Description

The following error might appear randomly in the Sitecore logs:

ERROR Exception in alarm clock event subscriber. 
Exception: System.InvalidOperationException 
Message: Collection was modified; enumeration operation may not execute. 
Source: mscorlib 
   at System.Collections.Hashtable.HashtableEnumerator.MoveNext() 
   at Sitecore.Collections.SafeDictionary`2.d__7.MoveNext() 
   at Sitecore.Pipelines.HealthMonitor.HealthMonitor.DumpRenderingsStatistics(PipelineArgs args) 
   at (Object , Object[] ) 
   at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args) 
   at Sitecore.Services.AlarmClock.Heartbeat_Beat(Object sender, EventArgs e)

The Sitecore HealthMonitor periodically enumerates the page rendering statistics collection to save the information into a file. The issue might occur if another thread tries to modify the collection concurrently, meaning, if a page is being rendered while the statistics are being saved.

The issue affects Sitecore XP 8.0.3 - 8.2.2, Sitecore XP 9.0.1, and later versions.

Solution

To resolve the issue, consider one of the following options:

• Upgrade your solution to Sitecore XP 10.3.0 or later.
• For Sitecore XP 8.2.2 and earlier, and for Sitecore 9.0 (Initial Release to Update-2), download and install the patch compatible with the affected product version found on this page: https://github.com/sitecoresupport/Sitecore.Support.126747/releases.

Alternatively, to mitigate the issue, proceed with the following options:

• To stop saving rendering statistics into a file, disable the DumpRenderingsStatistics functionality by creating a configuration patch file in the \App_Config\Include\zzz folder, as shown in the following:
  

  <?xml version="1.0" encoding="utf-8" ?>
  <configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
   <sitecore>
    <pipelines>
     <healthMonitor>
      <processor method="DumpRenderingsStatistics">
       <patch:delete/>
      </processor>
     </healthMonitor>
    </pipelines>
   </sitecore>
  </configuration>

• To stop collecting rendering statistics, set the Statistics.CollectRenderingData setting value to false:
  

  <!-- COLLECT RENDERING DATA
  Indicates if renderings statistics should be collected.
  Default value: true
  -->
  <setting name="Statistics.CollectRenderingData" value="false"/>

  Note: The values must be patched by creating a configuration patch in the \App_Config\Include\zzz folder.