如何在 kotlin 协程上进行指数退避重试

Question

我正在使用 kotlin 协程进行网络请求，使用扩展方法调用 class 进行这样的改造

public suspend fun <T : Any> Call<T>.await(): T {

  return suspendCancellableCoroutine { continuation -> 

    enqueue(object : Callback<T> {

        override fun onResponse(call: Call<T>?, response: Response<T?>) {
            if (response.isSuccessful) {
                val body = response.body()
                if (body == null) {
                    continuation.resumeWithException(
                            NullPointerException("Response body is null")
                    )
                } else {
                    continuation.resume(body)
                }
            } else {
                continuation.resumeWithException(HttpException(response))
            }
        }

        override fun onFailure(call: Call<T>, t: Throwable) {
            // Don't bother with resuming the continuation if it is already cancelled.
            if (continuation.isCancelled) return
            continuation.resumeWithException(t)
        }
    })

      registerOnCompletion(continuation)
  }
}

然后从调用方我使用上面这样的方法

private fun getArticles()  = launch(UI) {

    loading.value = true
    try {
        val networkResult = api.getArticle().await()
        articles.value =  networkResult

    }catch (e: Throwable){
        e.printStackTrace()
        message.value = e.message

    }finally {
        loading.value = false
    }

}

我想在某些情况下以指数方式重试此 api 调用，即（IOException）我该如何实现？？

Answer 1

我建议为您的重试逻辑编写一个帮助程序 higher-order function。您可以使用以下实现作为开始：

suspend fun <T> retryIO(
    times: Int = Int.MAX_VALUE,
    initialDelay: Long = 100, // 0.1 second
    maxDelay: Long = 1000,    // 1 second
    factor: Double = 2.0,
    block: suspend () -> T): T
{
    var currentDelay = initialDelay
    repeat(times - 1) {
        try {
            return block()
        } catch (e: IOException) {
            // you can log an error here and/or make a more finer-grained
            // analysis of the cause to see if retry is needed
        }
        delay(currentDelay)
        currentDelay = (currentDelay * factor).toLong().coerceAtMost(maxDelay)
    }
    return block() // last attempt
}

使用这个功能非常简单：

val networkResult = retryIO { api.getArticle().await() }

您可以根据具体情况更改重试参数，例如：

val networkResult = retryIO(times = 3) { api.doSomething().await() }

您还可以完全更改 retryIO 的实现以满足您的应用程序的需要。例如，您可以硬编码所有重试参数，摆脱对重试次数的限制，更改默认值等。

Answer 2

您可以通过简单的用法尝试这种简单但非常敏捷的方法：

编辑： 在单独的答案中添加了更复杂的解决方案。

class Completion(private val retry: (Completion) -> Unit) {
    fun operationFailed() {
        retry.invoke(this)
    }
}

fun retryOperation(retries: Int, 
                   dispatcher: CoroutineDispatcher = Dispatchers.Default, 
                   operation: Completion.() -> Unit
) {
    var tryNumber = 0

    val completion = Completion {
        tryNumber++
        if (tryNumber < retries) {
            GlobalScope.launch(dispatcher) {
                delay(TimeUnit.SECONDS.toMillis(tryNumber.toLong()))
                operation.invoke(it)
            }
        }
    }

    operation.invoke(completion)
}

像这样使用它：

retryOperation(3) {
    if (!tryStuff()) {
        // this will trigger a retry after tryNumber seconds
        operationFailed()
    }
}

