如何实现Azure Web App实例优雅重启？解决.NET大字符串内存问题

Hey there! Let's tackle this graceful restart scenario for your Azure Web App running a .NET app—great question, since handling large strings and memory constraints makes this super important. Here's a step-by-step approach to get this working:

1. 先配置Azure层面的优雅关闭基础 #

Azure Web App has built-in graceful shutdown settings you should enable first:

• Head to your Web App in the Azure Portal → Configuration → General Settings
• Look for the "Graceful shutdown timeout" option, set a value that matches your longest typical request processing time (e.g., 300 seconds)
  This tells Azure to stop routing new requests to the instance when a restart is triggered, while giving existing requests time to finish before forcing a shutdown.

2. 在.NET应用中监听关闭信号并拒绝新请求 #

Azure's default settings help, but you'll need to add code to your ASP.NET Core app to explicitly block new requests during shutdown and wrap up ongoing work. Here's how:

示例：注册优雅关闭服务 #

Create a hosted service to track shutdown state and handle cleanup:

public class GracefulShutdownService : IHostedService
{
    private readonly ILogger<GracefulShutdownService> _logger;
    private readonly IHostApplicationLifetime _appLifetime;
    private bool _isShuttingDown = false;
    private int _activeRequests = 0;

    public GracefulShutdownService(ILogger<GracefulShutdownService> logger, 
                                  IHostApplicationLifetime appLifetime)
    {
        _logger = logger;
        _appLifetime = appLifetime;
    }

    public Task StartAsync(CancellationToken cancellationToken)
    {
        _appLifetime.ApplicationStopping.Register(InitiateShutdown);
        return Task.CompletedTask;
    }

    private void InitiateShutdown()
    {
        _isShuttingDown = true;
        _logger.LogInformation("Instance entering graceful shutdown mode—no new requests accepted.");

        // Wait for active requests to finish (adjust timeout as needed)
        var waitTime = TimeSpan.FromSeconds(45);
        var startTime = DateTime.UtcNow;
        while (_activeRequests > 0 && DateTime.UtcNow - startTime < waitTime)
        {
            Thread.Sleep(500);
        }
    }

    public void IncrementActiveRequests() => Interlocked.Increment(ref _activeRequests);
    public void DecrementActiveRequests() => Interlocked.Decrement(ref _activeRequests);
    public bool IsShuttingDown() => _isShuttingDown;

    public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;
    }

示例：添加中间件拦截新请求 #

Register the service and add a middleware to block new requests during shutdown:

var builder = WebApplication.CreateBuilder(args);

// Register required services
builder.Services.AddHostedService<GracefulShutdownService>();

var app = builder.Build();

// Middleware to reject new requests when shutting down
app.Use(async (context, next) =>
{
    var shutdownService = context.RequestServices.GetRequiredService<GracefulShutdownService>();
    
    if (shutdownService.IsShuttingDown())
    {
        context.Response.StatusCode = StatusCodes.Status503ServiceUnavailable;
        await context.Response.WriteAsync("Instance is restarting. Please try again shortly.");
        return;
    }

    // Track active requests
    shutdownService.IncrementActiveRequests();
    try
    {
        await next();
    }
    finally
    {
        shutdownService.DecrementActiveRequests();
    }
});

// Your existing middleware/routing setup here...

app.Run();

3. 触发优雅重启的正确方式 #

• Manual restart via Portal: Go to your Web App → Instances, select the target instance, then click "Restart". Azure will trigger the graceful shutdown flow before restarting.
• Automated restart via CLI: Use this command to restart a specific instance (follows graceful shutdown rules):

az webapp restart --name <your-webapp-name> --resource-group <your-resource-group> --instance <instance-id>

4. Bonus: Fix that large string memory issue #

Since you mentioned oversized strings causing memory problems, here are quick fixes to reduce pressure:

• Use Stream instead of strings for handling large content (avoids loading entire payloads into memory)
• Enable server-side GC in your runtimeconfig.json:

{
  "runtimeOptions": {
    "configProperties": {
      "System.GC.Server": true
    }
  }
}

• Offload large string processing to background tasks (like Azure Functions) to keep your Web App instances lean.

内容的提问来源于stack exchange，提问作者Minh