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# Synchronization and Flow Processing
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Arrow FX Coroutines provides advanced synchronization primitives and Flow extensions for coordinating concurrent operations and processing streams of data with timing and parallel capabilities.
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## Synchronization Primitives
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### CountDownLatch
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```kotlin { .api }
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class CountDownLatch(private val initial: Long) {
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    fun count(): Long
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    suspend fun await()
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    fun countDown()
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}
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```
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A synchronization primitive that allows coroutines to wait until a specified number of countdown signals have been received.
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#### CountDownLatch Usage
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```kotlin
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val latch = CountDownLatch(3)
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// Start multiple coroutines
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launch { 
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    performTask1()
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    latch.countDown()
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}
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launch { 
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    performTask2()
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    latch.countDown()
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}
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launch { 
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    performTask3()
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    latch.countDown()
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}
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// Wait for all tasks to complete
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latch.await()
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println("All tasks completed!")
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```
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#### Producer-Consumer Pattern
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```kotlin
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class DataProcessor {
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    private val latch = CountDownLatch(1)
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    private var processedData: String? = null
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    suspend fun processData(input: String) {
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        // Simulate processing
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        delay(1000)
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        processedData = "Processed: $input"
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        latch.countDown()
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    }
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    suspend fun getResult(): String {
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        latch.await()
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        return processedData!!
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    }
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}
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val processor = DataProcessor()
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launch { processor.processData("important data") }
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val result = processor.getResult()
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```
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### CyclicBarrier
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```kotlin { .api }
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class CyclicBarrier(val capacity: Int, barrierAction: () -> Unit = {}) {
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    val capacity: Int
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    suspend fun reset()
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    suspend fun await()
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}
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class CyclicBarrierCancellationException : CancellationException
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```
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A synchronization primitive that allows a set of coroutines to wait for each other to reach a common barrier point.
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#### CyclicBarrier Usage
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```kotlin
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val barrier = CyclicBarrier(3) {
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    println("All workers reached the barrier!")
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}
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// Start workers
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repeat(3) { workerId ->
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    launch {
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        repeat(5) { phase ->
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            performWork(workerId, phase)
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            println("Worker $workerId completed phase $phase")
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            barrier.await() // Wait for all workers
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            println("Worker $workerId starting next phase")
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        }
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    }
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}
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```
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#### Batch Processing Pattern
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```kotlin
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class BatchProcessor<T>(private val batchSize: Int) {
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    private val barrier = CyclicBarrier(batchSize) {
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        println("Batch of $batchSize items ready for processing")
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    }
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    suspend fun submitItem(item: T) {
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        // Add item to batch
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        addToBatch(item)
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        // Wait for batch to fill
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        barrier.await()
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        // Process batch collectively
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        processBatch()
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        // Reset for next batch
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        barrier.reset()
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    }
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}
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```
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## Experimental AwaitAll API
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### AwaitAllScope
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```kotlin { .api }
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@ExperimentalAwaitAllApi
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class AwaitAllScope {
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    fun <A> async(
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        context: CoroutineContext = EmptyCoroutineContext,
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        start: CoroutineStart = CoroutineStart.DEFAULT,
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        block: suspend CoroutineScope.() -> A
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    ): Deferred<A>
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}
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```
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A scope that automatically awaits all async operations created within it.
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### AwaitAll Functions
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```kotlin { .api }
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@ExperimentalAwaitAllApi
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suspend fun <A> awaitAll(block: suspend AwaitAllScope.() -> A): A
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@ExperimentalAwaitAllApi
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suspend fun <A> awaitAll(context: CoroutineContext, block: suspend AwaitAllScope.() -> A): A
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```
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Execute a block where all async operations are automatically awaited.
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```kotlin
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@OptIn(ExperimentalAwaitAllApi::class)
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val results = awaitAll {
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    val deferred1 = async { fetchData1() }
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    val deferred2 = async { fetchData2() }
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    val deferred3 = async { fetchData3() }
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    // All deferreds are automatically awaited
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    // Results are available immediately
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    combineResults(deferred1.await(), deferred2.await(), deferred3.await())
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}
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```
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## Flow Extensions
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### Parallel Flow Processing
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```kotlin { .api }
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fun <A, B> Flow<A>.parMap(concurrency: Int = DEFAULT_CONCURRENCY, crossinline transform: suspend CoroutineScope.(A) -> B): Flow<B>
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fun <A, B> Flow<A>.parMapUnordered(concurrency: Int = DEFAULT_CONCURRENCY, crossinline transform: suspend (A) -> B): Flow<B>
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fun <A, B> Flow<A>.parMapNotNullUnordered(concurrency: Int = DEFAULT_CONCURRENCY, crossinline transform: suspend (A) -> B?): Flow<B>
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```
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Process Flow elements in parallel while controlling concurrency.
