Set a default Polly policy with Flurl

问题:

I'm currently using Polly and Flurl together, but I have a common retry policy that I have to add to every request. I notice that Polly allows you to set a default using AddPolicyHandler(...) but this requires an IHttpClientBuilder and I can't see any way of getting hold of this from Flurl.

I thought overloading DefaultHttpClientFactory might be the way to go, but that only gives me access to the HttpClient, not the IHttpClientBuilder.

I know I could make my own HttpClients and pass them into Flurl, but I'd rather avoid that if I can as I'd like Flurl to manage their lifecycle.

Is there currently a way of doing what I want to do?

回答1:

Great question. Flurl gives you all the necessary hooks to do this. First define a DelegatingHandler that takes a Polly policy:

public class PollyHandler : DelegatingHandler
{
    private readonly IAsyncPolicy<HttpResponseMessage> _policy;

    public PollyHandler(IAsyncPolicy<HttpResponseMessage> policy) {
        _policy = policy;
    }

    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken) {
        return _policy.ExecuteAsync(ct => base.SendAsync(request, ct), cancellationToken);
    }
}

Then create a custom IHttpClientFactory that returns your custom handler with the default handler as its InnerHandler:

public class PollyFactory : DefaultHttpClientFactory
{
    private readonly IAsyncPolicy<HttpResponseMessage> _policy;

    public PollyFactory(IAsyncPolicy<HttpResponseMessage> policy) {
        _policy = policy;
    }

    public override HttpMessageHandler CreateMessageHandler() {
        return new PollyHandler(_policy) {
            InnerHandler = base.CreateMessageHandler()
        };
    }
}

Finally, on app startup, define your policy and register it with Flurl:

var policy = Policy
    .Handle<HttpRequestException>()
    .OrResult<HttpResponseMessage>(r => !r.IsSuccessStatusCode)
    .RetryAsync(5);

FlurlHttp.Configure(settings => settings.HttpClientFactory = new PollyFactory(policy));

One important note is that this approach will not work with a policy that handles FlurlHttpException. That's because you're intercepting calls at the HttpMessageHandler level here. Flurl converts responses and errors to FlurlHttpExceptions higher up the stack, so those won't get trapped/retried with this approach. The policy in the example above traps HttpRequestException and HttpResponseMessage (with non-2XX status codes), which will work.