Allows to handle an event that fires when a WebJob process is about to close

WebJobShutdownWatcher.fs

module WebJobShutdownWatcher

open System
open System.IO

/// Allows to handle an event that fires when a WebJob process is about to close.
/// Based on a blog-post: http://blog.amitapple.com/post/2014/05/webjobs-graceful-shutdown
let subscribe handler =

    let shutdownFile = Environment.GetEnvironmentVariable "WEBJOBS_SHUTDOWN_FILE" 
    if shutdownFile <> null then

        let watcher = new FileSystemWatcher(path = (shutdownFile |> Path.GetDirectoryName), 
                                            NotifyFilter = (NotifyFilters.CreationTime ||| NotifyFilters.FileName ||| NotifyFilters.LastWrite),
                                            IncludeSubdirectories = false, 
                                            EnableRaisingEvents = true)

        Event.merge watcher.Created watcher.Changed
        |> Event.filter (fun e -> e.FullPath.IndexOf(shutdownFile, StringComparison.InvariantCultureIgnoreCase) >= 0)
        |> Event.add (handler >> watcher.Dispose)

    ()

// usage
// WebJobShutdownWatcher.subscribe (fun e -> printfn "%s" e.FullPath)