c# - 并发访问时出现 Azure CloudAppendBlob 错误

Question

我的理解是，Azure CloudAppendBlob 不会出现并发问题，因为您只能附加到此 Blob 存储，并且不需要比较电子标签。正如这篇文章所述:

http://blogs.msdn.com/b/windowsazurestorage/archive/2015/04/13/introducing-azure-storage-append-blob.aspx

具体来说:

In addition, Append Blob supports having multiple clients writing to the same blob without any need for synchronization (unlike block and page blob)

但是以下单元测试会引发:

412 the append position condition specified was not met.

堆栈跟踪

Microsoft.WindowsAzure.Storage.Blob.BlobWriteStream.Flush()
Microsoft.WindowsAzure.Storage.Blob.BlobWriteStream.Commit()
Microsoft.WindowsAzure.Storage.Blob.CloudAppendBlob.UploadFromStreamHelper
Microsoft.WindowsAzure.Storage.Blob.CloudAppendBlob.AppendFromStream
Microsoft.WindowsAzure.Storage.Blob.CloudAppendBlob.AppendFromByteArray
Microsoft.WindowsAzure.Storage.Blob.CloudAppendBlob.AppendText

这是单元测试。也许该服务将处理来自不同上下文的请求，但不会像这样并行处理？

    [TestMethod]
    public void test_append_text_concurrency()
    {
        AppendBlobStorage abs = new AppendBlobStorage(new    TestConnectConfig(), "testappendblob");

        string filename = "test-concurrent-blob";

        abs.Delete(filename);                       

        Parallel.Invoke(
            () => { abs.AppendText(filename, "message1\r\n"); },
            () => { abs.AppendText(filename, "message2\r\n"); }
        );

        string text = abs.ReadText(filename);

        Assert.IsTrue(text.Contains("message1"));
        Assert.IsTrue(text.Contains("message2"));
    }

AppendBlobStorage 中的方法

    public void AppendText(string filename, string text)
    {
        CloudAppendBlob cab = m_BlobStorage.BlobContainer.GetAppendBlobReference(filename);

        // Create if it doesn't exist
        if (!cab.Exists())
        {
            try
            {
                cab.CreateOrReplace(AccessCondition.GenerateIfNotExistsCondition(), null, null);
            }
            catch { }
        }

        // Append the text
        cab.AppendText(text);      
    }

也许我错过了一些东西。我尝试这样做的原因是因为我有多个网络作业，它们都可以写入此附加 blob，并且我认为这就是它的设计目的？

Answer 1

经过更多搜索后，看来这是一个实际问题。

我猜 AppendBlobStorage 是相当新的。 (目前 AppendBlobStorage 还存在其他问题。请参阅

http://blogs.msdn.com/b/windowsazurestorage/archive/2015/09/02/issue-in-azure-storage-client-library-5-0-0-and-5-0-1-preview-in-appendblob-functionality.aspx )

无论如何，我通过使用 AppendBlock 变体而不是这里建议的 AppendText 解决了这个问题:

https://azurekan.wordpress.com/2015/09/08/issues-with-adding-text-to-azure-storage-append-blob/

对appendtext方法的更改通过了上面定义的单元测试

    public void AppendText(string filename, string text)
    {
        if (string.IsNullOrWhiteSpace(filename))
            throw new ArgumentException("filename cannot be null or empty");

        if (!string.IsNullOrEmpty(text))
        {
            CloudAppendBlob cab = m_BlobStorage.BlobContainer.GetAppendBlobReference(filename);

            // Create if it doesn't exist
            if (!cab.Exists())
            {
                try
                {
                    cab.CreateOrReplace(AccessCondition.GenerateIfNotExistsCondition(), null, null);
                }
                catch (StorageException) { }
            }

            // use append block as append text seems to have an error at the moment.
            using (MemoryStream ms = new MemoryStream(Encoding.UTF8.GetBytes(text)))
            {
                cab.AppendBlock(ms);
            }
        }

    }