BoxLang File Watchers

Overview

BoxLang includes a runtime watcher service for real-time filesystem automation.

Watchers are ideal for:

• hot reload and development workflows
• build pipelines and asset regeneration
• file drop ingestion and ETL triggers
• reacting to create/modify/delete events

Application.bx Integration

You can register app-scoped watchers directly in Application.bx with this.watchers.

For full descriptor behavior (app discovery, multi-app isolation, lifecycle), see application-descriptor.

class {

    this.name = "MyApp"

    watcherListener = new app.listeners.HotReloadListener()

    this.watchers = {
        sourceWatcher : {
            paths : [ expandPath( "./src" ) ],
            listener : watcherListener,
            recursive : true,
            debounce : 250,
            atomicWrites : true,
            errorThreshold : 10
        }
    }

}

Supported watcher definition keys commonly include paths, listener, recursive, debounce, throttle, atomicWrites, and errorThreshold.

Event Model

Event kinds:

• created
• modified
• deleted
• overflow

Listener payload struct keys:

• kind
• path (absolute path; blank for overflow)
• relativePath (relative to watcher root; blank for overflow)
• watchRoot (blank for overflow)
• timestamp (ISO-8601)

Creating a Watcher

watcherNew() registers a watcher but does not automatically start it unless you chain .start() on the returned instance.

watcher = watcherNew(
    name = "sourceWatcher",
    paths = [ "./src" ],
    listener = ( event ) => {
        println( "[#event.kind#] #event.relativePath#" )
    },
    recursive = true,
    debounce = 250,
    throttle = 0,
    atomicWrites = true,
    errorThreshold = 10,
    force = false
).start()

Accepted paths forms:

• string path
• array of string paths

Listener Forms

1) Closure / Lambda

listener = ( event ) => {
    if ( event.kind == "modified" ) {
        recompileAsset( event.path )
    }
}

2) Struct Listener

listener = {
    onCreate: ( event ) => onCreated( event ),
    onModify: ( event ) => onModified( event ),
    onDelete: ( event ) => onDeleted( event ),
    onOverflow: ( event ) => fullResync(),
    onEvent: ( event ) => writeLog( text: "Watcher event: #event.kind#", log: "async" )
}

3) Class Name String or Class Instance

watcherNew(
    name = "prodWatcher",
    paths = [ "/srv/import" ],
    listener = "app.listeners.ImportWatcher"
)

watcherNew(
    name = "prodWatcher",
    paths = [ "/srv/import" ],
    listener = new ImportWatcher()
)

For class listeners, implement onEvent( required struct event ) as the baseline and optionally:

• onCreate( event )
• onModify( event )
• onDelete( event )
• onOverflow( event )
• onError( error )

Watcher BIFs

• watcherNew( name?, paths, listener, recursive=true, debounce=0, throttle=0, atomicWrites=true, delay=0, errorThreshold=10, force=false )
• watcherStart( name )
• watcherStop( name, force=false )
• watcherRestart( name )
• watcherGet( name )
• watcherGetAll()
• watcherList()
• watcherExists( name )
• watcherShutdown( name )
• watcherStopAll( force=false )
• watcherShutdownAll( force=false )

Lifecycle distinctions:

• stop = stop execution but keep registration
• shutdown = stop + unregister (destructive)

WatcherInstance API

watcherNew() and watcherGet() return WatcherInstance:

• start()
• stop( force=false )
• restart()
• isRunning() / isStopped()
• getState() / getStateAsString()
• getName()
• getWatchPaths()
• getListener()
• getStats()

getStats() includes:

• name
• state (CREATED, RUNNING, STOPPED)
• paths
• recursive
• debounce
• throttle
• atomicWrites
• errorThreshold
• consecutiveErrors

Tuning and Safety

• debounce: suppress rapid duplicate events on the same path.
• throttle: limit event frequency per path.
• atomicWrites: reduce noisy editor temp-write sequences.
• errorThreshold: auto-stop watcher after consecutive listener failures.
• force stop: faster stop via interruption and watch service close; may drop in-flight events.

Production Pattern

watcher = watcherNew(
    name = "inbound-files",
    paths = [ "/data/inbound" ],
    listener = new InboundFileListener(),
    recursive = true,
    debounce = 300,
    atomicWrites = true,
    errorThreshold = 5
).start()

if ( !watcher.isRunning() ) {
    watcher.restart()
}

Best Practices

• Keep listener logic fast; offload expensive work to executors/futures.
• Use class listeners for reusable, testable production handlers.
• Always log errors in onError and track consecutiveErrors.
• Use absolute paths in production to avoid cwd ambiguity.
• Use watcherStopAll() in graceful shutdown flows.

Troubleshooting

• No events firing: confirm path exists and is a directory.
• Too many duplicate events: increase debounce, enable atomicWrites, and consider throttle.
• Watcher disappears: check if watcherShutdown() or watcherShutdownAll() was invoked.
• Unexpected stop: inspect listener exceptions; watcher may have hit errorThreshold.