Azure blob availability during an overwrite

Question

Is a azure blob available for download whilst it is being overwritten with a new version?

From my tests using Cloud Storage Studio the download is blocked until the overwrite is completed, however my tests are from the same machine so I can't be sure this is correct.

If it isn't available during an overwrite, then I presume the solution (to maintain availability) would be to upload using a different blob name and then rename once complete. Does anyone have any better solution than this?

Answer 1

The blob is available during overwrite. What you see will depend on whether you are using a block blob or page blob however. For block blobs, you will download the older version until the final block commit. That final PutBlockList operation will atomically update the blob to the new version. I am not actually sure however for very large blobs that you are in the middle of downloading what happens when a PutBlockList atomically updates the blob. Choices are: a.) request continues with older blob, b.) the connection is broken, or c:) you start downloading bytes of new blob. What a fun thing to test!

If you are using page blobs (without a lease), you will read inconsistent data as the page ranges are updated underneath you. Each page range update is atomic, but it will look weird unless you lease the blob and keep other readers out (readers can snapshot a leased blob and read the state).

I might try to test the block blob update in middle of read scenario to see what happens. However, your core question should be answered: the blob is available.