No Sandbox Worker processes are currently available


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Our tester reported the following error logged in a Dynamics 365 online instance multiple times and it is a blocker. The Dynamics 365 instance consists of a Sales module and a PSA module.

Error: The plug-in execution failed because no Sandbox Worker processes are currently available.

Dynamics Sandbox Understanding (The Mechanism)

VarunReddyParam has an excellent write up on the topic. Therefore I am not going to repeat it here. In summary, partially trusted work load in Dynamics 365 follows two flows below.

w3wp > sandbox host > sandbox worker image

CrmAsyncService > sandbox host > sandbox worker image

Possible Causes

  • Microsoft platform issues. The host of my Dynamics 365 ran out of worker processes.
  • w3wp put too much load on the sandbox host.
  • CrmAsyncService puts too much load on the sandbox host.
  • Check SPN (Server Principle Name) if you are on Dynamics 365 on-premise. My friend Aung Khaing kindly supplied this comment.

The REAL Cause

This is GOLD from another friend - Mr. M.S.! He directly IMed me.

The real culprit is an “Async void” method. Below is a sequence of events leading up to the error.

Async void > excption > plugin context doesn’t exist anymore > error bubbles up to sandbox processing service > 3 errors will cause the service to restart > no more sandbox worker process during the restarting.

Solution: convert async code to sync code

Some sample code below

        private void archiveDocAsync(Entity targetEntity, IPluginExecutionContext context, IOrganizationService service)
                var postAsyncTask = client.PostAsync("", content);

                var httpResponseMessage = postAsyncTask.Result;
                var r = httpResponseMessage.Content.ReadAsStringAsync();
            catch (AggregateException ae)
                ae.Handle(inner =>

                    return true;



Narrow Down

I looked through all workflow system jobs. There were failed ones but none of them has the “no Sandbox Worker processes” error.

I didn’t have enought information to pin point the issue. So, I will need some Microsft help because only they can see see lower level errors and logs.

Microsoft Customer Support Service

I was hoping Microsoft CSS can

  • Narrow down if it is a platform issue or a customization issue.
  • If it is a customization issue, they could pin point which workflow or plugin was the culprit.

So a premium support ticket with an escalation to level A later, Microsoft recommended us to review our plugin code.

The Key Takeaways

There are three takeaways. Performance, performance, and performance.

Review your plugin where:

  • Plugins trigger on all entities.
  • Plugins trigger on all attributes.
  • Plugins code retrieves ALL fields.
  • Plugins takes too long to execute. For example, plugins process/validate/generate/upload files. Maybe convert it into an asynchronous workflow?

Fixing issues retrospetively is expensive. You should do the right thing in the first place.


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