CODEWITHSUNDEEP
c#azureazure-blob-storage
Anu Thomas Chandy
Anu Thomas Chandy

Reputation: 570

Reading Azure blob content while copying initiated through CopyBlob API is in progress

I used to initiate copy blob and while copying was in progress we were able to read few bytes (e.g. first 512 bytes) of the destination blob. See below code that initiate copy blob and then read the first 512 bytes while copying is in progress.

But now blobStream.Read always fills the data buffer with zero not with the actual bytes. I have tried with latest azure storage client lib also, but output is same. But as soon as the copy finishes I can read the actual bytes.

Someone please let me know whether this is a bug introduced in the latest azure storage service and possible workaround (if any)?

CloudBlobClient client = CloudStorageAccount.Parse(
    string.Format(
        "DefaultEndpointsProtocol={0};AccountName={1};AccountKey={2}",
        "http",
        "<destination-account-name>",
        "<destination-account-key>"
    )
).CreateCloudBlobClient();


var vhdsContainer = client.GetContainerReference("<destination-container-name>");
CloudBlob destinationBlob = vhdsContainer.GetPageBlobReference("test-1-b5c1d3f4-23d5-40e4-adc7-236553f8d62d-1.vhd");
destinationBlob.StartCopyFromBlob(
    new Uri("http://<source-storage>.blob.core.windows.net/<source-container-name>/test-4-d803ca0a-5d98-4be8-8895-2a9d15ec3974-1.vhd"), null, null, null);


CloudBlob destBlob = vhdsContainer.GetBlobReference("test-1-b5c1d3f4-23d5-40e4-adc7-236553f8d62d-1.vhd");
int maxWaitTime = 3*60000;//let's wait for a maximum of 3 minute
do
{
    destBlob.FetchAttributes();
    if (destBlob.CopyState.BytesCopied > 2048 || maxWaitTime <= 0)
    {
        break;
    }
    maxWaitTime -= 1000;
    Thread.Sleep(1000);
 }
 while (true);

 var data = new byte[512];
 using (BlobStream blobStream = destBlob.OpenRead())
 {
     blobStream.Seek(0, System.IO.SeekOrigin.Begin);
     blobStream.Read(data, 0, 512);
 }

Upvotes: 1

Views: 723

Answers (2)

Anu Thomas Chandy
Anu Thomas Chandy

Reputation: 570

Looks like user has to wait for copy to finish to read actual bytes from the destination blob.

The CopyState::BytesCopied can be used to track the number of bytes copied, user should not assume that destination blob is ready for read based on CopyState::BytesCopied. Read only after CopyState::Status become 'Success'.

Upvotes: 0

Gaurav Mantri
Gaurav Mantri

Reputation: 136196

As you may already know, copy blob operation is asynchronous. When you call StartCopyFromBlob, the operation is queued. There is no guarantee that the operation will start immediately. This could be the reason for not able to read meaningful data immediately after start copying operation.

One possible thing you could do is check the copy status periodically after calling the copy operation. Once you see that the copy operation has begun, then you can start reading from the destination blob.

To check if the copy operation has started, you could fetch blob attributes in a loop and check for CopyState property of the blob. Once you see some bytes are copied, you can break out of the loop. However keep in mind that you may end up looping in this loop for a long time if the copy operation does not start in a reasonable time frame.

So instead of doing:

Thread.Sleep(4000)

You could do something like:

int maxWaitTime = 10000;//let's wait for a maximum of 10 seconds
do
{
    destBlob.FetchAttributes();
    if (destBlob.CopyState.BytesCopied > 0 || maxWaitTime <= 0)
    {
        break;
    }
    maxWaitTime -= 1000;
    Thread.Sleep(1000);
}
while (true);

Upvotes: 2

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