Azure Blob Storage support for Lokad.ContentAddr.

The relevant types are AzureStoreFactory (for a multi-tenant store) and AzureStore (for a single-tenant store). These implement the IStoreFactory and IStore interfaces from the base Lokad.ContentAddr library.

Data Layout

This creates two containers in the Azure Blob Storage account:

• persist will contain the persistent blobs, named <realm>/<hash> (for instance, a blob AC03061D6376491889AE7B1D6661AC94 in realm 85055 would be named 85055/AC03061D6376491889AE7B1D6661AC94).

• staging will contain temporary blobs as they are being uploaded to the content-addressable store. This container can be safely emptied without losing any committed data.

This library supports two upload models for uploading blobs:

Uploading with the Lokad.ContentAddr library

This uses the IStore interface to push data to Azure Blob Storage. For example:

AzureStore store = ...;
store.WriteAsync(new byte[] {...}, cancel);

More information about the implementation details (not relevant to normal use of the library) follows:

Data is accumulated in-memory up to a certain point (around 4MB), after which a block blob is created in the staging container, and the data is written as separate blocks to that blob, to avoid consuming too much memory.

The hash of the blob is calculated on-the-fly.

Once the entire data has been provided, the hash becomes known. The library then determines a blob with the same hash already exists for the tenant. If it does, the temporary block blob is deleted. If it does not, the temporary block blob is committed, then copied to the persist container with the appropriate hash.

Uploading from a different source

It is also possible to give a different client an upload link with a shared access signature (for instance, an in-browser JavaScript uploader). This is done using the appropriate method on the store:

AzureStore store = ...;
string identifier = Guid.NewGuid().ToString();
TimeSpan life = TimeSpan.FromMinutes(10);
Url url = store.GetSignedUploadUrl(identifier, life);

This will create a Shared Access Signature that lasts 10 minutes, that allows the client to write to that specific blob in the staging container.

Once the client has finished uploading, it should contact the issuer of the signed URL in order to commit the upload to the store. The issuer then invokes:

// Same store and identifier as were used to generate the URL
AzureStore store = ...;
string identifier = ...; 
await store.CommitTemporaryBlob(identifier, cancellationToken);

This causes the store to download the entire uploaded blob, compute its hash, and then move it to the persist container (if a copy is not already present there).

Download links

It is possible to produce a short-lived URL that allows anyone to download a blob from the persist container.

    AzureStore store;
    IAzureBlobRef blob = store[new Hash("AC03061D6376491889AE7B1D6661AC94")];
    Url download = await blob.GetDownloadUrlAsync(
        now: DateTime.UtcNow,
        life: TimeSpan.FromMinutes(10),
        filename: "budget.xlsx",
        contentType: "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
        cancel: cancellationToken);

The URL contains a Content-Disposition: attachment header that causes browsers to interpret it as a downloadable file (with the file name specified through the filename argument of the function).