Accessing Azure SMB File Shares From Function App

William Beglen 25

I have an Azure function app that needs to move files around between Azure file shares. My function app contains a lot of code that uses System.IO (C#, .Net) file system classes. My issue is that I'm new to Azure, and I'm not sure how to access an Azure SMB share from within a function app using System.IO methods. Even the file enumeration methods return an 'access denied' exception. Since Azure file shares support SMB protocol, is there any reason why existing .Net System.IO classes can't be used inside an Azure environment by a function app rather than rewrite all of our existing file handline code to use the REST API interface client? Do I need to mount a file share inside of azure to use System.IO?

2 answers

Konstantinos Passadis 19,266 Reputation points MVP

2023-04-24T12:24:33.7833333+00:00

Hello @William Beglen Yes, you can use System.IO classes to access files in an Azure file share from within an Azure function app. However, you will need to configure the function app to authenticate to the file share and mount it as a drive letter or UNC path. Here are the steps to do this: Create a storage account and file share in Azure if you haven't already done so. Open the Function App settings in the Azure portal and go to the "Configuration" section.

Add a new application setting with the name "STORAGE_CONNECTION_STRING" and the value set to the connection string for your storage account. You can find the connection string in the "Access keys" section of the storage account. In your function app code, use the following code to authenticate to the file share and mount it as a drive letter or UNC path:

string storageConnectionString = Environment.GetEnvironmentVariable("STORAGE_CONNECTION_STRING"); CloudStorageAccount storageAccount = CloudStorageAccount.Parse(storageConnectionString); CloudFileClient fileClient = storageAccount.CreateCloudFileClient(); CloudFileShare fileShare = fileClient.GetShareReference("yourfilesharename"); fileShare.CreateIfNotExists(); CloudFileDirectory rootDir = fileShare.GetRootDirectoryReference(); rootDir.CreateIfNotExists(); string mappedDrive = Path.GetTempPath() + "yourfilesharename"; rootDir.Mount(mappedDrive); This code will mount the file share at a temporary directory on the function app, which can be accessed using standard System.IO classes. Note that if your file share is in a different Azure region than your function app, you may experience slower performance and higher latency due to network traffic. In this case, you may want to consider using the REST API interface client or a dedicated file transfer solution such as Azure File Sync or Azure Data Box.

Please mark the answer as Accepted in case it helped ! BR
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William Beglen 25 Reputation points

2023-04-24T20:43:19.9633333+00:00

Thank you for the response. I haven't been able to locate a 'Mount' method on the CloudFileDirectory class in the SDK documentation. I'm not sure what I'm missing here, although I note that the documentation I see online is marked with (deprecated). Do you have any links to another version's documentation that shows a Mount method?

Konstantinos Passadis 19,266 Reputation points MVP

2023-04-25T09:13:56.09+00:00

Hello @William Beglen yes here you are :

https://learn.microsoft.com/en-us/dotnet/api/overview/azure/storage.files.shares-readme?view=azure-dotnet

Please mark the answer as Accepted in case it helped !

BR

William Beglen 25 Reputation points

2023-04-28T13:39:40.6133333+00:00

I'm afraid that I haven't found any references to CloudFileDirectory.Mount() in the link supplied or in any of my searches. It doesn't seem to exist.
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MuthuKumaranMurugaachari-MSFT 22,361 Reputation points

2023-04-28T14:58:37.5966667+00:00

William Beglen Thanks for posting your question in Microsoft Q&A. Yes, you need to mount file share before accessing it. Currently, Mount file shares are only supported for Linux Web Apps (and Windows Container Web apps), and you can follow doc: Mount file shares in setting the mount via CLI az webapp config storage-account add(or PowerShell command).

Then you can use System.IO to access files with the mount-path and I found article https://www.serverlessnotes.com/docs/mounting-file-shares-on-linux-azure-function-apps created by MVP that would help with your scenario. Also, .NET SDK can be used to access file shares as described in Access the file share programmatically (after setting up Configure Azure Storage connection strings). I hope this helps with your question and let me know if you have any questions or face issues.

If you found the answer to your question helpful, please take a moment to mark it as "Yes" for others to benefit from your experience. Or simply add a comment tagging me and would be happy to answer your questions.
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William Beglen 25 Reputation points

2023-04-28T19:57:34.29+00:00

Thank you for your reply.

Yes, I can successfully use the .NET SDK (ShareDirectoryClient, etc.) to access the files, however, here is my dilemma:

We have lots of code that uses System.IO we'd like to be able to reuse if possible. Using existing System.IO code would save us tons of time and expense.

