http - azure Blob : "The condition specified using HTTP conditional header(s) is not met"

Question

当我运行我的应用程序时，我遇到了这个异常。它也会发生在真正的 Azure blob 存储中。

我已经用 Fiddler 捕获了导致此问题的请求:

GET http://127.0.0.1:10000/devstoreaccount1/ebb413ed-fdb5-49f2-a5ac-74faa7e2d3bf/8844c3ec-9e4b-43ec-88b2-58eddf65fc0a/perro?timeout=90 HTTP/1.1
x-ms-version: 2009-09-19
User-Agent: WA-Storage/6.0.6002.18006
x-ms-range: bytes=0-524304
If-Match: 0x8CDA190BD304DD0
x-ms-date: Wed, 23 Feb 2011 16:49:18 GMT
Authorization: SharedKey devstoreaccount1:5j3IScY9UJLN3o1ICWKwVEazO4/IDJG796sdZKqHlR4=
Host: 127.0.0.1:10000

这是响应:

HTTP/1.1 412 The condition specified using HTTP conditional header(s) is not met.
Content-Length: 252
Content-Type: application/xml
Server: Windows-Azure-Blob/1.0 Microsoft-HTTPAPI/2.0
x-ms-request-id: fbff9d15-65c8-4f21-9088-c95e4496c62c
x-ms-version: 2009-09-19
Date: Wed, 23 Feb 2011 16:49:18 GMT

<?xml version="1.0" encoding="utf-8"?><Error><Code>ConditionNotMet</Code><Message>The condition specified using HTTP conditional header(s) is not met.
RequestId:fbff9d15-65c8-4f21-9088-c95e4496c62c
Time:2011-02-23T16:49:18.8790478Z</Message></Error>

当我使用从此行检索的流时会发生这种情况:

blob.OpenRead();

为什么ETAG会介意读操作？我怎样才能避免这个问题？

每次我启动多个并行任务在 blob 存储上执行操作时，都会发生这种情况。

如果我使用:

blob.OpenRead(new BlobRequestOptions() { AccessCondition = AccessCondition.IfMatch("*") });

我得到了这个异常，没有内部异常(在它有一个带有详细信息的 WebException 之前)，要么是 Fiddler 中的失败行:

Microsoft.WindowsAzure.StorageClient.StorageClientException was unhandled
  Message=The conditionals specified for this operation did not match server.
  Source=mscorlib
  StackTrace:
    Server stack trace: 
       at Microsoft.WindowsAzure.StorageClient.Tasks.Task`1.get_Result()
       at Microsoft.WindowsAzure.StorageClient.Tasks.Task`1.ExecuteAndWait()
       at Microsoft.WindowsAzure.StorageClient.TaskImplHelper.ExecuteImpl[T](Func`2 impl)
       at Microsoft.WindowsAzure.StorageClient.BlobReadStream.Read(Byte[] buffer, Int32 offset, Int32 count)
       at System.IO.BinaryReader.ReadBytes(Int32 count)
       at System.Runtime.Serialization.Formatters.Binary.SerializationHeaderRecord.Read(__BinaryParser input)
       at System.Runtime.Serialization.Formatters.Binary.__BinaryParser.ReadSerializationHeaderRecord()
       at System.Runtime.Serialization.Formatters.Binary.__BinaryParser.Run()
       at System.Runtime.Serialization.Formatters.Binary.ObjectReader.Deserialize(HeaderHandler handler, __BinaryParser serParser, Boolean fCheck, Boolean isCrossAppDomain, IMethodCallMessage methodCallMessage)
       at System.Runtime.Serialization.Formatters.Binary.BinaryFormatter.Deserialize(Stream serializationStream, HeaderHandler handler, Boolean fCheck, Boolean isCrossAppDomain, IMethodCallMessage methodCallMessage)
       ...........

提前致谢。

Answer 1

Buff...谜题解决了!

好吧，当您执行 CloudBlob.OpenRead() 时，客户端库正在执行两个操作:

首先，获取 blob 阻止列表:

GET /devstoreaccount1/etagtest/test2.txt?comp=blocklist&blocklisttype=Committed&timeout=90 HTTP/1.1
x-ms-version: 2009-09-19
User-Agent: WA-Storage/6.0.6002.18006
x-ms-date: Wed, 23 Feb 2011 22:21:01 GMT
Authorization: SharedKey devstoreaccount1:SPOBe/IUrZJvoPXnAdD/Twnppvu37+qrUbHnaBHJY24=
Host: 127.0.0.1:10000

HTTP/1.1 200 OK
Transfer-Encoding: chunked
Content-Type: application/xml
Last-Modified: Wed, 23 Feb 2011 22:20:33 GMT
ETag: 0x8CDA1BF0593B660
Server: Windows-Azure-Blob/1.0 Microsoft-HTTPAPI/2.0
x-ms-request-id: ecffddf2-137f-403c-9595-c8fc2847c9d0
x-ms-version: 2009-09-19
x-ms-blob-content-length: 4
Date: Wed, 23 Feb 2011 22:21:02 GMT

注意响应中的 ETag。

第二，我猜想开始检索它，现在注意请求中的ETag:

GET /devstoreaccount1/etagtest/test2.txt?timeout=90 HTTP/1.1
x-ms-version: 2009-09-19
User-Agent: WA-Storage/6.0.6002.18006
x-ms-range: bytes=0-525311
If-Match: 0x8CDA1BF0593B660
x-ms-date: Wed, 23 Feb 2011 22:21:02 GMT
Authorization: SharedKey devstoreaccount1:WXzXRv5e9+p0SzlHUAd7iv7jRHXvf+27t9tO4nrhY5Q=
Host: 127.0.0.1:10000

HTTP/1.1 206 Partial Content
Content-Length: 4
Content-Type: text/plain
Content-Range: bytes 0-3/4
Last-Modified: Wed, 23 Feb 2011 22:20:33 GMT
ETag: 0x8CDA1BF0593B660
Server: Windows-Azure-Blob/1.0 Microsoft-HTTPAPI/2.0
x-ms-request-id: db1e221d-fc61-4837-a255-28b1547cb5d7
x-ms-version: 2009-09-19
x-ms-lease-status: unlocked
x-ms-blob-type: BlockBlob
Date: Wed, 23 Feb 2011 22:21:02 GMT

如果另一个 WebRole 在调用之间在 blob 中执行某些操作，会发生什么情况？ 是的，竞争条件。

解决方案:使用CloudBlob.DownloadToStream()，该方法仅发出一次调用:

GET /devstoreaccount1/etagtestxx/test2.txt?timeout=90 HTTP/1.1
x-ms-version: 2009-09-19
User-Agent: WA-Storage/6.0.6002.18006
x-ms-date: Wed, 23 Feb 2011 22:34:02 GMT
Authorization: SharedKey devstoreaccount1:VjXIO2kbjCIP4UeiXPtxDxmFLeoYAKOqiRv4SV3bZno=
Host: 127.0.0.1:10000

HTTP/1.1 200 OK
Content-Length: 4
Content-Type: text/plain
Last-Modified: Wed, 23 Feb 2011 22:33:47 GMT
ETag: 0x8CDA1C0DEB562D0
Server: Windows-Azure-Blob/1.0 Microsoft-HTTPAPI/2.0
x-ms-request-id: 183a05bb-ea47-4811-8768-6a62195cdb64
x-ms-version: 2009-09-19
x-ms-lease-status: unlocked
x-ms-blob-type: BlockBlob
Date: Wed, 23 Feb 2011 22:34:02 GMT

明天早上我会在工作中实践这一点，看看会发生什么。