Initial version

EonaCat.LogStack/EonaCatLoggerCore/Flows/FailoverFlow.cs

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+using EonaCat.LogStack.Core;
+using EonaCat.LogStack.Flows;
+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace EonaCat.LogStack.Flows
+{
+    // This file is part of the EonaCat project(s) which is released under the Apache License.
+    // See the LICENSE file or go to https://EonaCat.com/License for full license details.
+
+    public sealed class FailoverFlow : FlowBase
+    {
+        private readonly IFlow _primary;
+        private readonly IFlow _secondary;
+
+        public FailoverFlow(IFlow primary, IFlow secondary)
+            : base($"Failover({primary.Name})", primary.MinimumLevel)
+        {
+            _primary = primary ?? throw new ArgumentNullException(nameof(primary));
+            _secondary = secondary ?? throw new ArgumentNullException(nameof(secondary));
+        }
+
+        public override async Task<WriteResult> BlastAsync(LogEvent logEvent, CancellationToken cancellationToken = default)
+        {
+            var result = await _primary.BlastAsync(logEvent, cancellationToken).ConfigureAwait(false);
+
+            if (result == WriteResult.Success)
+            {
+                return result;
+            }
+
+            return await _secondary.BlastAsync(logEvent, cancellationToken).ConfigureAwait(false);
+        }
+
+        public override async Task FlushAsync(CancellationToken cancellationToken = default)
+        {
+            await _primary.FlushAsync(cancellationToken);
+            await _secondary.FlushAsync(cancellationToken);
+        }
+
+        public override async ValueTask DisposeAsync()
+        {
+            await _primary.DisposeAsync();
+            await _secondary.DisposeAsync();
+        }
+    }
+}