HTTPConnectionPool+Backoff.swift

//===----------------------------------------------------------------------===//
//
// This source file is part of the AsyncHTTPClient open source project
//
// Copyright (c) 2021 Apple Inc. and the AsyncHTTPClient project authors
// Licensed under Apache License v2.0
//
// See LICENSE.txt for license information
// See CONTRIBUTORS.txt for the list of AsyncHTTPClient project authors
//
// SPDX-License-Identifier: Apache-2.0
//
//===----------------------------------------------------------------------===//

import NIOCore
#if canImport(Darwin)
    import func Darwin.pow
#else
    import func Glibc.pow
#endif

extension HTTPConnectionPool {
    /// Calculates the delay for the next connection attempt after the given number of failed `attempts`.
    ///
    /// Our backoff formula is: 100ms * 1.25^(attempts - 1) that is capped of at 1 minute.
    /// This means for:
    ///   -  1 failed attempt :  100ms
    ///   -  5 failed attempts: ~300ms
    ///   - 10 failed attempts: ~930ms
    ///   - 15 failed attempts: ~2.84s
    ///   - 20 failed attempts: ~8.67s
    ///   - 25 failed attempts: ~26s
    ///   - 29 failed attempts: ~60s (max out)
    ///
    /// - Parameter attempts: number of failed attempts in a row
    /// - Returns: time to wait until trying to establishing a new connection
    static func calculateBackoff(failedAttempt attempts: Int) -> TimeAmount {
        // Our backoff formula is: 100ms * 1.25^(attempts - 1) that is capped of at 1minute
        // This means for:
        //   -  1 failed attempt :  100ms
        //   -  5 failed attempts: ~300ms
        //   - 10 failed attempts: ~930ms
        //   - 15 failed attempts: ~2.84s
        //   - 20 failed attempts: ~8.67s
        //   - 25 failed attempts: ~26s
        //   - 29 failed attempts: ~60s (max out)

        let start = Double(TimeAmount.milliseconds(100).nanoseconds)
        let backoffNanoseconds = Int64(start * pow(1.25, Double(attempts - 1)))

        let backoff: TimeAmount = min(.nanoseconds(backoffNanoseconds), .seconds(60))

        // Calculate a 3% jitter range
        let jitterRange = (backoff.nanoseconds / 100) * 3
        // Pick a random element from the range +/- jitter range.
        let jitter: TimeAmount = .nanoseconds((-jitterRange...jitterRange).randomElement()!)
        let jitteredBackoff = backoff + jitter
        return jitteredBackoff
    }
}