Moving to Suave meant a new logger was required as the original logger used the .NET Core LoggerFactory. Generally for logging I’ve used log4net or flat files. In this case I decided to try using the System.Diagnostics.Trace tools – a little old fashioned perhaps – but supported in .NET 4.6.1 as well as .NET Core (future-proofing!).

Creating a trace involves two parts:
1. Creating a TraceSource instance and calling methods on it;
2. Adding listeners to the config file

In this case it is configured to write warnings and higher to the console, and everything to a file log (AmApi.log).

    <trace autoflush="true" />
      <source name="Log" switchValue="All" switchType="System.Diagnostics.SourceSwitch">
          <add name="console" type="System.Diagnostics.ConsoleTraceListener">
            <filter type="System.Diagnostics.EventTypeFilter" initializeData="Warning"/>
          <add name="logToFileListener"/>
          <remove name="Default"/>
      <add name="logToFileListener" type="System.Diagnostics.TextWriterTraceListener" initializeData="AmApi.log" />     

Suave has built in logging capabilities, such as the colored text displayed in the console. It allows these logs to be accessed by creating a logging adapter and configuring it to be used. This is described here, although the interface definition is more advanced than given on that page, as illustrated by the adapter shown below. This implementation calls a method, SuaveLog, on the main logger class that understands and converts Suave log levels to Diagnostics.TraceEventType.

type SuaveLoggerAdapter() =
    let _log (msg:Suave.Logging.Message) = 
        use strWriter = new System.IO.StringWriter()
        let txt = Suave.Logging.TextWriterTarget(Suave.Logging.LogLevel.Verbose, strWriter) :> Suave.Logging.Logger
        txt.logSimple msg
        Logger.SuaveLog (strWriter.ToString()) msg.level

    interface Suave.Logging.Logger with
        member __.logSimple msg = _log msg
        member __.log level msgFactory = _log (msgFactory level)
        member __.logWithAck level msgFactory = async { do _log (msgFactory level) }

Configuration is done in the startWebServer method. I wanted to preserve the existing logging capabilities, particularly the console, so a CombiningTarget was used. CombiningTarget sends the log messages to multiple loggers.

let defaultLog = Suave.Logging.Targets.create Suave.Logging.LogLevel.Info
let logger = Suave.Logging.CombiningTarget([ defaultLog; Util.SuaveLoggerAdapter() ])
let config = { defaultConfig with logger = logger }
startWebServer config handleRequest

To complete the picture the logging class and instance are shown here. I’ve been lazy and used the underscore here to denote the intention to keep the class private. Alternatively an interface could have been created and a private class defined to implement it. A singleton was also considered but one never knows when it might be useful to split logs so I try to avoid that approach.

type _Logger() = 
    let log = new System.Diagnostics.TraceSource("Log")

    let _log (eventType:Diagnostics.TraceEventType) (msg:string) =
        log.TraceEvent(eventType, 0, (sprintf "%s: %s" (DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss.fff K")) msg))

    override this.Finalize() = 

    member this.Info msg = _log Diagnostics.TraceEventType.Information msg
    member this.Warn msg = _log Diagnostics.TraceEventType.Warning msg
    member this.Error msg = _log Diagnostics.TraceEventType.Error msg

    member this.SuaveLog msg (level:Suave.Logging.LogLevel) = 
        let traceEventType = match level with
                                | Suave.Logging.LogLevel.Verbose -> Diagnostics.TraceEventType.Verbose
                                | Suave.Logging.LogLevel.Debug   -> Diagnostics.TraceEventType.Verbose
                                | Suave.Logging.LogLevel.Info    -> Diagnostics.TraceEventType.Information
                                | Suave.Logging.LogLevel.Warn    -> Diagnostics.TraceEventType.Warning
                                | Suave.Logging.LogLevel.Error   -> Diagnostics.TraceEventType.Error
                                | Suave.Logging.LogLevel.Fatal   -> Diagnostics.TraceEventType.Critical
        _log traceEventType msg

let Logger = _Logger()