In a dotnet core app, if one configures mssql as a sink via appsettings, how can you configure a "backup" sink such as a file? So if serilog cant write to a given sink, it tries to write to the 2nd.
Is the Selflog what I need to look at?
Serilog doesn't have a feature like that, as of this writing, but it should be possible to implement by creating your own Sink that wraps the primary sinks and manages the error handling to perform the fallback to "backup" sink(s).
You might want to look at the code of Serilog.Sinks.Async for inspiration, as it shows you a way of wrapping multiple sinks into one which could be a starting point.