[API Proposal]: Allow retrieval of HttpRequestMessage from Polly.Context #4957
Description
Background and motivation
Clients using new resilience APIs based on top of Polly v8 might want to access HttpRequestMessage associated with the attempt being executed. This would allow the following scenarios:
Metering Enrichment
Client can decide to add additional metrics extracted from HttpRequestMessage using metering enrichment.
Custom Routing
For hedging or retries, the client might decide to change the URL for secondary attempts. This would also enable dynamic URL resolution.
Access to HttpRequestMessage in callbacks
For logging/other purposes the client uses callbacks and can be interested in the current request message.
API Proposal
namespace Polly;
public static class HttpResilienceContextExtensions
{
public static HttpRequestMessage? GetRequestMessage(this ResilienceContext context);
public static void SetRequestMessage(this ResilienceContext context, HttpRequestMessage? requestMessage);
}API Usage
services
.AddHttpClient("my-client", client => client.BaseAddress = new Uri("https://primary-endpoint"))
.AddStandardHedgingHandler()
.Configure(options =>
{
options.Hedging.OnHedging = args =>
{
// Retrieve the request message
HttpRequestMessage request = args.ActionContext.GetRequestMessage();
UriBuilder uriBuilder = new(request.RequestUri);
uriBuilder.Host = "secondary-endpoint";
// Override the request URI
request.RequestUri = uriBuilder.Uri;
return default;
};
});The example above demonstrates how the new API can be used to change the URL of outgoing request. Similar approach can be used for retries or telemetry enrichment.
Alternative Designs
The above example can be rewritten as:
ResiliencePropertyKey<HttpRequestMessage> requestMessageKey = new("Resilience.Http.RequestMessage");
services
.AddHttpClient("my-client", client => client.BaseAddress = new Uri("https://primary-endpoint"))
.AddStandardHedgingHandler()
.Configure(options =>
{
options.Hedging.OnHedging = args =>
{
// Retrieve the request message
HttpRequestMessage? request = args.ActionContext.Properties.GetValue(requestMessageKey, null!);
UriBuilder uriBuilder = new(request.RequestUri);
uriBuilder.Host = "secondary-endpoint";
// Override the request URI
request.RequestUri = uriBuilder.Uri;
return default;
};
});However, this relies on some internal naming:
https://github.com/dotnet/extensions/blob/a8e17517bd5150cc0f992b66aa6591c7c6fbdfc4/src/Libraries/Microsoft.Extensions.Http.Resilience/Internal/ResilienceKeys.cs#L13C92-L13C122
It would be better to have built-in support for retrieving HttpRequestMessage from ResilienceContext as I suspect this will be a common scenario.
Risks
Caller retrieving and modifying HttpRequestMessage associated with the current attempt in a non-compatible way or doing some changes that will corrupt the request.