您显然可以在此基础上构建更多内容。

Answer 3

这是我之前回答的更复杂和方便的版本，希望它能帮助别人：

class RetryOperation internal constructor(
    private val retries: Int,
    private val initialIntervalMilli: Long = 1000,
    private val retryStrategy: RetryStrategy = RetryStrategy.LINEAR,
    private val retry: suspend RetryOperation.() -> Unit
) {
    var tryNumber: Int = 0
        internal set

    suspend fun operationFailed() {
        tryNumber++
        if (tryNumber < retries) {
            delay(calculateDelay(tryNumber, initialIntervalMilli, retryStrategy))
            retry.invoke(this)
        }
    }
}

enum class RetryStrategy {
    CONSTANT, LINEAR, EXPONENTIAL
}

suspend fun retryOperation(
    retries: Int = 100,
    initialDelay: Long = 0,
    initialIntervalMilli: Long = 1000,
    retryStrategy: RetryStrategy = RetryStrategy.LINEAR,
    operation: suspend RetryOperation.() -> Unit
) {
    val retryOperation = RetryOperation(
        retries,
        initialIntervalMilli,
        retryStrategy,
        operation,
    )

    delay(initialDelay)

    operation.invoke(retryOperation)
}

internal fun calculateDelay(tryNumber: Int, initialIntervalMilli: Long, retryStrategy: RetryStrategy): Long {
    return when (retryStrategy) {
        RetryStrategy.CONSTANT -> initialIntervalMilli
        RetryStrategy.LINEAR -> initialIntervalMilli * tryNumber
        RetryStrategy.EXPONENTIAL -> 2.0.pow(tryNumber).toLong()
    }
}

用法：

coroutineScope.launch {
    retryOperation(3) {
        if (!tryStuff()) {
            Log.d(TAG, "Try number $tryNumber")
            operationFailed()
        }
    }
}

Answer 4

这里是 Flow 和 retryWhen 函数的例子

RetryWhen 分机：

fun <T> Flow<T>.retryWhen(
    @FloatRange(from = 0.0) initialDelay: Float = RETRY_INITIAL_DELAY,
    @FloatRange(from = 1.0) retryFactor: Float = RETRY_FACTOR_DELAY,
    predicate: suspend FlowCollector<T>.(cause: Throwable, attempt: Long, delay: Long) -> Boolean
): Flow<T> = this.retryWhen { cause, attempt ->
    val retryDelay = initialDelay * retryFactor.pow(attempt.toFloat())
    predicate(cause, attempt, retryDelay.toLong())
}

用法：

flow {
    ...
}.retryWhen { cause, attempt, delay ->
    delay(delay)
    ...
}

Answer 5

流版本https://github.com/hoc081098/FlowExt

package com.hoc081098.flowext

import kotlin.time.Duration
import kotlin.time.ExperimentalTime
import kotlinx.coroutines.delay
import kotlinx.coroutines.flow.Flow
import kotlinx.coroutines.flow.FlowCollector
import kotlinx.coroutines.flow.emitAll
import kotlinx.coroutines.flow.flow
import kotlinx.coroutines.flow.retryWhen

@ExperimentalTime
public fun <T> Flow<T>.retryWithExponentialBackoff(
  initialDelay: Duration,
  factor: Double,
  maxAttempt: Long = Long.MAX_VALUE,
  maxDelay: Duration = Duration.INFINITE,
  predicate: suspend (cause: Throwable) -> Boolean = { true }
): Flow<T> {
  require(maxAttempt > 0) { "Expected positive amount of maxAttempt, but had $maxAttempt" }
  return retryWhenWithExponentialBackoff(
    initialDelay = initialDelay,
    factor = factor,
    maxDelay = maxDelay
  ) { cause, attempt -> attempt < maxAttempt && predicate(cause) }
}

@ExperimentalTime
public fun <T> Flow<T>.retryWhenWithExponentialBackoff(
  initialDelay: Duration,
  factor: Double,
  maxDelay: Duration = Duration.INFINITE,
  predicate: suspend FlowCollector<T>.(cause: Throwable, attempt: Long) -> Boolean
): Flow<T> = flow {
  var currentDelay = initialDelay

  retryWhen { cause, attempt ->
    predicate(cause, attempt).also {
      if (it) {
        delay(currentDelay)
        currentDelay = (currentDelay * factor).coerceAtMost(maxDelay)
      }
    }
  }.let { emitAll(it) }
}

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