ASP.NET并行调用API遇连接意外关闭问题求助

AI 小北

2026-6-19

问题：ASP.NET MVC调用Core Web API批量生成PDF时请求过多导致连接异常

场景说明

主应用为ASP.NET MVC（.NET 4.8），调用ASP.NET Core 8 Web API生成PDF文档。批量生成时先同步完成所有国家的数据准备与映射，再按国家并行发送请求到服务端；服务端通过Razor生成强类型HTML，借助IronPDF库转换为PDF后返回主应用。

核心代码

主应用代码

public async Task PublishDocuments(List<string> countries)
{
    var requests = new Dictionary<string, PublishRequest>();

    foreach (var country in countries)
    {
        // 同步准备数据并添加到requests字典
    }

    using var httpClientHandler = new HttpClientHandler();
    httpClientHandler.ServerCertificateCustomValidationCallback = (sender, cert, chain, sslPolicyErrors) => true;

    using var httpClient = new HttpClient(httpClientHandler);
    httpClient.Timeout = TimeSpan.FromMinutes(15);

    var publishTasks = requests.Select(x => GetAndSaveFile(x.Value, x.Key, httpClient));
    await Task.WhenAll(publishTasks);

    // 后续数据库操作
}

private async Task GetAndSaveFile(PublishRequest request, string fullFilePath, HttpClient httpClient)
{
    var res = await GetDocument(request, httpClient);
    File.WriteAllBytes(fullFilePath, res);
}

private async Task<byte[]> GetDocument(PublishRequest request, HttpClient httpClient)
{
    var serializedRequest = JsonConvert.SerializeObject(request);

    var content = new StringContent(serializedRequest, System.Text.Encoding.UTF8, "application/json");
    var response = await httpClient.PostAsync("https://localhost:7000/publish", content);

    return await response.Content.ReadAsByteArrayAsync();
}

服务端代码

app.MapPost("/publish", async (PublishRequest request, HttpContext context) =>
{
    // 使用Razor生成强类型HTML，依赖IronPDF转PDF
    var htmlContent = await RenderRazorViewToStringAsync(context, "Document", request);
    var renderer = new ChromePdfRenderer();

    renderer.RenderingOptions.EnableJavaScript = true;
    renderer.RenderingOptions.WaitFor.JavaScript(500);

    var pdf = renderer.RenderHtmlAsPdf(htmlContent);

    context.Response.ContentType = "application/pdf";
    context.Response.StatusCode = 200;
    context.Response.Headers.Add("Content-Disposition", "attachment; filename=generated.pdf");
    await context.Response.Body.WriteAsync(pdf.BinaryData);
}).WithRequestTimeout(TimeSpan.FromMinutes(10));

异常信息

请求数量超过阈值时出现两类错误：

PublishDocuments方法中抛出：

System.Net.Http.HttpRequestException: 'An error occurred while sending the request.'

at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()

at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()

at PublishService.d__22.MoveNext() in...

内部异常：

The underlying connection was closed: The connection was closed unexpectedly.

at System.Net.HttpWebRequest.EndGetResponse(IAsyncResult asyncResult)

at System.Net.Http.HttpClientHandler.GetResponseCallback(IAsyncResult ar)

GetDocument方法中抛出：

Error while copying content to a stream.

at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()

at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()

at PublishService.d__22.MoveNext() in ...

内部异常：

Unable to read data from the transport connection: The connection was closed.

at System.Net.ConnectStream.IOError(Exception exception, Boolean willThrow)

at System.Net.ConnectStream.InternalWrite(Boolean async, Byte[] buffer, Int32 offset, Int32 size, AsyncCallback callback, Object state)

at System.Net.ConnectStream.BeginWrite(Byte[] buffer, Int32 offset, Int32 size, AsyncCallback callback, Object state)

at System.Threading.Tasks.TaskFactory1.FromAsyncImpl[TArg1,TArg2,TArg3](Func6 beginMethod, Func2 endFunction, Action1 endAction, TArg1 arg1, TArg2 arg2, TArg3 arg3, Object state, TaskCreationOptions creationOptions)

at System.Net.Http.ByteArrayContent.SerializeToStreamAsync(Stream stream, TransportContext context)

at System.Net.Http.HttpContent.CopyToAsync(Stream stream, TransportContext context)

已尝试无效方案

将HttpClient实例超时设置为15分钟；
曾设置System.Net.ServicePointManager.Expect100Continue = false（无效后移除）；
复用HttpClient实例而非每个请求创建新实例；
服务端API方法设置10分钟请求超时。

