Azure Block Blob PUT fails when using HTTPS

Question

I have written a Cygwin app that uploads (using the REST API PUT operation) Block Blobs to my Azure storage account, and it works well for different size blobs when using HTTP. However, use of SSL (i.e. PUT using HTTPS) fails for Blobs greater than 5.5MB. Blobs less than 5.5MB upload correctly. Anything greater and I find that the TCP session (as seen by Wireshark) reports a dwindling window size that goes to 0 once the aforementioned number of bytes are transferred. The failure is repeatable and consistent. As a point of reference, PUT operations against my Google/AWS/HP cloud storage accounts work fine when using HTTPS for various object sizes, which suggests the problem is not my client but specific to the HTTPS implementation on the MSAZURE storage servers.

If I upload the 5.5MB blob as two separate uploads of 4MB and 1.5MB followed by a PUT Block List, the operation succeeds as long as the two uploads used separate HTTPS sessions. Notice the emphasis on separate. That same operation fails if I attempt to maintain an HTTPS session across both uploads.

Any ideas on why I might be seeing this odd behavior with MS Azure? Same PUT operation with HTTPS works ok with AWS/Google/HP cloud storage servers.

Answer 1

Thank you for reporting this and we apologize for the inconvenience. We have managed to recreate the issue and have filed a bug. Unfortunately we cannot share a timeline for the fix at this time, but we will respond to this forum when the fix has been deployed. In the meantime, a plausible workaround (and a recommended best practice) is to break large uploads into smaller chunks (using the Put Block and Put Block List APIs), thus enabling the client to parallelize the upload.

Answer 2

This bug has now been fixed and the operation should now complete as expected.