can and occasionally will happen.

In addition to that, to reduce the frequency of the situation, hopefully Microsoft will release a change in the near future so that failed calls to the trigger method are automatically retried.

How To Configure A Logic App To Use An Internal API App

I have been developing Microsoft Azure App Service functionality over the last few weeks and have stumbled over many hurdles. The Logic Apps development experience is still definitely in ‘preview’.

One issue that especially concerned me was a problem I had with a Logic App when trying to use a custom API App that had its Access Level set to Internal. This setting lives in All settings => Application settings => Access Level, with the possible options being:

  • Public (anonymous);
  • Internal; or
  • Public (authenticated).

After configuring the Logic App to use the custom API App and running the Logic App, the following error message is shown in the Outputs Link of the Logic App run => Action.

1"status": 403,
2"source": "https://[omitted].azurewebsites.net/api/service/invoke/myinternalaccess.apiapp/DoSomething?api-version=2015-01-14",
3"message": "Permissions for service \"MyInternalAccess.ApiApp\" are set to internal but this request was external."

Clearly, the Logic App was behaving like an external caller of the API App.

The way to fix this is by modifying the code of the Logic App. There were two issues in the code.

Issue 1

In the "parameters" node, there is a parameter for a token for your API App.

 1"parameters": {
 2  "/subscriptions/[...]/resourcegroups/[...]/providers/Microsoft.AppService/apiapps/myinternalaccess.apiapp/token": {
 3    "type": "String",
 4    "metadata": {
 5      "token": {
 6        "name": "/subscriptions/[...]/resourcegroups/[...]/providers/Microsoft.AppService/apiapps/myinternalaccess.apiapp/token"
 7      }
 8    }
 9  },

The code above is missing a "defaultValue" node. In order to fix this issue, add in "defaultValue": "", in after type. The parameter should now look like:

 1"parameters": {
 2  "/subscriptions/[...]/resourcegroups/[...]/providers/Microsoft.AppService/apiapps/myinternalaccess.apiapp/token": {
 3    "type": "String",
 4    "defaultValue": "", 
 5    "metadata": {
 6      "token": {
 7        "name": "/subscriptions/[...]/resourcegroups/[...]/providers/Microsoft.AppService/apiapps/myinternalaccess.apiapp/token"
 8      }
 9    }
10  },

After saving, exiting the code editor, refreshing, jumping up and down for good luck (given that the user experience is not entirely reliable), going back into the editor of the Logic App, waiting a little for things to load (this can be important!), and switching to the code editor, the defaultValue is then magically populated with a nice long set of characters.

Issue 2

The second issue is that the API App action must have an "authentication" node under the "inputs" node.

The API App action that did not work looked like the following:

 1"actions": {
 2  "myinternalaccess.apiapp": {
 3    "type": "ApiApp",
 4    "inputs": {
 5      "apiVersion": "2015-01-14",
 6      "host": {
 7        "id": "/subscriptions/[...]/resourcegroups/[...]/providers/Microsoft.AppService/apiapps/myinternalaccess.apiapp",
 8        "gateway": "https://[omitted].azurewebsites.net"
 9      },
10      "operation": "DoSomething",
11        "parameters": {}
12    },
13    "conditions": []
14  }

The fix is to add the following "authentication" node after the "parameters" node.

1"authentication": {
2  "type": "Raw",
3  "scheme": "Zumo",
4  "parameter": "@parameters('/subscriptions/[...]/resourcegroups/[...]/providers/Microsoft.AppService/apiapps/myinternalaccess.apiapp/token')"

The action should now look like:

 1"actions": {
 2  "myinternalaccess.apiapp": {
 3    "type": "ApiApp",
 4    "inputs": {
 5      "apiVersion": "2015-01-14",
 6      "host": {
 7        "id": "/subscriptions/[...]/resourcegroups/[...]/providers/Microsoft.AppService/apiapps/myinternalaccess.apiapp",
 8        "gateway": "https://[omitted].azurewebsites.net"
 9      },
10      "operation": "DoSomething",
11      "parameters": {},
12      "authentication": {
13        "type": "Raw",
14        "scheme": "Zumo",
15        "parameter": "@parameters('/subscriptions/[...]/resourcegroups/[...]/providers/Microsoft.AppService/apiapps/myinternalaccess.apiapp/token')"
16      }
17    },
18    "conditions": []
19  }

After saving these changes and doing a good-luck dance, the Logic App will now successfully call the API App.

Sometimes, if you go back into the editor, after it has loaded the Discard button is enabled, and if you switch into the code, you may notice that the authentication node is missing. Simply press the Discard button and the authentication node immediately reappears.

Hopefully Microsoft will fix many of the user experience issues with Logic Apps very soon!