可行解决办法

1. 限制并行请求数量

.NET 4.8中ServicePointManager.DefaultConnectionLimit默认值为2，批量并行请求会耗尽连接池，引发连接异常。可以：

全局调整连接限制（比如在Global.asax的Application_Start中）：

System.Net.ServicePointManager.DefaultConnectionLimit = 20; // 根据机器性能调整

使用SemaphoreSlim控制并发请求数，避免瞬间压垮服务端：

// 在PublishDocuments方法中修改任务创建逻辑
var semaphore = new SemaphoreSlim(10); // 限制同时10个请求
var publishTasks = requests.Select(async x =>
{
    await semaphore.WaitAsync();
    try
    {
        await GetAndSaveFile(x.Value, x.Key, httpClient);
    }
    finally
    {
        semaphore.Release();
    }
});
await Task.WhenAll(publishTasks);

2. 优化服务端资源占用

IronPDF的ChromePdfRenderer每次实例化会启动Chrome进程，批量请求会消耗大量CPU、内存：

复用全局单例的ChromePdfRenderer实例，减少资源开销：

// 在服务端启动类中定义全局实例
private static readonly ChromePdfRenderer _renderer = new ChromePdfRenderer();
static YourStartupClass()
{
    _renderer.RenderingOptions.EnableJavaScript = true;
    _renderer.RenderingOptions.WaitFor.JavaScript(500);
}

// 在接口中复用该实例
app.MapPost("/publish", async (PublishRequest request, HttpContext context) =>
{
    var htmlContent = await RenderRazorViewToStringAsync(context, "Document", request);
    var pdf = _renderer.RenderHtmlAsPdf(htmlContent);

    // 响应写入逻辑
}).WithRequestTimeout(TimeSpan.FromMinutes(10));

3. 规范服务端响应处理

替换直接写入Response.Body的方式，使用ASP.NET Core内置的Results.File返回，确保响应完整发送：

app.MapPost("/publish", async (PublishRequest request, HttpContext context) =>
{
    var htmlContent = await RenderRazorViewToStringAsync(context, "Document", request);
    var renderer = new ChromePdfRenderer();
    renderer.RenderingOptions.EnableJavaScript = true;
    renderer.RenderingOptions.WaitFor.JavaScript(500);
    var pdf = renderer.RenderHtmlAsPdf(htmlContent);

    return Results.File(pdf.BinaryData, "application/pdf", "generated.pdf");
}).WithRequestTimeout(TimeSpan.FromMinutes(10));

4. 启用HTTP/2优化连接

.NET 4.8的HttpClient支持HTTP/2，启用后可提升并发请求处理效率：

// 在主应用创建HttpClientHandler时配置
var httpClientHandler = new HttpClientHandler();
httpClientHandler.ServerCertificateCustomValidationCallback = (sender, cert, chain, sslPolicyErrors) => true;
httpClientHandler.SslProtocols = System.Security.Authentication.SslProtocols.Tls12 | System.Security.Authentication.SslProtocols.Tls13;
httpClientHandler.UseHttp2 = true;

5. 添加错误重试机制

针对偶发的连接异常，使用Polly库实现重试逻辑：

// 安装Polly NuGet包后配置重试策略
var retryPolicy = Policy
    .Handle<HttpRequestException>()
    .Or<IOException>()
    .WaitAndRetryAsync(3, retryAttempt => TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)));

// 在GetDocument方法中应用策略
private async Task<byte[]> GetDocument(PublishRequest request, HttpClient httpClient)
{
    return await retryPolicy.ExecuteAsync(async () =>
    {
        var serializedRequest = JsonConvert.SerializeObject(request);
        var content = new StringContent(serializedRequest, System.Text.Encoding.UTF8, "application/json");
        var response = await httpClient.PostAsync("https://localhost:7000/publish", content);
        response.EnsureSuccessStatusCode(); // 触发重试条件
        return await response.Content.ReadAsByteArrayAsync();
    });
}

内容的提问来源于stack exchange，提问作者dzenesiz

ASP.NET并行调用API遇连接意外关闭问题求助

场景说明 #

核心代码 #

主应用代码 #

服务端代码 #

异常信息 #

已尝试无效方案 #

可行解决办法 #

1. 限制并行请求数量 #

2. 优化服务端资源占用 #

3. 规范服务端响应处理 #

4. 启用HTTP/2优化连接 #

5. 添加错误重试机制 #