Fixed: ForEach-Object -Parallel Doesn't Send Requests at the Same Time

The same burst of 600 requests crashed my API from curl and passed from PowerShell, and I nearly wrote that it only happened on one operating system. It was my PowerShell command: asked for 300 at a time, ForEach-Object -Parallel never got past 87.

What I Was Doing

I was testing how an API behaves with more requests at once than its connection pool holds, on PowerShell 7.6.6. This should send 600 requests, 150 at a time:

PowerShell

1..600 | ForEach-Object -Parallel {
    try { (Invoke-WebRequest -Uri "http://localhost:5190/api/v1/products?pageSize=100" -SkipHttpErrorCheck).StatusCode } catch { "no answer" }
} -ThrottleLimit 150 | Group-Object | Select-Object Count, Name

What I Expected

The API to crash, as it had with seq 600 | xargs -P 150 and curl.

What I Got

Output

Count Name
----- ----
  600 200

All fine, three times out of three, and again with -ThrottleLimit 300.

What I Checked

The server's own count. Each request uses a database connection, so MySQL's Max_used_connections, read after a restart of the API, shows how many overlapped:

Peak connections, from a fresh start of the API

ThrottleLimit=150 run 1:  600 200; peak connections=87 alive=200
ThrottleLimit=150 run 2:  600 200; peak connections=82 alive=200
ThrottleLimit=150 run 3:  600 200; peak connections=79 alive=200
ThrottleLimit=300 run 1:  600 200; peak connections=83 alive=200
ThrottleLimit=300 run 2:  600 200; peak connections=80 alive=200
ThrottleLimit=300 run 3:  600 200; peak connections=77 alive=200

Asked for 150 or for 300, it peaked between 77 and 87. The API's pool holds 100, so these requests never had to wait for a connection, and the failure I was testing for never began.

Microsoft's documentation explains the cost: -ThrottleLimit "limits the number of parallel scripts running at a time", and a new runspace "can be a large performance and resource hit if the work you are doing is small compared to creating new runspaces". So it's a ceiling, not what you get.

Why It Happens

ForEach-Object -Parallel is built for work that's slow compared with starting a runspace. A request that takes a few milliseconds isn't, so earlier requests finish while new runspaces are still starting. For real overlap, you need something much cheaper per request.

The Fix

Use HttpClient, and start every request before waiting for any. GetAsync returns as soon as a request is on its way:

PowerShell

Add-Type -AssemblyName System.Net.Http
$client = [System.Net.Http.HttpClient]::new()
$requests = 1..600 | ForEach-Object { $client.GetAsync("http://localhost:5190/api/v1/products?pageSize=100") }
try { [System.Threading.Tasks.Task]::WaitAll([System.Threading.Tasks.Task[]]$requests) } catch { }
$requests | Group-Object { if ($_.IsFaulted) { "no answer" } else { [int]$_.Result.StatusCode } } | Select-Object Count, Name

Line by line:

• GetAsync starts a request and hands back a task. The loop finishes with all 600 running.
• WaitAll waits for them. It throws if any fail, and the catch lets the next line report them.
• Group-Object counts them by status. A task that failed has no status, so it counts as no answer.

After the Fix

This reproduced what curl found, 3 times out of 3:

Output

Count Name
----- ----
  599 no answer
    1 500

Common Mistakes

Mistake	What happens	Do this instead
Trusting -ThrottleLimit as the real concurrency	Your test passes, and proves nothing	Check on the server
Calling .Result inside the loop	Each request waits for the last	Start them all, then wait
A new HttpClient per request	You test socket setup, not your API	Share one

Troubleshooting

Symptom	Likely cause	Fix
Everything answers no answer	The API isn't running	Start it, and check the address
PowerShell can't find the HttpClient type	Windows PowerShell 5.1 needs the assembly loaded	Keep the Add-Type line
Requests time out at 100 seconds	HttpClient's default	Set $client.Timeout

Conclusion

-ThrottleLimit is a ceiling, and with requests this short, ForEach-Object -Parallel doesn't reach it. For a real burst, start every request with HttpClient, and check the result on the server.

Additional Resources

• ForEach-Object — -Parallel and -ThrottleLimit.
• HttpClient.GetAsync — what it returns.

Related

• Fixed: "Unhandled exception. System.AggregateException" with "Authentication to host failed. (I/O error occurred.)" Crashes a .NET C# API with MySql.Data
• How to Limit Concurrent Requests in ASP.NET Core
• How to See Thread Pool Starvation with dotnet-counters