BoxLang Async Programming

Overview

BoxLang provides a comprehensive async framework built on Java's CompletableFuture and Project Loom virtual threads. The AsyncService manages executors, schedulers, and futures. All async primitives integrate seamlessly with the BoxLang runtime.

Creating BoxFutures

BoxFuture extends CompletableFuture with BoxLang-friendly chaining.

futureNew() — Primary BIF (v1.4.0+)

// Create a future from a function (runs asynchronously)
var future = futureNew( () => fetchDataFromAPI() )

// Create a completed future with a value
var future = futureNew( "Hello World" )

// Create an empty future (complete later)
var future = futureNew()

// Create with a specific executor
var future = futureNew( () => heavyCalculation(), "cpu-tasks" )

asyncRun() — Simplified Async Execution

// Run a function asynchronously on the default io-tasks executor
var future = asyncRun( () => fetchDataFromAPI() )

// Run with specific executor
var future = asyncRun( () => processCPUWork(), "cpu-tasks" )

// Get the result (blocks until complete)
var result = future.get()

// With timeout
var result = future.get( 5, "seconds" )

Chaining with then, thenAsync, and onError

• then() — runs the transformation on the same thread (fast, lightweight transforms)
• thenAsync() — runs the transformation on an executor thread (for heavy/I/O work)

asyncRun( () => fetchUser( userId ) )
    .then( (user) => enrichWithProfile( user ) )          // same thread
    .thenAsync( (user) => sendWelcomeEmail( user ) )      // executor thread
    .then( (user) => user )
    .onError( (error) => {
        logError( error.message )
        return getDefaultUser()
    })
    .get()

Core Result Methods

// Blocking retrieval
var result = future.get()
var result = future.get( 5000 )              // timeout in ms
var result = future.get( 5, "seconds" )      // timeout with unit

// Safe retrieval with defaults
var result = future.getOrDefault( "fallback" )
var result = future.joinOrDefault( 0 )       // join() variant with default

// Get as Attempt object (functional error handling)
var attempt = future.getAsAttempt()
if ( attempt.isPresent() ) {
    doSomething( attempt.get() )
}

Parallel Execution with asyncAll

asyncAll() runs multiple operations concurrently and returns a BoxFuture<Array> — call .get() once to receive all results in order:

// Returns BoxFuture<Array> — .get() resolves to [result1, result2, result3]
var results = asyncAll([
    () => fetchOrders( userId ),
    () => fetchProfile( userId ),
    () => fetchPreferences( userId )
]).get()

var [orders, profile, prefs] = results

// Mix lambdas, closures, and pre-created futures
var results = asyncAll([
    () => fetchOrders( userId ),           // function
    futureNew( () => fetchProfile( id ) ), // pre-created future
    () => fetchPreferences( userId )       // function
]).get()

Race to the Finish with asyncAny

asyncAny() returns the result of whichever future completes first (v1.4.0+):

var fastestResult = asyncAny([
    () => fetchFromPrimaryDB(),
    () => fetchFromReplicaDB(),
    () => fetchFromCache()
]).get()

Parallel Collection Processing with asyncAllApply

Apply a function to every element of an array or struct in parallel (v1.4.0+):

// Array processing in parallel
var userIds = [ 1, 2, 3, 4, 5 ]
var profiles = asyncAllApply(
    userIds,
    ( id ) => fetchUserProfile( id )  // each item processed in parallel
)
// profiles = [ profile1, profile2, profile3, profile4, profile5 ]

// Struct processing in parallel
var config = { db: "prod-db", cache: "redis", queue: "rabbit" }
var validated = asyncAllApply(
    config,
    ( item ) => validateConfig( item.key, item.value )  // item = { key, value }
)

Named Executors

// Three pre-configured runtime executors:
// "io-tasks"        — virtual threads (default, best for I/O-bound work)
// "cpu-tasks"       — scheduled pool, 20 threads (best for CPU-bound work)
// "scheduled-tasks" — scheduled pool, 20 threads (for cron/periodic tasks)

// Pass executor name as second arg to asyncRun / futureNew
asyncRun( () => cpuIntensiveWork(), "cpu-tasks" )
futureNew( () => fetchData(), "io-tasks" )