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#### Ordered Parallel Processing
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```kotlin
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val processedFlow = sourceFlow
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    .parMap(concurrency = 10) { item ->
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        expensiveOperation(item)
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    }
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    .collect { processedItem ->
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        // Items arrive in original order
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        println("Processed: $processedItem")
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    }
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```
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#### Unordered Parallel Processing
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```kotlin
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val processedFlow = sourceFlow
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    .parMapUnordered(concurrency = 5) { item ->
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        asyncOperation(item)
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    }
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    .collect { processedItem ->
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        // Items arrive as soon as they're processed
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        println("Completed: $processedItem")
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    }
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```
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### Flow Repetition
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```kotlin { .api }
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fun <A> Flow<A>.repeat(): Flow<A>
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```
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Repeat a Flow forever.
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```kotlin
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val heartbeatFlow = flowOf("ping")
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    .repeat()
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    .collect { 
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        println("Heartbeat: $it")
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        delay(1000)
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    }
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```
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### Timed Flow Operations
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```kotlin { .api }
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fun <A> Flow<A>.metered(period: Duration): Flow<A>
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fun <A> Flow<A>.metered(periodInMillis: Long): Flow<A>
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fun <A, B> Flow<A>.mapIndexed(crossinline f: suspend (index: Int, value: A) -> B): Flow<B>
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```
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Control the timing and indexing of Flow emissions.
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#### Rate-Limited Processing
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```kotlin
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val rateLimitedFlow = dataFlow
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    .metered(Duration.ofSeconds(1)) // One item per second
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    .collect { item ->
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        processItem(item)
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    }
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```
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#### Indexed Processing
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```kotlin
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val indexedResults = sourceFlow
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    .mapIndexed { index, item ->
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        "Item $index: $item"
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    }
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    .collect { indexedItem ->
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        println(indexedItem)
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    }
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```
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### Fixed Rate Flow Generation
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```kotlin { .api }
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fun fixedRate(period: Duration, dampen: Boolean = true, timeStamp: () -> ComparableTimeMark = { TimeSource.Monotonic.markNow() }): Flow<Unit>
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fun fixedRate(periodInMillis: Long, dampen: Boolean = true, timeStamp: () -> ComparableTimeMark = { TimeSource.Monotonic.markNow() }): Flow<Unit>
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```
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Create a Flow that emits at fixed intervals.
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#### Periodic Tasks
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```kotlin
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val periodicTask = fixedRate(Duration.ofMinutes(5))
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    .collect {
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        performMaintenanceTask()
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    }
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```
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#### With Dampening
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```kotlin
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// Dampen = true: Delays if processing takes longer than period
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val dampenedFlow = fixedRate(Duration.ofSeconds(10), dampen = true)
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    .collect {
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        longRunningTask() // Won't overlap if it takes > 10 seconds
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    }
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// Dampen = false: Strict timing regardless of processing time
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val strictFlow = fixedRate(Duration.ofSeconds(10), dampen = false)
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    .collect {
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        quickTask() // Overlapping execution possible
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    }
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```
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## Advanced Synchronization Patterns
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### Multi-Stage Pipeline
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```kotlin
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class PipelineStage<T>(private val capacity: Int) {
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    private val inputBarrier = CyclicBarrier(capacity)
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    private val outputBarrier = CyclicBarrier(capacity)
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    suspend fun process(items: List<T>): List<T> {
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        // Wait for all inputs
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        inputBarrier.await()
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        // Process in parallel
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        val results = items.parMap { item ->
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            processItem(item)
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        }
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        // Wait for all processing to complete
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        outputBarrier.await()
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        return results
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    }
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}
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```
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### Coordinated Resource Access
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```kotlin
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class CoordinatedResourcePool<T>(
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    private val resources: List<T>,
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    private val maxConcurrentUsers: Int
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) {
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    private val accessBarrier = CyclicBarrier(maxConcurrentUsers)
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    suspend fun <R> useResource(operation: suspend (T) -> R): R {
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        accessBarrier.await() // Wait for access slot
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        return try {
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            val resource = acquireResource()
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            operation(resource)
