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

Question

I got this exception when I run my application. It happens also in the real Azure blob storage.

I've caught with Fiddler the request that creates this problem:

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

And this is the response:

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>

It happens when I use the Stream retrieved from this line:

blob.OpenRead();

Why the ETAG minds in a read operation? How may I avoid this problem?

It happens every time I launch several parallel tasks doing things on the blob storage.

If i use:

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

I got this exception with no inner one (before it had a WebException with the details), either a fail line in Fiddler :

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)
       ...........

Thanks in advance.

vtortola · Accepted Answer · 2011-02-23 22:36:30Z

Bufff... mistery solved!

Well, when you do a CloudBlob.OpenRead(), the client library is doing two operations:

First, get the blob block list:

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

Attention to the ETag in the response.

Second, I guess that start to retrieve it, and now attention to the ETag in the request:

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

What happen if another WebRole do something in the blob between call? YES a race condition.

Solution: Use CloudBlob.DownloadToStream(), that method only issues one call:

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

I will put this on practice tomorrow morning at work and see what happen.

You should notify MS that their server is sending invalid ETags, and that they shouldn't return a partial response (206) when the client didn't ask for it.

Anu Thomas Chandy · Accepted Answer · 2013-10-09 17:07:28Z

3

You can still use OpenRead, you need to pass OperationContext instance like below code:

// cloudBlob instance of CloudPageBlob

 OperationContext context = new OperationContext();
 context.SendingRequest += (sender, e) => { 
     e.Request.Headers["if-match"] = "*";
 };

 using (AutoResetEvent waitHandle = new AutoResetEvent(false))
 {
     cloudBlob.StreamMinimumReadSizeInBytes = 16385;
     var result = cloudBlob.BeginOpenRead(null, null, context,
         ar => waitHandle.Set(),
         null);
     waitHandle.WaitOne();
     using (Stream blobStream = vhd.EndOpenRead(result))
     {
         var k = blobStream.ReadByte();
     }
 }

answered Oct 9, 2013 at 17:07

Anu Thomas Chandy

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Comments

Julian Reschke · Accepted Answer · 2011-02-23 18:47:57Z

1

One thing that comes to mind is that the ETag in

If-Match: 0x8CDA190BD304DD0

is malformed; a valid (strong) etag is always in double quotes.

Dunno whether this has something to do with your problem, though.

answered Feb 23, 2011 at 18:47

Julian Reschke

42.5k8 gold badges103 silver badges102 bronze badges

3 Comments

vtortola Over a year ago

Actually I've read somewhere that HTTP 412 is returned when the etag has an invalid format. What I don't know is where that ETAG comes from.

vtortola Over a year ago

here: social.msdn.microsoft.com/Forums/en-US/windowsazuredata/thread/…

Julian Reschke Over a year ago

I'd expect a 400 (bad request) for a malformed etag, but I can see why a server would return the 412 (when it's permissive parsing the header)

Jay Dadhaniya · Accepted Answer · 2021-11-18 06:37:31Z

If you don't want to store blob data in your memory with DownloadToStream and still want to use blob read then you can add access condition on a read operation which matches any Etag available on the reference blob Like below

var accessCondition = new AccessCondition();
var blobRequestOptions = new BlobRequestOptions();
var operationContext = new OperationContext();

// Added match of any ETag access condition so that it will not cause any issue due to ongoing concurrent modification on the same blob
operationContext.SendingRequest += (sender, e) =>
{
    if (e.Request.Headers.Contains("if-match"))
    {
        e.Request.Headers.Remove("if-match");
    }
    e.Request.Headers.Add("if-match", "*");
};

var blobStream = await blobRef.OpenReadAsync(accessCondition, blobRequestOptions, operationContext);

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