Serilog logs only from one IIS-hosted ASP.NET Core application when using custom Application Pool identities

[PranavBongane]

🔹 Question:

I'm working with two separate ASP.NET Core 8 applications:

• APIGateway

• IdentityService

Both are deployed on the same Windows Server and hosted in IIS under their own dedicated Application Pools. I have configured Serilog in both applications to log to the same physical file (C:\Logs\Logs.log) with rollingInterval set to Day, and shared: true to allow concurrent access.

Here’s a simplified snippet of the Serilog config used in both apps (appsettings.json):

json

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"Serilog": { 
  "Enrich": [ "FromLogContext" ], 
  "WriteTo": [ 
    { 
      "Name": "File", 
      "Args": { 
        "path": "C:\\Logs\\Logs.log", 
        "rollingInterval": "Day", 
        "outputTemplate": "[{Timestamp:yyyy-MM-dd HH:mm:ss,fff}] {Message}{NewLine}{Exception}", 
        "shared": true 
      } 
    }, 
    { 
      "Name": "Console", 
      "Args": { 
        "outputTemplate": "[{Timestamp:yyyy-MM-dd HH:mm:ss,fff}] {Message}{NewLine}{Exception}" 
      } 
    } 
  ] 
} 

✅ What works:

• When both application pools are configured to run as LocalSystem, both apps log correctly to the same file.

• When only one application runs under a custom identity, that app logs fine.

• The issue only appears when both apps use separate custom identities.

• Logging also works when both apps run under the default ApplicationPoolIdentity user.

❌ What doesn’t work:

When both apps use custom application pool identities (e.g., DOMAIN\GatewayUser and DOMAIN\IdentityUser), only one of the two logs to the file. The other silently fails to write any logs — no errors or exceptions are thrown, and Serilog’s SelfLog doesn't report anything obvious either.

🔍 What I've tried:

Ensured both custom user accounts have Modify permissions on the C:\Logs folder and the existing Logs.log file.

Verified file access with icacls to confirm proper NTFS permissions.

Enabled Serilog self-logging using:

csharp

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Serilog.Debugging.SelfLog.Enable(msg => Debug.WriteLine(msg));
Confirmed that each application is using the correct configuration and file path.

Restarted both application pools and ensured they’re not accessing the file simultaneously at startup.

⚙️ Server Environment:

• Windows Server 2019

• IIS 10

• .NET 8

• Serilog.Sinks.File (v5.0.0)

App pools are Integrated mode, running separately for each app

🤔 My Goal:

I want both applications, running under different app pools, to be able to log to the same file reliably with same identity user.

❓ What I Need Help With:

1. Is this a known file sharing/locking issue with Serilog when using different users?

2. Are there additional permissions or security settings required beyond NTFS Modify access?

3. Is there a recommended approach to configure Serilog or IIS to make this setup work reliably across different identities?