Fetch Azure blob metadata only if the blob exists

Question

I need to fetch the metadata for an Azure blob if it exists and would like to achieve this with minimal REST calls (by the storage SDK to the storage service)

I know I can do something like

CloudBlockBlob.ExistsAsync() and then
CloudBlockBlob.FetchAttributesAsync() if the blob exists

I tried to combine these 2 calls into one

CloudBlockBlob.FetchAttributesAsync(AccessCondition.GenerateIfExistsCondition(),new BlobRequestOptions(), new OperationContext());

Docs on 'AccessCondition.GenerateIfExistsCondition()' say -

Constructs an access condition such that an operation will be performed only if the resource exists.

but it still fails with a 404 not found.

Any idea if what I want to achieve is even possible and what I might be doing wrong?

juunas · Accepted Answer · 2019-01-09 17:40:39Z

1

Looking at the documentation for the action: https://learn.microsoft.com/en-us/rest/api/storageservices/get-blob-properties.

It's basically a HEAD request to the blob, and there is no mention of If-Match etc. for headers.

So I think the most optimal way of doing it is just calling FetchAttributesAsync. If that causes a 404, then the blob did not exist. It only does 1 HTTP request.

answered Jan 9, 2019 at 17:40

juunas

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4 Comments

alwayslearning Over a year ago

Would need to wrap it in a try catch. Wanted to avoid generating exceptions as far as possible 🙂

juunas Over a year ago

Right, that would indeed be nice. If you could tell the Storage SDK to not throw, but return null if not found etc..

alwayslearning Over a year ago

I did check the decomiled code. It adds the if-match header with a value of '*'. Don't know how that's processed by the server though

juunas Over a year ago

The AccessCondition can be specified so you can specify a lease id if you have acquired a lease on the blob.

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