Troubleshoot API blank responses

Summary

This article explains how to troubleshoot an API that returns blank responses and restore request forwarding in Azure API Management.

This is the first scenario of the Azure API Management troubleshooting series lab. Make sure you follow the lab setup instructions as per the API Management troubleshooting series lab instructions.

Original product version:   API Management Service
Original KB number:   4464936

Symptoms

Blank API consists of two operations: GetHeaders and GetMyIp. GetMyIp returns the value of the X-FORWARDED-FOR header, and GetHeaders returns all the request header values. GetMyIp returns the expected output, but suddenly GetHeaders started returning a blank response (no response body).

The expected output of the GetHeaders API should be similar to the following example:

{
  "headers": {
    "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8", 
    "Accept-Encoding": "gzip, deflate", 
    "Accept-Language": "en-US,en;q=0.5", 
    "Connection": "close", 
    "Cookie": "_gauges_unique_day=1; _gauges_unique_month=1; _gauges_unique_year=1; _gauges_unique=1", 
    "Host": "eu.httpbin.org", 
    "Upgrade-Insecure-Requests": "1", 
    "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:59.0) Gecko/20100101 Firefox/59.0"
  }
}

Troubleshooting

To troubleshoot this problem, the best approach is to collect an API Management inspector trace to inspect request processing inside the API Management pipeline.

Notice that the forward-request policy is missing. The forward-request policy forwards the incoming request to the backend service specified in the request context. Removing this policy results in the request not being forwarded to the backend service. The policies in the outbound section are evaluated immediately upon the successful completion of the policies in the inbound section.

Check the <backend> section of the GetHeaders operation under Blank-API. Notice that the forward-request policy is removed.

Add the forward-request policy in the backend section or add the <base /> element so that it inherits the forward-request policy from the parent level (API level). This change should resolve the problem.

For more information, see the forward-request policy.