Kotlin

Getting started with resilience4j-kotlin

Introduction

Integration for Kotlin coroutines. This module adds extensions for executing and decorating suspend functions as well as custom operators for the coroutine reactive primitive Flow.

Extension functions that accept Kotlin suspend functions are provided for RateLimiter, Retry, CircuitBreaker, TimeLimiter, and semaphore-based Bulkheads. No extensions for ThreadPoolBulkhead or Caches are currently provided.

For Flow, there are operators provided for RateLimiter, Retry, CircuitBreaker, TimeLimiter, and semaphore-based Bulkheads. No extensions for ThreadPoolBulkhead or Caches are currently provided.

Setup

Add the Kotlin module of Resilience4j to your compile dependency, along with whichever core modules are needed:

Groovy

repositories {
    jCenter()
} 

dependencies {
	compile "io.github.resilience4j:resilience4j-kotlin:${resilience4jVersion}"
  // also have a dependency on the core module(s) needed - for example, retry:
	compile "io.github.resilience4j:resilience4j-retry:${resilience4jVersion}"
}

Usage

Two extension functions are declared for each of CircuitBreaker, RateLimiter, Retry, and TimeLimiter: one to execute a suspend function, and one to decorate a suspend function.

Usage - Suspending Functions

Two extension functions are declared for each of CircuitBreaker, RateLimiter, Retry, and TimeLimiter: one to execute a suspend function, and one to decorate a suspend function.

Kotlin

val circuitBreaker = CircuitBreaker.ofDefaults()
val result = circuitBreaker.executeSuspendFunction {
   // call suspending functions here
}

val function = circuitBreaker.decorateSuspendFunction {
   // call suspending functions here
}
val result = function()

The suspend functions suspend where usage of the normal methods would block. For example, calls to a RateLimiter which need to be delayed to fit within the rate limit suspend before the given function is executed.
No changes are made to the coroutine context of the given suspend functions.

Usage - Flow

There are extensions for Flow<T> defined for each of CircuitBreaker, RateLimiter, Retry, and TimeLimiter. The order in which operators are chain is important and directly correlates to the order in which resilience primitives are evaluated. Considering the following example, flowOf(1).retry(...).timeLimiter(...), all of the retry attempts must completed within the duration of the supplied TimeLimiter.

Kotlin

val retry = Retry.ofDefaults()
val rateLimiter = RateLimiter.ofDefaults()
val timeLimiter = TimeLimiter.ofDefaults()
val circuitBreaker = CircuitBreaker.ofDefaults()
  
flowOf(1, 2, 3)
  .retry(retry)
  .rateLimiter(rateLimiter)
  .timeLimiter(timeLimiter)
  .circuitBreaker(circuitBreaker)
  .collect { println(it) }

Bulkhead

Decorating a suspend function or Flow with a Bulkhead special attention needs to be paid to configuration value of maxWaitTime. If maxWaitTime is non-zero, the call will block until the max wait time is reached or permission is obtained. This blocking is restricted to the dispatcher Dispatchers.IO. Although this helps mitigate some of the performance impact introduced by the usage of the blocking API, it is not recommended to use this extension function with Bulkheads with non-zero max wait times.

If the coroutine scope is cancelled, either normally or exceptionally, the acquired permission will be released. After which no success or failure events will be recorded.

No extension functions for thread-pool based bulkheads are provided.

CircuitBreaker

Decorating a suspend function or Flow with a CircuitBreaker does not add any additional suspension points. In the case of decorated Flows, attempts to collect will throw a CallNotPermittedException if the circuit breaker state is OPEN.

If the coroutine scope is cancelled, either normally or exceptionally, the acquired permission will be released. After which no success or failure events will be recorded.

Rate Limiter

Suspend functions decorated with a RateLimiter use delay() in order to suspend before executing the decorated function if the rate limit has been reached.

Flows decorated with a RateLimiter will invoke delay() on attempts to collect if the rate limit has been reached.

Retry

Suspend functions and Flows decorated with a Retry use delay() in order to suspend between retries.

Time Limiter

The TimeLimiter extension functions simply uses withTimeout() from kotlinx-coroutines, using the timeout from the receiver's configuration. Specifically, this means:

• On timeout, a TimeoutCancellationException is raised, rather than a TimeoutException as with methods for non-suspending functions.
• When a timeout occurs, the coroutine is cancelled, rather than the thread being interrupted as with methods for non-suspending functions.
• After the timeout, the given block can only be stopped at a cancellable suspending function call.
• The cancelRunningFuture configuration setting is ignored - on timeout, the suspend function is always cancelled even if the cancelRunningFuture is set to false.

Flows decorated with a TimeLimiter will ensure that the entire Flow is consumed within the configured limit once collection has begun.