Polly Timeout 乐观使用 HttpClientFactory。 如何使用取消令牌？

Question

I would like to understand how to implement an optimistic timeout policy with Polly by using the HttpClientFactory .

In the examples on the net, the timeout policy is used when calling asynchronous methods to be able to pass the cancellation token. But if I set the timeout policy from configure services, as indicated in the guide ), how do I manage the cancellation token?

In example code, from guide linked, I see this:

var timeoutPolicy = Policy.TimeoutAsync<HttpResponseMessage>(10); // Timeout for an individual try

serviceCollection.AddHttpClient("GitHub", client =>
{
    client.BaseAddress = new Uri("https://api.github.com/");
    client.DefaultRequestHeaders.Add("Accept", "application/vnd.github.v3+json");
    client.Timeout = TimeSpan.FromSeconds(60); // Overall timeout across all tries
})
.AddPolicyHandler(retryPolicy)
.AddPolicyHandler(timeoutPolicy);

Answer 1

You should chain the retry and timeout policy into a combined policy.

You have two options:

Wrap method

.AddPolicyHandler(retryPolicy.Wrap(timeoutPolicy))

• timeoutPolicy is the inner policy so it applied for each and every attempt separately
• retryPolicy is the outer policy, so it's overarching the timeout policy
• Note the ordering matters (I will detail it in a later section)

PolicyWrap class

.AddPolicyHandler(Policy.Wrap(retryPolicy,timeoutPolicy))

• the first argument is the most outer policy
• the last argument is the most inner policy

---

Ordering does matter

You should be aware that the following two combined policies are very different:

Policy.Wrap(retryPolicy,timeoutPolicy)
Policy.Wrap(timeoutPolicy, retryPolicy)

• In the first case you have a local timeout, which is applied for each and every retry attempt
• In the second case you have a global timeout, which is applied for the overall retry activities

Pushing this idea forward you can avoid to set the Timeout property of HttpClient by defining a global timeout as well:

var localTimeoutPolicy = Policy.TimeoutAsync<HttpResponseMessage>(10);
var globalTimeoutPolicy = Policy.TimeoutAsync<HttpResponseMessage>(60);
var resilientStrategy = Policy.Wrap(globalTimeoutPolicy, retryPolicy, localTimeoutPolicy);

serviceCollection.AddHttpClient("GitHub", client =>
{
    client.BaseAddress = new Uri("https://api.github.com/");
    client.DefaultRequestHeaders.Add("Accept", "application/vnd.github.v3+json");
})
.AddPolicyHandler(resilientStrategy);