// Access an executor by name
var executor = executorGet( "io-tasks" )

Async Pipelines

// Sequential pipeline
var result = futureNew( () => loadRawData() )
    .then( (data) => parseData( data ) )
    .then( (parsed) => validate( parsed ) )
    .then( (valid) => persist( valid ) )
    .onError( (e) => rollback() )
    .get()

// Mix sequential and parallel stages
var pipeline = asyncRun( () => fetchConfig() )
    .then( (config) => {
        // Fan out: run two tasks in parallel with the config
        var results = asyncAll([
            () => buildReport( config ),
            () => sendNotifications( config )
        ]).get()
        return results
    })
    .get()

Scheduled Tasks

Create a BoxLang Scheduler class (Scheduler.bx) and register it in boxlang.json:

// schedulers/MyScheduler.bx
class {

    property name="scheduler"
    property name="logger"

    function configure() {
        scheduler.setSchedulerName( "MyApp-Scheduler" )
        scheduler.setTimezone( "UTC" )

        // Register tasks with fluent DSL
        scheduler.task( "cleanupExpiredSessions" )
            .call( () => sessionService.cleanup() )
            .every( 15, "minutes" )
            .onFailure( (task, error) => logError( error.message ) )

        scheduler.task( "dailyReport" )
            .call( () => reportService.generate() )
            .every( 1, "day" )
            .startOn( "00:00" )
    }

    void function onStartup() {
        logger.info( "Scheduler started: #scheduler.getSchedulerName()#" )
    }

    void function onShutdown() {
        logger.info( "Scheduler shutting down" )
    }

    void function onAnyTaskError( required task, required exception ) {
        logger.error( "Task '#task.getName()#' failed: #exception.message#" )
    }
}

Register in boxlang.json:

{
    "scheduler": {
        "schedulers": [ "/path/to/schedulers/MyScheduler.bx" ]
    }
}

Or run from CLI:

boxlang schedule /path/to/schedulers/MyScheduler.bx

The thread Component

For explicit thread management:

// Start a named thread
thread name="backgroundWorker" action="run" {
    // Code runs in a new thread
    processLargeDataset( datasetId )
}

// Start multiple threads
thread name="worker1" action="run" {
    processChunk( chunk1 )
}
thread name="worker2" action="run" {
    processChunk( chunk2 )
}

// Wait for threads to finish
thread action="join" name="worker1,worker2" timeout=30000

// Access thread results
var result1 = cfthread.worker1.result
var result2 = cfthread.worker2.result

Distributed Locking with bx:lock

bx:lock prevents race conditions and supports distributed locking:

// Exclusive lock (one thread at a time)
bx:lock name="userUpdate_#userId#" type="exclusive" timeout=10 {
    // Only one request can be here at a time for this userId
    user = userService.update( userId, data )
}

// Read lock (multiple readers, exclusive writer)
bx:lock name="configCache" type="readonly" timeout=5 {
    var config = cacheGet( "appConfig" )
}
bx:lock name="configCache" type="exclusive" timeout=5 {
    cachePut( "appConfig", freshConfig )
}

// Scoped locks (application, session, request)
bx:lock scope="application" type="exclusive" timeout=10 {
    if ( !application.initialized ) {
        initializeApp()
        application.initialized = true
    }
}

File and Directory Watchers (v1.12+)

// Watch a directory for changes
var watcher = directoryWatcher(
    path     = expandPath( "./config" ),
    onChange = (event) -> {
        // event.type: "create", "modify", "delete"
        // event.path: full path to changed file
        reloadConfig( event.path )
    },
    filter   = "*.json",
    recurse  = false
)

watcher.start()

// Stop later
watcher.stop()

Error Handling in Async Code

// onError continues the chain with a recovery value
var result = asyncRun( () => flakyOperation() )
    .onError( (err) => {
        logError( err.message )
        return defaultValue   // chain continues with this
    })
    .then( (val) => process( val ) )
    .get()

// exceptionally (Java-style)
var future = asyncRun( () => riskyWork() )
future.exceptionally( (err) => fallback )

// Check completion state
if ( future.isDone() ) { ... }
if ( future.isCompletedExceptionally() ) { ... }
if ( future.isCancelled() ) { ... }

