Azure function can't read a static file occasionally

Question

Azure function can't read a static file occasionally

Zhengxiao (Edward) Wang 0 Microsoft Employee

I'm using a timer triggered azure function in .NET.

Azure function info:
runtime version: 4.29.1.21919
file system: windows

.NET version: .NET6.0

I have a static json file which is published to the azure function runtime environment. It is placed in the root of the project and I use the following code to get its path:

Path.Combine(context.FunctionAppDirectory, 'myconfig.json')

The weird thing is that the azure function occasionally throws FileNotFound error on this file. Sometimes the azure function can read the file and sometimes it can't, without any code or azure function configuration change. Does anyone have any idea about why this happens?

---Update on 2024-02-26: BTW, I'm using the Azure function hosted in Premium Plan.

I used the same way to read configuration file in other azure functions hosted in Consumption Plan and such error doesn't happen ever.

So I suspect the premium plan is the one to blame. But because my azure function requires ~3GB RAM and the Consumption Plan has 1.5 GB memory limit so I have to choose the Premium Plan.

3 answers

Your answer

Answer 1

Sedat SALMAN 14,155 MVP

Azure Functions may not guarantee the persistent storage of files across executions.

To overcome this problem please read the following articles

https://scale-tone.github.io/2019/09/02/how-to-serve-static-content-with-azure-functions

https://learn.microsoft.com/en-us/azure/azure-functions/functions-reference-csharp

https://learn.microsoft.com/en-us/azure/azure-functions/dotnet-isolated-process-guide

Zhengxiao (Edward) Wang 0 Reputation points Microsoft Employee

2024-02-06T14:39:22.2733333+00:00

@Sedat SALMAN Could you be more specific? It seems that the article https://scale-tone.github.io/2019/09/02/how-to-serve-static-content-with-azure-functions you shared suggests the same way as I did to read a static file. The other two are too general and do not directly solve the problem I'm facing.

Answer 2

Albert Tanure 0 Microsoft Employee

Hi Zhengxiao, your question is interesting. I would like to understand a bit more your approach to load this file from the root folder. If the case is to maintain configurations, a good approach is to use the Azure App Configuration. It's a good way to manage configurations of your application. Kind Regards, Albert Tanure

Zhengxiao (Edward) Wang 0 Reputation points Microsoft Employee

2024-02-19T03:34:20.2666667+00:00

I do use Azure App Configuration to store some simple configurations. But for some complex configurations like a large JSON object, using Azure App Configuration makes it hard to view and modify it. So I stored it in a JSON file and read it from the file.

Answer 3

MayankBargali-MSFT 70,861

@Zhengxiao (Edward) Wang Thanks for reaching out.

To answer your question on how to read any file you can use the below. I am using the below code to get the file path and it is working as expected and didn't observe any issue. Environment.GetEnvironmentVariable("HOME") + @"\site\wwwroot{director(if any)\youfilename}

In case if you are still facing the issue then please let me know.

Please 'Accept Answer' if it helped so that it can help others in the community looking for help on similar topics.

Zhengxiao (Edward) Wang 0 Reputation points Microsoft Employee

2024-02-20T08:55:41.7033333+00:00

Hi , thanks for helping! Do you have any idea why Path.Combine(context.FunctionAppDirectory, 'myconfig.json') is not always working? It resolves to c:\site\wwwroot\myconfig.json So I believe it should be the same as your suggested approach Environment.GetEnvironmentVariable("HOME") + @"\site\wwwroot{director(if any)\youfilename}
MayankBargali-MSFT 70,861 Reputation points

2024-02-20T13:13:58.03+00:00

@Zhengxiao (Edward) Wang Ideally it should return the same value but it looks like the context.FunctionAppDirectory is returning the null value or the incorrect path. You can try logging the path it is trying to access and validate the same at your end.

Are you deploying this file as part of package or uploading it separately? The only reason I could think about the fileshare that is mounted with the backend VM that hosts the function app will change periodically and the contents might be lost in case if you have uploaded/updated separately.
Zhengxiao (Edward) Wang 0 Reputation points Microsoft Employee

2024-02-21T05:44:43.73+00:00

@MayankBargali-MSFT I've already logged the path and it showed 'c:\site\wwwroot\myconfig.json'. It looks good to me.

I use this command func azure functionapp publish myFunc to deploy the azure function. No more uploading/updating action. So I believe the configuration file is deployed as a part of the package.

Backend VM changing is a really good point. Do you know how can I check which VM is backing the azure function?
MayankBargali-MSFT 70,861 Reputation points

2024-02-22T04:20:25.16+00:00

@Zhengxiao (Edward) Wang You cannot check it from your end, and this would need to further troubleshoot it to know the cause why the file is not accessible, and you are getting Not found intermittently. You can try updating the code and see if it resolves the issue, if not then I suggest engaging us over one on one to review it further.
Zhengxiao (Edward) Wang 0 Reputation points Microsoft Employee

2024-02-23T02:56:00.66+00:00

@MayankBargali-MSFT Here is the log file when the FileNotFound error happens I find that there is only host.json file on under c:\home\site\wwwroot when the error happens. However, normally, there should be many dll and json files under c:\home\site\wwwroot
MayankBargali-MSFT 70,861 Reputation points

2024-02-27T04:24:17.7333333+00:00

@Zhengxiao (Edward) Wang This would further need to validate from the logs why it is resulting in file not found error intermittently and one on one support ticket will further help to validate it.

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Azure function can't read a static file occasionally

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