Maintaining cache control property on a file when it is returned as Stream from VirtualPathProvider

问题

I have implemented a VirtualPathProvider to return Theme files (images,css) for an Azure web site from the Azure CDN. It is working fine apart from one thing: the files that are coming from the CDN all have their cache control property set to "private" and so are never cached.

The actual blobs have their properties set correctly and if I access one by it's direct URL (i.e. not through the VPP) then the cache control is correct.

The problem seems to be in the Open() method of the VirtualFile class I have to implement to return the file as a Stream?

public override Stream Open()
    {
        CloudBlobClient client = new CloudBlobClient(cdnURL);
        CloudBlob blob = client.GetBlobReference(blobURL);
        blob.FetchAttributes();
        MemoryStream stream = new MemoryStream();
        BlobRequestOptions options = new BlobRequestOptions();
        options.BlobListingDetails = BlobListingDetails.Metadata;
        blob.DownloadToStream(stream,options);
        stream.Seek(0, SeekOrigin.Begin);
        return stream;
    }

Searching on this and I find most people have the problem the other way - i.e. files are cached when they don't want them to be. However none of the examples I can find are referencing a file from another URL. They all seem to use databases or just different physical paths.

回答1:

Thanks to this answer on the asp.net forum http://forums.asp.net/post/4716700.aspx

I have resolved the issue by adding in my Open() method:

HttpContext.Current.Response.Cache.SetCacheability(HttpCacheability.Public);
HttpContext.Current.Response.Cache.AppendCacheExtension("max-age=86400");

回答2:

I think you may have missed a crucial point in how the CDN gains it's advantage. The CDN helps by putting the resources closer to the client requesting the file. i.e when the client requests the file it goes straight to the CDN URL. What seems to be happening here is you're downloading the file from the CDN to you code that's running web server and then returning the stream to the client from there.

Please correct me if I'm wrong.

It's also worth noting that the cache properties are not part of the file stream that you're returning, they're additional properties that can be found in CloudBlob.Properties.CacheControl

来源：https://stackoverflow.com/questions/8224075/maintaining-cache-control-property-on-a-file-when-it-is-returned-as-stream-from

标签

caching

azure

cdn

cache-control

virtualpathprovider