Registering a Subscriber

Basic Registration

// In Program.cs or a dedicated extension method
builder.Services.AddTransient<INodeEventSubscriber, AuditNodeSubscriber>();
builder.Services.AddTransient<INodeEventSubscriber, MetricsNodeSubscriber>();
builder.Services.AddTransient<INodeEventSubscriber, ErrorAlertSubscriber>();

Lifetime Considerations

Registration Order = Execution Order

Subscribers are called in the exact order they are registered. Register more critical subscribers first (e.g., audit logging before alerting) so that even if alerting fails, the audit record is still written.

// Recommended order: audit first, then metrics, then alerting
services.AddTransient<INodeEventSubscriber, AuditNodeSubscriber>();
services.AddTransient<INodeEventSubscriber, MetricsNodeSubscriber>();
services.AddTransient<INodeEventSubscriber, SlackAlertSubscriber>();

Extension Method Pattern (Recommended)

For subscribers shipped in a library or module, expose a registration extension:

// In your library's ServiceCollectionExtensions.cs
public static class NodeObservabilityExtensions
{
    public static IServiceCollection AddNodeObservability(
        this IServiceCollection services,
        Action<NodeObservabilityOptions>? configure = null)
    {
        var options = new NodeObservabilityOptions();
        configure?.Invoke(options);

        services.AddSingleton(options);
        services.AddTransient<INodeEventSubscriber, AuditNodeSubscriber>();

        if (options.EnableMetrics)
            services.AddTransient<INodeEventSubscriber, MetricsNodeSubscriber>();

        if (options.EnableAlerts)
            services.AddTransient<INodeEventSubscriber, ErrorAlertSubscriber>();

        return services;
    }
}

// Usage in Program.cs
builder.Services.AddNodeObservability(opts =>
{
    opts.EnableMetrics = true;
    opts.EnableAlerts  = true;
});

Conditional Registration (Feature Flags)

var config = builder.Configuration;

if (config.GetValue<bool>("Features:NodeAuditLog"))
    builder.Services.AddTransient<INodeEventSubscriber, AuditNodeSubscriber>();

if (config.GetValue<bool>("Features:NodeMetrics"))
    builder.Services.AddTransient<INodeEventSubscriber, MetricsNodeSubscriber>();

Testing a Subscriber

[Fact]
public async Task AuditSubscriber_WritesRecord_OnBeforeExecute()
{
    // Arrange
    var repo = new FakeAuditRepository();
    var subscriber = new AuditNodeSubscriber(repo);
    var args = new NodeExecutionEventArgs
    {
        NodeId      = "node-1",
        NodeType    = "HttpRequest",
        ExecutionId = "exec-123",
        ProcessId   = "proc-456",
        TenantId    = "tenant-789",
        StartedAt   = DateTimeOffset.UtcNow,
        Context     = FakeNodeContext.Create()
    };

    // Act
    await subscriber.OnBeforeExecuteAsync(args, CancellationToken.None);

    // Assert
    Assert.Single(repo.Entries);
    Assert.Equal(AuditStatus.InProgress, repo.Entries[0].Status);
    Assert.Equal("node-1", repo.Entries[0].NodeId);
}