How do I append to a file in an Azure storage file

问题:

I want to write entries to a log file stored in Azure file storage. I currently have this:

var log = "My log entry";

var client = _storageAccount.CreateCloudFileClient();
var share = client.GetShareReference(Config.LogShare);
share.CreateIfNotExists();
var root = share.GetRootDirectoryReference();
var logfile = root.GetFileReference("log.txt");
if (!logfile.Exists()) logfile.Create(0);

// What goes here to append to the file...?

I can see plenty of examples of how to do this with Blobs, or how to upload an entire file, but how do I just append to an existing file?

I have tried this:

var buffer = Encoding.GetEncoding("UTF-8").GetBytes(log.ToCharArray());

using (var fileStream = logfile.OpenWrite(0)) {
    fileStream.Write(buffer, (int)logfile.Properties.Length, buffer.Length);
}

But then I get this error:

The remote server returned an error: (416) The range specified is invalid for the current size of the resource..

回答1:

Azure file storage REST API doesn't support appending to an existing file. To achieve this, please mount the file share to your machine as a drive, and append to the file just like simple local files.

Actually, I don't think you really need appending functionality per your code above. You can specify the file size in CloudFile.OpenWrite() / CloudFile.Create(), or try CloudFile.UploadFromStream() instead of CloudFile.OpenWrite().

回答2:

I managed to work this out myself. You just need to increase the size of the file by the number of new bytes you want to write to it, and then write the new data to that new empty space at the end of the file, like this:

var client = _storageAccount.CreateCloudFileClient();
var share = client.GetShareReference(Config.LogShare);
share.CreateIfNotExists();
var root = share.GetRootDirectoryReference();
var logfile = root.GetFileReference("log.txt");
if (!logfile.Exists()) logfile.Create(0);

var buffer = Encoding.UTF8.GetBytes($"{log}\r\n");

logfile.Resize(logfile.Properties.Length + buffer.Length);

using (var fileStream = logfile.OpenWrite(null)) {
    fileStream.Seek(buffer.Length * -1, SeekOrigin.End);
    fileStream.Write(buffer, 0, buffer.Length);
}

回答3:

This error could also be due to multi-threaded access. I bet if you tried to lock the file before you access it, you will not face this problem. There are many ways to update the file. Since you already managed to get the share, the root, the folder and the file.. Here is a portion of my code that worked for me.

if (!fileLock.IsWriteLockHeld) fileLock.EnterWriteLock();


        try
        {
            using (var stream = new MemoryStream(content, false))
            {
                file.UploadFromStream(stream, null, options);
            }
        }
        catch (Exception ex)
        {
            File.AppendAllText(FileName, ex.ToString());
        }
        finally
        {
            if (fileLock.IsWriteLockHeld)
                fileLock.ExitWriteLock();
        }

Where fileLock is declared as:

  protected ReaderWriterLockSlim fileLock = new ReaderWriterLockSlim();

Having said that, I am not saying that this is the best way ever to do it. The two things I would like you to keep in mind : 1-Lock the resource that is likely to be accessed by more than one thread (That is so common in AZURE) 2- Get familiar with asynchronous methods that Azure provides.. use them when they suit well.

Coming back to your original problem about appending to the existing file.. All the methods of the CloudFile will overwrite the existing file. Cloud Files are not for frequent writing, and they indeed impact performance if you keep writing on them frequently, add the lock impact on performance, they will be horrible.

Cloud files are meant to store big bulk of data once and for all, if you want to add another bulk you have the choice of creating another file. Have all your data with the client till they reach some size and create an algorith to select the file name and upload them all at once.

回答4:

You can do this with blobs https://blogs.msdn.microsoft.com/windowsazurestorage/2015/04/13/introducing-azure-storage-append-blob/

Shame it doesn't work with files too