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Test retries and timeouts in .NET apps that use Microsoft.Extensions.Http.Resilience

Tip

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At a glance
Goal: Confirm that your .NET HTTP resilience handler retries, backs off, and times out as expected
Time: 15 minutes
Plugins: GenericRandomErrorPlugin, RetryAfterPlugin, LatencyPlugin
Prerequisites: Set up Dev Proxy, a .NET app that uses Microsoft.Extensions.Http.Resilience

You added AddStandardResilienceHandler() to your HttpClient. How do you know it works? The APIs you call rarely fail on demand, and unit tests that mock HttpMessageHandler skip the resilience pipeline that you want to test.

Dev Proxy sits between your app and the API. It returns errors and slow responses to your app, so your app runs unchanged and the resilience handler reacts to real HTTP responses. You see every attempt in the Dev Proxy output.

What the standard resilience handler does

Before you test, know what to expect. With default options, AddStandardResilienceHandler():

Behavior Default
Retries on HTTP 500 and above, 408, 429, HttpRequestException, and TimeoutRejectedException
Number of retries 3, with exponential backoff and jitter, starting at 2 seconds
Retry-After header Honored. The handler waits for the time the API asks for.
Attempt timeout 10 seconds per attempt
Total timeout 30 seconds for the request, including all retries

For the full list of strategies and their defaults, see Standard resilience handler defaults.

Route your app through Dev Proxy

.NET uses the system proxy, so when you start Dev Proxy, it intercepts your app's requests without code changes. For more information, see Use Dev Proxy with .NET applications.

Simulate transient errors

Create a Dev Proxy configuration that fails requests to your API with the errors that the handler retries. This example uses https://api.contoso.com. Replace it with the URL of the API that your app calls.

File: devproxyrc.json

{
  "$schema": "https://raw.githubusercontent.com/dotnet/dev-proxy/main/schemas/v3.3.1/rc.schema.json",
  "plugins": [
    {
      "name": "RetryAfterPlugin",
      "enabled": true,
      "pluginPath": "~appFolder/plugins/DevProxy.Plugins.dll"
    },
    {
      "name": "GenericRandomErrorPlugin",
      "enabled": true,
      "pluginPath": "~appFolder/plugins/DevProxy.Plugins.dll",
      "configSection": "transientErrors"
    }
  ],
  "urlsToWatch": [
    "https://api.contoso.com/*"
  ],
  "transientErrors": {
    "$schema": "https://raw.githubusercontent.com/dotnet/dev-proxy/main/schemas/v3.3.1/genericrandomerrorplugin.schema.json",
    "errorsFile": "transient-errors.json",
    "rate": 50
  }
}

Caution

Add the RetryAfterPlugin before the GenericRandomErrorPlugin in your configuration file. If you add it after, the GenericRandomErrorPlugin fails the request before the RetryAfterPlugin can check it.

In the errors file, define a throttling response and two server errors. The @dynamic value sets the Retry-After header and tells the RetryAfterPlugin to check that your app waits that long before it calls the API again.

File: transient-errors.json

{
  "$schema": "https://raw.githubusercontent.com/dotnet/dev-proxy/main/schemas/v3.3.1/genericrandomerrorplugin.errorsfile.schema.json",
  "errors": [
    {
      "request": {
        "url": "https://api.contoso.com/*"
      },
      "responses": [
        {
          "statusCode": 429,
          "headers": [
            {
              "name": "Retry-After",
              "value": "@dynamic"
            }
          ]
        },
        {
          "statusCode": 500
        },
        {
          "statusCode": 503
        }
      ]
    }
  ]
}

Start Dev Proxy and run your app.

devproxy --config-file devproxyrc.json

With a 50% failure rate, most requests recover after one or two retries. In the Dev Proxy output, check that:

  • After a 429 response, the next attempt to the same URL comes after the Retry-After time. Dev Proxy uses 5 seconds by default. If your app calls the API too early, the RetryAfterPlugin reports it and throttles the request.
  • Your app doesn't send more attempts than you configured.
  • Requests that you don't want to retry, like a POST that creates a record, are sent only once. To exclude them, call options.Retry.DisableForUnsafeHttpMethods() or options.Retry.DisableFor(...).

Test what happens when retries run out

Retries hide short failures. You also need to know what your app does when the API keeps failing. Start Dev Proxy with a 100% failure rate:

devproxy --config-file devproxyrc.json --failure-rate 100

Dev Proxy shows 4 attempts for each request: the original request and 3 retries. After the last retry, the standard handler doesn't throw. It returns the last error response to your code. Check what your app does with it. For example, EnsureSuccessStatusCode() throws an HttpRequestException, and GetStringAsync() throws as well. Make sure that your app shows a useful message or falls back, instead of crashing or showing a generic error.

Test timeouts

Slow APIs trigger a different path in the handler. To test it, use the LatencyPlugin to delay responses beyond the 10-second attempt timeout.

File: devproxyrc.json

{
  "$schema": "https://raw.githubusercontent.com/dotnet/dev-proxy/main/schemas/v3.3.1/rc.schema.json",
  "plugins": [
    {
      "name": "LatencyPlugin",
      "enabled": true,
      "pluginPath": "~appFolder/plugins/DevProxy.Plugins.dll",
      "configSection": "slowApi"
    }
  ],
  "urlsToWatch": [
    "https://api.contoso.com/*"
  ],
  "slowApi": {
    "$schema": "https://raw.githubusercontent.com/dotnet/dev-proxy/main/schemas/v3.3.1/latencyplugin.schema.json",
    "minMs": 11000,
    "maxMs": 15000
  }
}

Start Dev Proxy and run your app. Each attempt takes longer than 10 seconds, so the attempt timeout cancels it and the handler retries. After 30 seconds, the total timeout cancels the request and your code gets a TimeoutRejectedException. Check that your app catches it and tells the user what happened.

Tip

To test the same scenarios with your own resilience settings, change the values in your AddStandardResilienceHandler(options => ...) call and run the same Dev Proxy configurations again.

Next step

Learn more about simulating throttling on any API.

Test that my application handles throttling properly

See also

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