如何指定HTTP状态代码304(NotModified)不是Amazon S3 GetObject API内部的错误条件

背景

我正在尝试使用S3作为一些"相当"静态XML文档的"无限大"大型缓存层。我希望确保客户端应用程序(将在数千台机器上同时运行，并且每小时多次请求XML文档(只有在自上次客户端应用程序下载这些文档以来其内容发生了更改时才下载这些XML文档。

方法

在AmazonS3上，我们可以使用HTTPETAG。默认情况下，AmazonS3对象的ETAG设置为该对象的MD5哈希。

然后，我们可以在GetObjectRequest.ETagToNotMatch属性中指定XML文档的MD5哈希。这确保了当我们进行AmazonS3.GetObject调用(或者在我的情况下是异步版本AmazonS3.BeginGetObject和AmazonS3.EndGetObject(时，如果所请求的文档具有与GetObjectRequest.ETagToNotMatch中包含的相同的MD5哈希，则S3自动返回HTTP状态代码304(NotModified(，并且XML文档的实际内容是而不是下载的。

问题

然而，问题在于，当调用AmazonS3.GetObject(或其异步等价物(时，Amazon.Net API实际上将HTTP状态代码304(NotModified(视为错误，并重试获取请求三次，然后最终抛出Amazon.S3.AmazonS3Exception: Maximum number of retry attempts reached : 3。

显然，我可以将此实现更改为使用AmazonS3.GetObjectMetaData，然后比较ETAG和使用AmazonS3.GetObject(如果它们不匹配(，但当文件过时时，会有两个对S3的请求，而不是一个。无论XML文档是否需要下载，我都希望有一个请求。

有什么想法吗？这是bug还是我遗漏了什么？有没有什么方法可以将重试次数减少到一次并"处理"异常(尽管我对这条路线感到"恶心"(。

实施

我使用的是AWS SDK for.NET(1.3.14版(。

以下是我的实现(稍微减少以保持更短(：

public Task<GetObjectResponse> DownloadString(string key, string etag = null) {
    var request = new GetObjectRequest { Key = key, BucketName = Bucket };
    if (etag != null) {
        request.ETagToNotMatch = etag;
    }
    var task = Task<GetObjectResponse>.Factory.FromAsync(_s3Client.BeginGetObject, _s3Client.EndGetObject, request, null);
    return task;
}

然后我这样称呼它：

var dlTask          = s3Manager.DownloadString("new one", "d7db7bc318d6eb9222d728747879b52e");
var responseTasks   = new[]
    {
        dlTask.ContinueWith(x => _log.Error("Error downloading string.", x.Exception), TaskContinuationOptions.OnlyOnFaulted),
        dlTask.ContinueWith(x => _log.Warn("Downloading string was cancelled."), TaskContinuationOptions.OnlyOnCanceled),
        dlTask.ContinueWith(x => _log.Info(string.Format("Done with download: {0}", x.Result.ETag)), TaskContinuationOptions.OnlyOnRanToCompletion)
    };
try {
    Task.WaitAny(responseTasks);
} catch (AggregateException aex) {
    _log.Error("Error while processing download string.", aex);
}
_log.Info("Exiting...");

