UA-.NETStandard/Libraries/Opc.Ua.Client/Session/ReconnectPolicy.cs

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 *
 * OPC Foundation MIT License 1.00
 *
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 * obtaining a copy of this software and associated documentation
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using System;
using System.Threading;

namespace Opc.Ua.Client
{
    /// <summary>
    /// Backoff strategy for reconnection delays.
    /// </summary>
    public enum BackoffStrategy
    {
        /// <summary>
        /// Constant delay between attempts.
        /// </summary>
        Constant,

        /// <summary>
        /// Linearly increasing delay.
        /// </summary>
        Linear,

        /// <summary>
        /// Exponentially increasing delay.
        /// </summary>
        Exponential
    }

    /// <summary>
    /// Default reconnect policy with configurable backoff,
    /// jitter, and retry limits.
    /// </summary>
    public class ReconnectPolicy : IReconnectPolicy
    {
        /// <summary>
        /// Default initial delay (1 second).
        /// </summary>
        public static readonly TimeSpan DefaultInitialDelay = TimeSpan.FromSeconds(1);

        /// <summary>
        /// Default max delay (30 seconds).
        /// </summary>
        public static readonly TimeSpan DefaultMaxDelay = TimeSpan.FromSeconds(30);

        /// <summary>
        /// Default maximum total reconnect time (5 minutes).
        /// </summary>
        public static readonly TimeSpan DefaultMaxTotalReconnectTime = TimeSpan.FromMinutes(5);

        /// <summary>
        /// Initialize a reconnect policy with default options.
        /// </summary>
        public ReconnectPolicy()
            : this(new ReconnectPolicyOptions())
        {
        }

        /// <summary>
        /// Initialize a reconnect policy from an options snapshot.
        /// </summary>
        public ReconnectPolicy(ReconnectPolicyOptions options)
        {
            if (options == null)
            {
                throw new ArgumentNullException(nameof(options));
            }
            InitialDelay = options.InitialDelay;
            MaxDelay = options.MaxDelay;
            MaxRetries = options.MaxRetries;
            Strategy = options.Strategy;
            JitterFactor = options.JitterFactor;
            MaxTotalReconnectTime = options.MaxTotalReconnectTime;
        }

        /// <summary>
        /// Initial delay between reconnect attempts.
        /// </summary>
        public TimeSpan InitialDelay { get; set; } = DefaultInitialDelay;

        /// <summary>
        /// Maximum delay between attempts.
        /// </summary>
        public TimeSpan MaxDelay { get; set; } = DefaultMaxDelay;

        /// <summary>
        /// Maximum number of retries (0 = unlimited).
        /// </summary>
        public int MaxRetries { get; set; }

        /// <summary>
        /// Backoff strategy.
        /// </summary>
        public BackoffStrategy Strategy { get; set; } = BackoffStrategy.Exponential;

        /// <summary>
        /// Jitter factor (0.0 = no jitter, 0.1 = ±10%).
        /// </summary>
        public double JitterFactor { get; set; } = 0.1;

        /// <summary>
        /// Maximum total elapsed time for one reconnect cycle across
        /// outer ManagedSession retries and channel-manager retries.
        /// </summary>
        public TimeSpan MaxTotalReconnectTime { get; set; } = DefaultMaxTotalReconnectTime;

        /// <inheritdoc/>
        public TimeSpan? GetNextDelay(int attempt, CancellationToken ct = default)
        {
            if (MaxRetries > 0 && attempt >= MaxRetries)
            {
                return null;
            }

            double delayMs = Strategy switch
            {
                BackoffStrategy.Constant => InitialDelay.TotalMilliseconds,
                BackoffStrategy.Linear => InitialDelay.TotalMilliseconds * (attempt + 1),
                BackoffStrategy.Exponential => InitialDelay.TotalMilliseconds * Math.Pow(2, attempt),
                _ => InitialDelay.TotalMilliseconds
            };

            delayMs = Math.Min(delayMs, MaxDelay.TotalMilliseconds);

            if (JitterFactor > 0)
            {
                int jitter = (int)(delayMs * JitterFactor);
                delayMs += UnsecureRandom.Shared.Next(-jitter, jitter + 1);
            }

            return TimeSpan.FromMilliseconds(Math.Max(delayMs, 0));
        }

        /// <inheritdoc/>
        public void Reset()
        {
            // Stateless — no internal state to reset.
        }
    }
}