Using local storage with BLOB storage (part 2) - Updating your HTTP handler to use local storage & Checking properties of a BLOB without downloading it

3. Updating your HTTP handler to use local storage

You’re going to modify your HTTP handler to check in local storage to determine whether the requested file exists already. If the file doesn’t exist, then you’ll retrieve the MP3 file from BLOB storage and store it in local storage. Finally, you’ll write the file back to the client from your local filesystem. The following code shows how you modify the ProcessRequest method in this listing  to use local storage.

public void ProcessRequest(HttpContext context)
{
   string blobName = context.Request.Path.Trim('/');

   var mp3Cache = RoleEnvironment.GetLocalResource("mp3Cache");
   string localFilePath = mp3Cache.RootPath + blobName;

  if (!File.Exists(localFilePath))
  {
     var blobData = GetBlob("chrisoriginals", blobName);
     File.WriteAllBytes(localFilePath, blobData);
  }

  context.Response.ContentType = "audio/mpeg";
  context.Response.WriteFile(localFilePath);
}

Using the local storage mechanism in your HTTP handler improves performance by serving the requested file from local storage, rather than always having to retrieve the file from BLOB storage first. Although performance is improved, if the file is changed in BLOB storage, the file that you’ll serve will be out of date. Let’s look at how you can keep the performance improvement but serve the latest content even if the file has changed in BLOB storage.

4. Checking properties of a BLOB without downloading it

If you were to use the HTTP HEAD verb instead of the GET verb, you could check the properties of the file without downloading the file. Figure 2 shows the output of a HEAD request.

In figure 2, the Last-Modified tag shows us the last time the file was updated in BLOB storage. By comparing the header value to the local value in your file properties, you know whether the file has changed.

The listing that follows shows the code for the console application shown in figure 2.

Listing 1. Showing the output of a HEAD request

In the code example shown in listing 1, you make the same request that you’ve made before, but it’s a HEAD request instead of a GET request. You’re making a HEAD request for the file podcast01.mp3 in the ChrisOriginals container in your development storage account . At you loop through all the returned headers, outputting the header key and value to the console screen.

Using the Storageclient Library

In listing 1, you retrieved the last modified time using the REST API directly, although you could’ve used the StorageClient library. The following code performs a HEAD request that returns the last modified time:

blob.FetchAttributes();
var lastModifiedTime = blob.Properties.LastModifiedUtc;

FetchAttributes is the StorageClient library equivalent of HEAD. It returns all the properties and custom metadata of the BLOB, without downloading the actual file. Table 1 lists the BLOB properties and their descriptions.

Now that you’re familiar with BLOB properties and how to retrieve them without downloading the file, let’s look at how you can use this knowledge to improve the performance of your handler.