svrnty-mcp-gateway/tests/Svrnty.MCP.Gateway.Infrastructure.Tests/Routing/RoundRobinStrategyTests.cs

using Xunit;
using Svrnty.MCP.Gateway.Infrastructure.Routing;
using Svrnty.MCP.Gateway.Core.Models;

namespace Svrnty.MCP.Gateway.Infrastructure.Tests.Routing;

/// <summary>
/// Unit tests for RoundRobinStrategy following TDD approach.
/// Tests round-robin server selection.
/// </summary>
public class RoundRobinStrategyTests
{
    [Fact]
    public async Task SelectServerAsync_WithOneServer_ReturnsServer()
    {
        // Arrange
        var strategy = new RoundRobinStrategy();
        var servers = new List<ServerInfo>
        {
            new ServerInfo { Id = "server-1", IsHealthy = true }
        };
        var context = new RoutingContext();

        // Act
        var selected = await strategy.SelectServerAsync(servers, context);

        // Assert
        Assert.NotNull(selected);
        Assert.Equal("server-1", selected.Id);
    }

    [Fact]
    public async Task SelectServerAsync_WithMultipleServers_RotatesServers()
    {
        // Arrange
        var strategy = new RoundRobinStrategy();
        var servers = new List<ServerInfo>
        {
            new ServerInfo { Id = "server-1", IsHealthy = true },
            new ServerInfo { Id = "server-2", IsHealthy = true },
            new ServerInfo { Id = "server-3", IsHealthy = true }
        };
        var context = new RoutingContext();

        // Act
        var selected1 = await strategy.SelectServerAsync(servers, context);
        var selected2 = await strategy.SelectServerAsync(servers, context);
        var selected3 = await strategy.SelectServerAsync(servers, context);
        var selected4 = await strategy.SelectServerAsync(servers, context);

        // Assert - should rotate through all servers
        Assert.Equal("server-1", selected1.Id);
        Assert.Equal("server-2", selected2.Id);
        Assert.Equal("server-3", selected3.Id);
        Assert.Equal("server-1", selected4.Id); // Back to first
    }

    [Fact]
    public async Task SelectServerAsync_WithNoHealthyServers_ReturnsNull()
    {
        // Arrange
        var strategy = new RoundRobinStrategy();
        var servers = new List<ServerInfo>
        {
            new ServerInfo { Id = "server-1", IsHealthy = false },
            new ServerInfo { Id = "server-2", IsHealthy = false }
        };
        var context = new RoutingContext();

        // Act
        var selected = await strategy.SelectServerAsync(servers, context);

        // Assert
        Assert.Null(selected);
    }

    [Fact]
    public async Task SelectServerAsync_WithEmptyList_ReturnsNull()
    {
        // Arrange
        var strategy = new RoundRobinStrategy();
        var servers = new List<ServerInfo>();
        var context = new RoutingContext();

        // Act
        var selected = await strategy.SelectServerAsync(servers, context);

        // Assert
        Assert.Null(selected);
    }

    [Fact]
    public async Task SelectServerAsync_OnlySelectsHealthyServers()
    {
        // Arrange
        var strategy = new RoundRobinStrategy();
        var servers = new List<ServerInfo>
        {
            new ServerInfo { Id = "server-1", IsHealthy = true },
            new ServerInfo { Id = "server-2", IsHealthy = false },
            new ServerInfo { Id = "server-3", IsHealthy = true }
        };
        var context = new RoutingContext();

        // Act
        var selected1 = await strategy.SelectServerAsync(servers, context);
        var selected2 = await strategy.SelectServerAsync(servers, context);
        var selected3 = await strategy.SelectServerAsync(servers, context);

        // Assert - should only select healthy servers
        Assert.Equal("server-1", selected1.Id);
        Assert.Equal("server-3", selected2.Id);
        Assert.Equal("server-1", selected3.Id); // Back to first healthy
    }

    [Fact]
    public async Task SelectServerAsync_WithMixedHealth_SkipsUnhealthy()
    {
        // Arrange
        var strategy = new RoundRobinStrategy();
        var servers = new List<ServerInfo>
        {
            new ServerInfo { Id = "server-1", IsHealthy = false },
            new ServerInfo { Id = "server-2", IsHealthy = true },
            new ServerInfo { Id = "server-3", IsHealthy = false },
            new ServerInfo { Id = "server-4", IsHealthy = true }
        };
        var context = new RoutingContext();

        // Act
        var selected1 = await strategy.SelectServerAsync(servers, context);
        var selected2 = await strategy.SelectServerAsync(servers, context);

        // Assert
        Assert.Equal("server-2", selected1.Id);
        Assert.Equal("server-4", selected2.Id);
    }

    [Fact]
    public async Task SelectServerAsync_IsThreadSafe()
    {
        // Arrange
        var strategy = new RoundRobinStrategy();
        var servers = new List<ServerInfo>
        {
            new ServerInfo { Id = "server-1", IsHealthy = true },
            new ServerInfo { Id = "server-2", IsHealthy = true }
        };
        var context = new RoutingContext();

        // Act - call concurrently from multiple tasks
        var tasks = Enumerable.Range(0, 100)
            .Select(_ => strategy.SelectServerAsync(servers, context))
            .ToList();

        var results = await Task.WhenAll(tasks);

        // Assert - all calls should complete successfully
        Assert.Equal(100, results.Length);
        Assert.All(results, r => Assert.NotNull(r));
    }
}