// Cancel a future
future.cancel( true )

Async with Timeout and Fallback

// orTimeout — throws TimeoutException after delay
var result = asyncRun( () => slowExternalAPI() )
    .orTimeout( 3, "seconds" )
    .onError( (err) => getCachedResult() )
    .get()

// completeOnTimeout — resolves with default value instead of throwing
var result = asyncRun( () => slowExternalAPI() )
    .completeOnTimeout( getDefaultData(), 3, "seconds" )
    .get()

Virtual Threads Best Practices

// Virtual threads (io-tasks, default) — ideal for:
// - HTTP calls, database queries, file I/O, any blocking I/O
asyncRun( () => httpClient.get( url ) )             // good — io-tasks
asyncRun( () => queryExecute( sql ) )               // good — io-tasks

// CPU pool — ideal for CPU-intensive work:
// - Image processing, encryption, data transformation
asyncRun( () => encryptLargeFile( path ), "cpu-tasks" )
futureNew( () => processLargeDataset( data ), "cpu-tasks" )

Executor Types and Runtime Configuration

Pre-Configured Runtime Executors

BoxLang provides three pre-configured executors accessible via executorGet():

// Access pre-configured runtime executors
var ioExecutor        = executorGet( "io-tasks" )        // Virtual threads for I/O
var cpuExecutor       = executorGet( "cpu-tasks" )       // Scheduled pool for CPU work
var scheduledExecutor = executorGet( "scheduled-tasks" ) // For scheduled tasks

// Use directly
var future = ioExecutor.submit( () => fetchFromAPI( url ) )
var result = future.get()

Runtime Executor Configuration (boxlang.json):

{
    "executors": {
        "io-tasks":        { "type": "virtual" },
        "cpu-tasks":       { "type": "scheduled", "threads": 10 },
        "scheduled-tasks": { "type": "scheduled", "threads": 10 }
    }
}

Creating Custom Executors

BoxLang supports seven executor types via executorNew():

// Virtual threads — best for I/O-bound tasks (default, JVM 21+)
var ioPool = executorNew( "virtual", "my-io-pool" )

// Fixed thread pool — CPU-bound tasks with predictable concurrency
var cpuPool = executorNew( type="fixed", name="cpu-pool", threads=8 )

// Cached pool — grows/shrinks dynamically for bursty workloads
var dynPool = executorNew( "cached", "burst-pool" )

// Single thread — sequential guaranteed-order execution
var seqPool = executorNew( "single", "sequential-processor" )

// Fork-join — recursive divide-and-conquer algorithms
var fjPool = executorNew( type="fork_join", name="fj-pool", parallelism=8 )

// Work-stealing — auto load-balancing across threads
var wsPool = executorNew( type="work_stealing", name="load-balanced", parallelism=8 )

Scheduled Execution API

The scheduled executor type provides scheduleOnce, scheduleAtFixedRate, and scheduleWithFixedDelay:

var executor = executorNew( type="scheduled", name="scheduler", threads=5 )

// Run once after a delay
var future = executor.scheduleOnce(
    () => performMaintenance(),
    5,        // delay value
    "seconds" // time unit
)

// Run at a fixed rate (regardless of task duration)
var future = executor.scheduleAtFixedRate(
    () => healthCheck(),
    0,        // initial delay
    30,       // period
    "seconds"
)

// Run with fixed delay BETWEEN executions (waits for task to finish first)
var future = executor.scheduleWithFixedDelay(
    () => processQueue(),
    10,       // initial delay
    5,        // delay between completions
    "seconds"
)

// Submit a callable and get result
var resultFuture = executor.submit( () => complexCalculation() )
var result = resultFuture.get()

// Shutdown executor when done
executor.shutdown()

Executor Statistics

var stats = executor.getStatistics()
writeOutput( "Active threads:   #stats.activeCount#" )
writeOutput( "Completed tasks:  #stats.completedTaskCount#" )
writeOutput( "Pool size:        #stats.poolSize#" )
writeOutput( "Is shutdown:      #executor.isShutdown()#" )

References

• Async Programming
• Scheduling
• Threading
• Locking