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        } finally {
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            releaseResource()
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            accessBarrier.reset()
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        }
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    }
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}
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```
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### Flow-Based Event Processing
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```kotlin
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class EventProcessor {
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    fun processEvents(eventFlow: Flow<Event>) = eventFlow
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        .parMapUnordered(concurrency = 20) { event ->
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            when (event.type) {
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                EventType.HIGH_PRIORITY -> processImmediately(event)
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                EventType.NORMAL -> processNormal(event)
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                EventType.BATCH -> processBatch(event)
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            }
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        }
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        .metered(Duration.ofMillis(100)) // Rate limit output
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        .collect { result ->
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            publishResult(result)
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        }
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}
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```
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## Integration Examples
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### Synchronization with Resource Management
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```kotlin
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resourceScope {
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    val database = databaseResource.bind()
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    val latch = CountDownLatch(3)
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    // Start parallel database operations
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    launch { 
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        database.updateUsers()
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        latch.countDown()
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    }
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    launch {
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        database.updateProducts()
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        latch.countDown()
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    }
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    launch {
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        database.updateOrders()
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        latch.countDown()
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    }
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    // Wait for all updates to complete
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    latch.await()
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    // Perform final operation
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    database.generateReport()
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}
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```
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### Flow Processing with Error Handling
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```kotlin
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val processedResults = either<ProcessingError, List<Result>> {
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    dataFlow
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        .parMapUnordered(concurrency = 10) { item ->
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            validateAndProcess(item).bind()
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        }
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        .metered(Duration.ofSeconds(1))
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        .toList()
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}
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```
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### Complex Coordination Example
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```kotlin
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class WorkflowCoordinator(private val stages: Int) {
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    private val stageBarriers = (0 until stages).map { 
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        CyclicBarrier(10) // 10 workers per stage
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    }
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    suspend fun executeWorkflow(data: List<WorkItem>) {
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        data.chunked(10).forEachIndexed { stageIndex, batch ->
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            batch.parMap { item ->
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                processAtStage(item, stageIndex)
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                stageBarriers[stageIndex].await()
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            }
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        }
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    }
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}
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```
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## Experimental APIs
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### AwaitAllScope (Experimental)
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```kotlin { .api }
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@ExperimentalAwaitAllApi
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class AwaitAllScope(scope: CoroutineScope) : CoroutineScope by scope {
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    fun <T> async(
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        context: CoroutineContext = EmptyCoroutineContext,
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        start: CoroutineStart = CoroutineStart.DEFAULT,
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        block: suspend CoroutineScope.() -> T
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    ): Deferred<T>
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}
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@ExperimentalAwaitAllApi
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suspend fun <A> awaitAll(block: suspend AwaitAllScope.() -> A): A
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@ExperimentalAwaitAllApi
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suspend fun <A> CoroutineScope.awaitAll(block: suspend AwaitAllScope.() -> A): A
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@RequiresOptIn(level = RequiresOptIn.Level.WARNING, message = "This API is work-in-progress and is subject to change.")
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@Retention(AnnotationRetention.BINARY) 
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@Target(AnnotationTarget.FUNCTION, AnnotationTarget.CLASS)
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annotation class ExperimentalAwaitAllApi
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```
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Experimental scope for automatic await management of async operations.
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#### AwaitAllScope Usage
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```kotlin
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@OptIn(ExperimentalAwaitAllApi::class)
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suspend fun fetchDataFromMultipleSources(): CombinedData = awaitAll {
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    // All async calls within this scope are automatically awaited
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    val userData = async { userService.getData() }
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    val settingsData = async { settingsService.getData() }
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    val notificationData = async { notificationService.getData() }
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    // Results are automatically awaited when accessed
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    CombinedData(
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        user = userData.await(),
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        settings = settingsData.await(),
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        notifications = notificationData.await()
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    )
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}
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```
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**⚠️ Warning**: This API is experimental and subject to change in future versions.