然后生成以下日志文件输出：

2011-10-11 13:21:20,376 [11] INFO  Amazon.S3.AmazonS3Client - Received response for GetObject (id 2ee99002-d148-4572-b19b-29259534f48f) with status code NotModified in 00:00:01.6140812.
2011-10-11 13:21:20,385 [11] INFO  Amazon.S3.AmazonS3Client - Request for GetObject is being redirect to https://s3.amazonaws.com/x/new%20one.
2011-10-11 13:21:20,789 [11] INFO  Amazon.S3.AmazonS3Client - Retry number 1 for request GetObject.
2011-10-11 13:21:22,329 [11] INFO  Amazon.S3.AmazonS3Client - Received response for GetObject (id 2ee99002-d148-4572-b19b-29259534f48f) with status code NotModified in 00:00:01.1400356.
2011-10-11 13:21:22,329 [11] INFO  Amazon.S3.AmazonS3Client - Request for GetObject is being redirect to https://s3.amazonaws.com/x/new%20one.
2011-10-11 13:21:23,929 [11] INFO  Amazon.S3.AmazonS3Client - Retry number 2 for request GetObject.
2011-10-11 13:21:26,508 [11] INFO  Amazon.S3.AmazonS3Client - Received response for GetObject (id 2ee99002-d148-4572-b19b-29259534f48f) with status code NotModified in 00:00:00.9790314.
2011-10-11 13:21:26,508 [11] INFO  Amazon.S3.AmazonS3Client - Request for GetObject is being redirect to https://s3.amazonaws.com/x/new%20one.
2011-10-11 13:21:32,908 [11] INFO  Amazon.S3.AmazonS3Client - Retry number 3 for request GetObject.
2011-10-11 13:21:40,604 [11] INFO  Amazon.S3.AmazonS3Client - Received response for GetObject (id 2ee99002-d148-4572-b19b-29259534f48f) with status code NotModified in 00:00:01.2950718.
2011-10-11 13:21:40,605 [11] INFO  Amazon.S3.AmazonS3Client - Request for GetObject is being redirect to https://s3.amazonaws.com/x/new%20one.
2011-10-11 13:21:40,621 [11] ERROR Amazon.S3.AmazonS3Client - Error for GetResponse
Amazon.S3.AmazonS3Exception: Maximum number of retry attempts reached : 3
   at Amazon.S3.AmazonS3Client.pauseOnRetry(Int32 retries, Int32 maxRetries, HttpStatusCode status, String requestAddr, WebHeaderCollection headers, Exception cause)
   at Amazon.S3.AmazonS3Client.handleHttpResponse[T](S3Request userRequest, HttpWebRequest request, HttpWebResponse httpResponse, Int32 retries, TimeSpan lengthOfRequest, T& response, Exception& cause, HttpStatusCode& statusCode)
   at Amazon.S3.AmazonS3Client.getResponseCallback[T](IAsyncResult result)
2011-10-11 13:21:40,635 [10] INFO  Example.Program - Exiting...
2011-10-11 13:21:40,638 [19] ERROR Example.Program - Error downloading string.
System.AggregateException: One or more errors occurred. ---> Amazon.S3.AmazonS3Exception: Maximum number of retry attempts reached : 3
   at Amazon.S3.AmazonS3Client.pauseOnRetry(Int32 retries, Int32 maxRetries, HttpStatusCode status, String requestAddr, WebHeaderCollection headers, Exception cause)
   at Amazon.S3.AmazonS3Client.handleHttpResponse[T](S3Request userRequest, HttpWebRequest request, HttpWebResponse httpResponse, Int32 retries, TimeSpan lengthOfRequest, T& response, Exception& cause, HttpStatusCode& statusCode)
   at Amazon.S3.AmazonS3Client.getResponseCallback[T](IAsyncResult result)
   at Amazon.S3.AmazonS3Client.endOperation[T](IAsyncResult result)
   at Amazon.S3.AmazonS3Client.EndGetObject(IAsyncResult asyncResult)
   at System.Threading.Tasks.TaskFactory`1.FromAsyncCoreLogic(IAsyncResult iar, Func`2 endMethod, TaskCompletionSource`1 tcs)
   --- End of inner exception stack trace ---
---> (Inner Exception #0) Amazon.S3.AmazonS3Exception: Maximum number of retry attempts reached : 3
   at Amazon.S3.AmazonS3Client.pauseOnRetry(Int32 retries, Int32 maxRetries, HttpStatusCode status, String requestAddr, WebHeaderCollection headers, Exception cause)
   at Amazon.S3.AmazonS3Client.handleHttpResponse[T](S3Request userRequest, HttpWebRequest request, HttpWebResponse httpResponse, Int32 retries, TimeSpan lengthOfRequest, T& response, Exception& cause, HttpStatusCode& statusCode)
   at Amazon.S3.AmazonS3Client.getResponseCallback[T](IAsyncResult result)
   at Amazon.S3.AmazonS3Client.endOperation[T](IAsyncResult result)
   at Amazon.S3.AmazonS3Client.EndGetObject(IAsyncResult asyncResult)
   at System.Threading.Tasks.TaskFactory`1.FromAsyncCoreLogic(IAsyncResult iar, Func`2 endMethod, TaskCompletionSource`1 tcs)<---