C# の HttpClient で HTTP 503 の処理をテストする

$ curl -i https://api.testingapis.com/api/status/503
HTTP/1.1 503 Service Unavailable
content-type: application/json; charset=utf-8
retry-after: 5

{"status":503,"statusText":"Service Unavailable","category":"server-error","message":"This is a 503 response. Use /api/status/{code} to request any status."}

using System.Net.Http.Json;
using System.Text.Json;

using var http = new HttpClient { BaseAddress = new Uri("https://api.testingapis.com") };

// A 503 is a normal response, not an exception.
using var response = await http.GetAsync("/api/status/503");
Console.WriteLine($"Status: {(int)response.StatusCode} {response.StatusCode}");
Console.WriteLine($"IsSuccessStatusCode: {response.IsSuccessStatusCode}");
Console.WriteLine($"Retry-After: {response.Headers.RetryAfter?.Delta?.TotalSeconds} seconds");

// EnsureSuccessStatusCode turns it into one, with the status attached.
try
{
    response.EnsureSuccessStatusCode();
}
catch (HttpRequestException ex)
{
    Console.WriteLine($"HttpRequestException: StatusCode={(int?)ex.StatusCode}");
}

// GetFromJsonAsync throws before reading the body.
try
{
    await http.GetFromJsonAsync<JsonElement>("/api/Products/1?error=503");
}
catch (HttpRequestException ex)
{
    Console.WriteLine($"GetFromJsonAsync threw: {(int?)ex.StatusCode}");
}

Status: 503 ServiceUnavailable
IsSuccessStatusCode: False
Retry-After: 5 seconds
HttpRequestException: StatusCode=503
GetFromJsonAsync threw: 503

$ curl -i "https://api.testingapis.com/api/Products/1?error=503"
HTTP/1.1 503 Service Unavailable
x-simulated: true

{"status":503,"error":"Service Unavailable","message":"Simulated error. Use ?error={code} to test different status codes.","simulated":true}

using System.Net;

/// Retries 503 and 429, waiting as long as Retry-After asks (capped), up to maxAttempts.
public sealed class RetryHandler(int maxAttempts, TimeSpan maxDelay) : DelegatingHandler
{
    protected override async Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request, CancellationToken cancellationToken)
    {
        for (var attempt = 1; ; attempt++)
        {
            var response = await base.SendAsync(request, cancellationToken);
            var retryable = response.StatusCode is HttpStatusCode.ServiceUnavailable
                                                or HttpStatusCode.TooManyRequests;
            if (!retryable || attempt == maxAttempts)
            {
                return response;
            }

            // Retry-After is either a number of seconds or a date. Without it, back off 0.5 s, 1 s, 2 s...
            var wait = response.Headers.RetryAfter?.Delta
                ?? (response.Headers.RetryAfter?.Date - DateTimeOffset.UtcNow)
                ?? TimeSpan.FromMilliseconds(500 * Math.Pow(2, attempt - 1));
            if (wait > maxDelay) wait = maxDelay;
            if (wait < TimeSpan.Zero) wait = TimeSpan.Zero;

            Console.WriteLine($"  attempt {attempt}: {(int)response.StatusCode}, waiting {wait.TotalSeconds:F1}s");
            response.Dispose();
            await Task.Delay(wait, cancellationToken);
        }
    }
}

using System.Diagnostics;

using var http = new HttpClient(
    new RetryHandler(maxAttempts: 3, maxDelay: TimeSpan.FromSeconds(2)) { InnerHandler = new HttpClientHandler() })
{
    BaseAddress = new Uri("https://api.testingapis.com"),
};

var watch = Stopwatch.StartNew();
using var response = await http.GetAsync("/api/status/503");
Console.WriteLine($"After retries: {(int)response.StatusCode} in {watch.Elapsed.TotalSeconds:F1}s");

watch.Restart();
using var ok = await http.GetAsync("/api/Products/1");
Console.WriteLine($"Healthy endpoint: {(int)ok.StatusCode} in {watch.Elapsed.TotalSeconds:F1}s");

attempt 1: 503, waiting 2.0s
  attempt 2: 503, waiting 2.0s
After retries: 503 in 4.3s
Healthy endpoint: 200 in 0.0s