System.IO classes have some functionality that already suits our purposes well, such as DirectoryInfo.GetFiles(wildcard).

There needs to be interaction and file movement between Azure File Shares and external file shares anyway, so if System.IO could be made to work with both (via SMB) if we prevent the expense of writing 2 versions of every file-handling module.

I'm surprised that this is supported for Linux-based function apps but not Windows, however, if this is the case I suppose I could create a Linux-based App Service and remove/modify any Windows-specific code (like Windows.Identity). I currently have a Windows-based app service plan (premium) function app. Would this be your recommendation (Linux app service)?

MuthuKumaranMurugaachari-MSFT 22,361 Reputation points

2023-04-28T20:07:50+00:00

William Beglen Can you elaborate more about external file shares, and how it is integrated with Azure? That would be helpful.

William Beglen 25 Reputation points

2023-04-28T20:28:43.69+00:00

Our current production systems transport files to a variety of servers using UNC paths, but we also have a lot of movement on the local Windows servers that run our automated processes. In both cases System.IO methods work fine. We have other methods of of transport as well (SFTP, etc.) - sometimes a vendor pulling from our server via SFTP (thus we deliver files via an UNC path to the SFTP server) and other times we push or pull to/from their SFTP server. But what I'm most concerned with at this time is having to rewrite a great deal of code that uses System.IO. I've successfully used the .NET SDK for Azure file shares, however, there isn't a 1:1 match in functionality and certainly it's a good bit of code refactoring. Using System.IO over SMB would solve this particular issue and would allow us to easily phase in our clients to Azure over time.

If I absolutely must refactor for Azure File Share client classes I will, and then our clients could probably just map a local drive to the appropriate share in Azure - but all of this becomes much more simple if we can just re-use our System.IO based file-handling code for our first phase Azure implementation.

MuthuKumaranMurugaachari-MSFT 22,361 Reputation points

2023-05-02T13:42:14.04+00:00

William Beglen Thanks for sharing the details. If you have files in Azure File Share and mount it as described in the answer above to Linux WebApps, then you can access it via System.IO classes. Here are the steps I followed (based on article https://www.serverlessnotes.com/docs/mounting-file-shares-on-linux-azure-function-apps):

Created a Azure Function app with Premium plan, Linux Operation System and .NET 6 runtime.

Created a Azure Storage account with FileShare named testfileshare which contains file 1.png and mounted file share to Azure Function using CLI command az webapp config storage-account add

Created Http Triggered function with the following code snippet to access file paths via System.IO and deployed it to the Azure Function.

using System.IO; using System.Threading.Tasks; using Microsoft.AspNetCore.Mvc; using Microsoft.Azure.WebJobs; using Microsoft.Azure.WebJobs.Extensions.Http; using Microsoft.AspNetCore.Http; using Microsoft.Extensions.Logging; using Newtonsoft.Json; namespace FileShareTest { public static class FileShareTest { [FunctionName("FileShareTest")] public static async Task<IActionResult> Run( [HttpTrigger(AuthorizationLevel.Anonymous, "get", "post", Route = null)] HttpRequest req, ILogger log) { log.LogInformation("C# HTTP trigger function processed a request."); string[] filePaths = Directory.GetFiles(@"/testfileshare"); return new OkObjectResult(filePaths); } } }

Run it in the portal and got the paths successfully.

This is alternative to access file in Azure File Shares instead of .NET SDK and I hope this helps with your questions.

MuthuKumaranMurugaachari-MSFT 22,361 Reputation points

2023-05-02T14:28:38.1633333+00:00

William Beglen There is an ongoing issue in Linux Consumption plan related to mounting file shares (https://learn.microsoft.com/en-us/answers/questions/1246843/cant-access-mounted-file-share-on-azure-function-p) and hence I have tested with Premium plan in the above comment.

William Beglen 25 Reputation points

2023-05-02T14:50:27.78+00:00

I appreciate your assistance on this issue. So to be clear, an Azure SMB file share can only be accessed by Linux WebApp-based Azure functions, and then only if the share is mounted - correct? There is no way to use an SMB file share from a Windows WebApp-based function either mounted or unmounted (like using a full-blown UNC path or something similar, correct?

MuthuKumaranMurugaachari-MSFT 22,361 Reputation points

2023-05-02T15:00:24.71+00:00

William Beglen You can access file shares using the path only if it is mounted. And yes, it is not yet supported in Azure Functions Windows WebApps. However, this is supported in App Service and check out doc: Mount Azure Storage as a local share in App Service.

You are welcome, happy to help :)
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