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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.Threading;
using System.Threading.Tasks;

namespace Opc.Ua.Client
{
    /// <summary>
    /// Handles server redundancy detection and failover target
    /// selection for a managed session.
    /// </summary>
    /// <remarks>
    /// See OPC UA Part 4 §6.6.2 for the server redundancy model.
    /// </remarks>
    public interface IServerRedundancyHandler
    {
        /// <summary>
        /// Reads redundancy information from the connected session.
        /// </summary>
        /// <param name="session">The active session to read from.</param>
        /// <param name="ct">Cancellation token.</param>
        /// <returns>
        /// A <see cref="ServerRedundancyInfo"/> describing the server's
        /// redundancy configuration.
        /// </returns>
        ValueTask<ServerRedundancyInfo> FetchRedundancyInfoAsync(
            ISession session,
            CancellationToken ct = default);

        /// <summary>
        /// Selects the best server to fail over to from the redundant
        /// server set.
        /// </summary>
        /// <param name="redundancyInfo">
        /// The redundancy information previously obtained via
        /// <see cref="FetchRedundancyInfoAsync"/>.
        /// </param>
        /// <param name="currentEndpoint">
        /// The endpoint the session is currently connected to.
        /// </param>
        /// <returns>
        /// A <see cref="ConfiguredEndpoint"/> for the best failover
        /// target, or <see langword="null"/> if no failover target is
        /// available (e.g., transparent redundancy or no suitable
        /// backup server).
        /// </returns>
        ConfiguredEndpoint? SelectFailoverTarget(
            ServerRedundancyInfo redundancyInfo,
            ConfiguredEndpoint currentEndpoint);
    }
}