Load testing POST request with JMeter getting 500 error

Question

I'm trying to set up load testing (for the very first time) for a containerized Web App that uses a JMeter script to call a specific POST request. I was able to get it working when calling a GET request, but my POST just returns a 500 server error and I'm not sure why.

I know it's not a problem with the function itself because:

1. I put a logging statement as the very first line of the function and it isn't being called,
2. and when I set the log level to trace in my host.json I see that the invocation request is getting sent but no response comes back.

That looks to me like there's a problem with how I'm making the API call and therefore with my JMeter script.

Here is my script. Does anyone see what I'm doing wrong?

<?xml version="1.0" encoding="UTF-8"?>
<jmeterTestPlan version="1.2" properties="5.0" jmeter="5.4.1">
  <hashTree>
    <TestPlan guiclass="TestPlanGui" testclass="TestPlan" testname="Azure Load Testing" enabled="true">
      <boolProp name="TestPlan.functional_mode">true</boolProp>
      <boolProp name="TestPlan.tearDown_on_shutdown">true</boolProp>
      <boolProp name="TestPlan.serialize_threadgroups">false</boolProp>
      <elementProp name="TestPlan.user_defined_variables" elementType="Arguments" guiclass="ArgumentsPanel" testclass="Arguments" testname="User Defined Variables" enabled="true">
        <collectionProp name="Arguments.arguments"/>
      </elementProp>
      <stringProp name="TestPlan.user_define_classpath"></stringProp>
    </TestPlan>
    <hashTree>
      <ThreadGroup guiclass="ThreadGroupGui" testclass="ThreadGroup" testname="Web endpoint test" enabled="true">
        <stringProp name="ThreadGroup.on_sample_error">continue</stringProp>
        <elementProp name="ThreadGroup.main_controller" elementType="LoopController" guiclass="LoopControlPanel" testclass="LoopController" testname="Loop Controller" enabled="true">
          <boolProp name="LoopController.continue_forever">false</boolProp>
          <intProp name="LoopController.loops">1</intProp>
        </elementProp>
        <stringProp name="ThreadGroup.num_threads">1</stringProp>
        <stringProp name="ThreadGroup.ramp_time">1</stringProp>
        <boolProp name="ThreadGroup.scheduler">true</boolProp>
        <stringProp name="ThreadGroup.duration">20</stringProp>
        <stringProp name="ThreadGroup.delay"></stringProp>
        <boolProp name="ThreadGroup.same_user_on_next_iteration">true</boolProp>
      </ThreadGroup>
      <hashTree>
        <HTTPSamplerProxy guiclass="HttpTestSampleGui" testclass="HTTPSamplerProxy" testname="HTTP request" enabled="true">
          <boolProp name="HTTPSampler.postBodyRaw">true</boolProp>
          <elementProp name="HTTPsampler.Arguments" elementType="Arguments">
            <collectionProp name="Arguments.arguments">
              <elementProp name="" elementType="HTTPArgument">
                <boolProp name="HTTPArgument.always_encode">false</boolProp>
                <stringProp name="Argument.value">{ &quot;redacted&quot;: &quot;JSON body&quot; }</stringProp>
                <stringProp name="Argument.metadata">=</stringProp>
              </elementProp>
            </collectionProp>
          </elementProp>
          <stringProp name="HTTPSampler.domain">redacted.azurewebsites.net</stringProp>
          <stringProp name="HTTPSampler.port">443</stringProp>
          <stringProp name="HTTPSampler.protocol">https</stringProp>
          <stringProp name="HTTPSampler.path">/api/calculate</stringProp>
          <stringProp name="HTTPSampler.method">POST</stringProp>
          <boolProp name="HTTPSampler.follow_redirects">true</boolProp>
          <boolProp name="HTTPSampler.auto_redirects">false</boolProp>
          <boolProp name="HTTPSampler.use_keepalive">true</boolProp>
          <boolProp name="HTTPSampler.DO_MULTIPART_POST">false</boolProp>
          <boolProp name="HTTPSampler.BROWSER_COMPATIBLE_MULTIPART">false</boolProp>
          <boolProp name="HTTPSampler.image_parser">false</boolProp>
          <boolProp name="HTTPSampler.concurrentDwn">false</boolProp>
          <stringProp name="HTTPSampler.concurrentPool">6</stringProp>
          <boolProp name="HTTPSampler.md5">false</boolProp>
          <intProp name="HTTPSampler.ipSourceType">0</intProp>
          <stringProp name="HTTPSampler.implementation">HttpClient4</stringProp>
        </HTTPSamplerProxy>
        <hashTree>
          <HeaderManager guiclass="HeaderPanel" testclass="HeaderManager" testname="HTTP Header Manager" enabled="true">
            <collectionProp name="HeaderManager.headers">
              <elementProp name="" elementType="Header">
                <stringProp name="Header.name">Accept</stringProp>
                <stringProp name="Header.value">application/json, text/plain, */*</stringProp>
              </elementProp>
              <elementProp name="Content-Type" elementType="Header">
                <stringProp name="Header.name">Content-Type</stringProp>
                <stringProp name="Header.value">application/json</stringProp>
              </elementProp>
            </collectionProp>
          </HeaderManager>
          <hashTree/>
        </hashTree>
      </hashTree>
    </hashTree>
  </hashTree>
</jmeterTestPlan>

Answer 1

In your JMeter script, you have defined "Accept" and "Content-Type" headers, which are common for JSON-based APIs. Ensure they are correctly set and that there are no typos.

Answer 2

I ended up capturing the calls I wanted with JMeter recording. The format of the resulting script was just different enough that I still can't pinpoint what was going wrong with posts but not gets, but using the new script is working.