svrnty-mcp-gateway/tests/Svrnty.MCP.Gateway.Infrastructure.Tests/Health/CircuitBreakerTests.cs

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

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

/// <summary>
/// Unit tests for CircuitBreaker following TDD approach.
/// Tests circuit breaker states: Closed, Open, HalfOpen.
/// </summary>
public class CircuitBreakerTests
{
    [Fact]
    public async Task ExecuteAsync_InClosedState_AllowsExecution()
    {
        // Arrange
        var breaker = new CircuitBreaker
        {
            FailureThreshold = 3
        };
        var executed = false;

        // Act
        await breaker.ExecuteAsync(async () =>
        {
            executed = true;
            await Task.CompletedTask;
        });

        // Assert
        Assert.True(executed);
        Assert.Equal(CircuitBreakerState.Closed, breaker.State);
    }

    [Fact]
    public async Task ExecuteAsync_AfterMultipleFailures_OpensCircuit()
    {
        // Arrange
        var breaker = new CircuitBreaker
        {
            FailureThreshold = 3,
            OpenTimeout = TimeSpan.FromSeconds(1)
        };

        // Act - trigger 3 failures
        for (int i = 0; i < 3; i++)
        {
            try
            {
                await breaker.ExecuteAsync<object>(async () =>
                {
                    await Task.CompletedTask;
                    throw new Exception("Simulated failure");
                });
            }
            catch
            {
                // Expected
            }
        }

        // Assert
        Assert.Equal(CircuitBreakerState.Open, breaker.State);
    }

    [Fact]
    public async Task ExecuteAsync_WhenOpen_ThrowsCircuitBreakerOpenException()
    {
        // Arrange
        var breaker = new CircuitBreaker
        {
            FailureThreshold = 1,
            OpenTimeout = TimeSpan.FromSeconds(10)
        };

        // Trip the breaker
        try
        {
            await breaker.ExecuteAsync<object>(async () =>
            {
                await Task.CompletedTask;
                throw new Exception("Failure");
            });
        }
        catch
        {
            // Expected
        }

        // Act & Assert - breaker should be open
        await Assert.ThrowsAsync<CircuitBreakerOpenException>(() =>
            breaker.ExecuteAsync(async () => await Task.CompletedTask));
    }

    [Fact]
    public async Task ExecuteAsync_AfterTimeout_TransitionsToHalfOpen()
    {
        // Arrange
        var breaker = new CircuitBreaker
        {
            FailureThreshold = 1,
            OpenTimeout = TimeSpan.FromMilliseconds(100),
            SuccessThreshold = 1 // Close after 1 success
        };

        // Trip the breaker
        try
        {
            await breaker.ExecuteAsync<object>(async () =>
            {
                await Task.CompletedTask;
                throw new Exception("Failure");
            });
        }
        catch
        {
            // Expected
        }

        Assert.Equal(CircuitBreakerState.Open, breaker.State);

        // Act - wait for timeout
        await Task.Delay(150);

        // Try another execution (should transition to half-open)
        var executed = false;
        await breaker.ExecuteAsync(async () =>
        {
            executed = true;
            await Task.CompletedTask;
        });

        // Assert
        Assert.True(executed);
        Assert.Equal(CircuitBreakerState.Closed, breaker.State); // Successful execution closes the circuit
    }

    [Fact]
    public async Task ExecuteAsync_InHalfOpen_SuccessfulCallClosesCircuit()
    {
        // Arrange
        var breaker = new CircuitBreaker
        {
            FailureThreshold = 1,
            OpenTimeout = TimeSpan.FromMilliseconds(100),
            SuccessThreshold = 1
        };

        // Trip the breaker
        try
        {
            await breaker.ExecuteAsync<object>(async () =>
            {
                await Task.CompletedTask;
                throw new Exception("Failure");
            });
        }
        catch
        {
            // Expected
        }

        await Task.Delay(150); // Wait for open timeout

        // Act - successful call in half-open
        await breaker.ExecuteAsync(async () => await Task.CompletedTask);

        // Assert
        Assert.Equal(CircuitBreakerState.Closed, breaker.State);
    }

    [Fact]
    public async Task ExecuteAsync_InHalfOpen_FailureReopensCircuit()
    {
        // Arrange
        var breaker = new CircuitBreaker
        {
            FailureThreshold = 1,
            OpenTimeout = TimeSpan.FromMilliseconds(100)
        };

        // Trip the breaker
        try
        {
            await breaker.ExecuteAsync<object>(async () =>
            {
                await Task.CompletedTask;
                throw new Exception("Failure");
            });
        }
        catch
        {
            // Expected
        }

        await Task.Delay(150); // Wait for open timeout (now in half-open)

        // Act - fail in half-open
        try
        {
            await breaker.ExecuteAsync<object>(async () =>
            {
                await Task.CompletedTask;
                throw new Exception("Another failure");
            });
        }
        catch
        {
            // Expected
        }

        // Assert
        Assert.Equal(CircuitBreakerState.Open, breaker.State);
    }

    [Fact]
    public async Task ExecuteAsync_WithReturnValue_ReturnsResult()
    {
        // Arrange
        var breaker = new CircuitBreaker();
        var expectedValue = 42;

        // Act
        var result = await breaker.ExecuteAsync(async () =>
        {
            await Task.CompletedTask;
            return expectedValue;
        });

        // Assert
        Assert.Equal(expectedValue, result);
    }

    [Fact]
    public void Reset_ClosesCircuitAndClearsFailures()
    {
        // Arrange
        var breaker = new CircuitBreaker { FailureThreshold = 1 };

        try
        {
            breaker.ExecuteAsync<object>(async () =>
            {
                await Task.CompletedTask;
                throw new Exception("Failure");
            }).GetAwaiter().GetResult();
        }
        catch
        {
            // Expected
        }

        Assert.Equal(CircuitBreakerState.Open, breaker.State);

        // Act
        breaker.Reset();

        // Assert
        Assert.Equal(CircuitBreakerState.Closed, breaker.State);
    }

    [Fact]
    public void Constructor_SetsDefaultValues()
    {
        // Act
        var breaker = new CircuitBreaker();

        // Assert
        Assert.Equal(CircuitBreakerState.Closed, breaker.State);
        Assert.Equal(5, breaker.FailureThreshold);
        Assert.Equal(3, breaker.SuccessThreshold);
        Assert.Equal(TimeSpan.FromSeconds(60), breaker.OpenTimeout);
    }

    [Fact]
    public async Task ExecuteAsync_TracksConcurrentFailures()
    {
        // Arrange
        var breaker = new CircuitBreaker { FailureThreshold = 5 };
        var failureCount = 0;

        // Act - cause 4 failures (not enough to trip)
        for (int i = 0; i < 4; i++)
        {
            try
            {
                await breaker.ExecuteAsync<object>(async () =>
                {
                    await Task.CompletedTask;
                    throw new Exception("Failure");
                });
            }
            catch
            {
                failureCount++;
            }
        }

        // Assert - should still be closed
        Assert.Equal(CircuitBreakerState.Closed, breaker.State);
        Assert.Equal(4, failureCount);
    }
}