Prognosis 4.0.0

Prognosis

A dependency-aware service health modeling library for .NET.

graph TD
    Store["🛒 Online Store"] -->|"Required"| Checkout["Checkout"]
    Store -->|"Important"| Search["Product Search"]
    Store -->|"Optional"| Reviews["Reviews"]
    Checkout -->|"Required"| Payment["Payment Gateway"]
    Checkout -->|"Required"| Inventory["Inventory"]
    Payment -->|"Important"| Fraud["Fraud Detection"]
    Search -->|"Required"| Index["Search Index"]

    style Store fill:#22c55e,color:#fff
    style Checkout fill:#22c55e,color:#fff
    style Payment fill:#22c55e,color:#fff
    style Inventory fill:#22c55e,color:#fff
    style Fraud fill:#22c55e,color:#fff
    style Search fill:#22c55e,color:#fff
    style Index fill:#22c55e,color:#fff
    style Reviews fill:#22c55e,color:#fff

How it works: each service reports its own health and declares dependencies with an importance level. Prognosis walks the graph and computes the effective status — a Required dependency failing makes the parent unhealthy, an Important one degrades it, and an Optional one is ignored. If Fraud Detection goes down, Payment Gateway becomes degraded, which degrades Checkout, which degrades the whole store. If Payment Gateway itself goes down, Checkout becomes unhealthy — and since it's Required, the store is unhealthy too. If Reviews go down? Nothing happens.

Packages

Key concepts

Health statuses

Ordered worst-is-highest so comparisons naturally surface the most severe status.

Dependency importance

Usage patterns

1. Implement IHealthAware on a class you own

Expose a HealthNode property — no forwarding boilerplate:

class CacheService : IHealthAware
{
    public HealthNode HealthNode { get; }

    public CacheService()
    {
        HealthNode = new DelegateHealthNode("Cache",
            () => IsConnected
                ? HealthStatus.Healthy
                : new HealthEvaluation(HealthStatus.Unhealthy, "Redis timeout"));
    }

    public bool IsConnected { get; set; } = true;
}

For services with fine-grained health attributes, use a CompositeHealthNode backed by sub-nodes:

class DatabaseService : IHealthAware
{
    public HealthNode HealthNode { get; }

    public bool IsConnected { get; set; } = true;
    public double AverageLatencyMs { get; set; } = 50;
    public double PoolUtilization { get; set; } = 0.3;

    public DatabaseService()
    {
        var connection = new DelegateHealthNode("Database.Connection",
            () => IsConnected
                ? HealthStatus.Healthy
                : new HealthEvaluation(HealthStatus.Unhealthy, "Connection lost"));

        var latency = new DelegateHealthNode("Database.Latency",
            () => AverageLatencyMs switch
            {
                > 500 => new HealthEvaluation(HealthStatus.Degraded,
                    $"Avg latency {AverageLatencyMs:F0}ms exceeds 500ms threshold"),
                _ => HealthStatus.Healthy,
            });

        var connectionPool = new DelegateHealthNode("Database.ConnectionPool",
            () => PoolUtilization switch
            {
                >= 1.0 => new HealthEvaluation(HealthStatus.Unhealthy,
                    "Connection pool exhausted"),
                >= 0.9 => new HealthEvaluation(HealthStatus.Degraded,
                    $"Connection pool at {PoolUtilization:P0} utilization"),
                _ => HealthStatus.Healthy,
            });

        HealthNode = new CompositeHealthNode("Database")
            .DependsOn(connection, Importance.Required)
            .DependsOn(latency, Importance.Important)
            .DependsOn(connectionPool, Importance.Required);
    }
}

The sub-nodes show up automatically in EvaluateAll, CreateReport, and the JSON output. Reason strings chain from the leaf all the way up:

Database.Latency: Degraded — Avg latency 600ms exceeds 500ms threshold
Database: Degraded — Database.Latency: Avg latency 600ms exceeds 500ms threshold
AuthService: Degraded — Database: Database.Latency: ...

2. Wrap a service you can't modify

Use DelegateHealthNode with a health-check delegate:

var emailHealth = new DelegateHealthNode("EmailProvider",
    () => client.IsConnected
        ? HealthStatus.Healthy
        : new HealthEvaluation(HealthStatus.Unhealthy, "SMTP connection refused"));

3. Compose the graph

Wire services together with DependsOn:

var authService = new DelegateHealthNode("AuthService")
    .DependsOn(database.HealthNode, Importance.Required)
    .DependsOn(cache.HealthNode, Importance.Important);

var app = new CompositeHealthNode("Application")
    .DependsOn(authService, Importance.Required)
    .DependsOn(notifications, Importance.Important);

Resilient dependencies

Use Importance.Resilient when a parent has multiple paths to the same capability (e.g. primary + replica database). Losing one degrades — but doesn't kill — the parent:

// If one goes down but the other is healthy, the parent is degraded (not unhealthy).
// If both go down, the parent becomes unhealthy.
var app = new CompositeHealthNode("Application")
    .DependsOn(primaryDb, Importance.Resilient)
    .DependsOn(replicaDb, Importance.Resilient);

Only Resilient-marked siblings participate in the resilience check — Required, Important, and Optional dependencies are unaffected.

Graph operations

var graph = HealthGraph.Create(app);

// Evaluate a single service (walks its dependencies)
HealthEvaluation eval = app.Evaluate();

// Snapshot the entire graph (depth-first post-order)
IReadOnlyList<HealthSnapshot> snapshots = graph.EvaluateAll();

// Package as a serialization-ready report with timestamp
HealthReport report = graph.CreateReport();

// Detect circular dependencies
IReadOnlyList<IReadOnlyList<string>> cycles = graph.DetectCycles();

// Diff two reports to find individual service changes
IReadOnlyList<StatusChange> changes = before.DiffTo(after);

Observable health monitoring

Every HealthNode node supports push-based notifications. Subscribe to individual services or monitor the full graph:

// Individual service notifications
database.HealthNode.StatusChanged.Subscribe(observer);

// Graph-level polling with HealthMonitor
await using var monitor = new HealthMonitor(graph, TimeSpan.FromSeconds(5));
monitor.Start();
monitor.ReportChanged.Subscribe(reportObserver);

// Manual poll (useful for testing or getting initial state)
monitor.Poll();

IObservable<T> is a BCL type — no System.Reactive dependency required. Add System.Reactive only when you want operators like DistinctUntilChanged() or Throttle().

Dependency injection

The Prognosis.DependencyInjection package provides a fluent builder for configuring the health graph within a hosted application:

builder.Services.AddPrognosis(health =>
{
    // Discover all IHealthAware implementations and wire [DependsOn<T>] attributes.
    health.ScanForServices(typeof(Program).Assembly);

    // Wrap a third-party service with a health delegate.
    // Name defaults to typeof(T).Name when omitted.
    health.AddDelegate<ThirdPartyEmailClient>("EmailProvider",
        client => client.IsConnected
            ? HealthStatus.Healthy
            : new HealthEvaluation(HealthStatus.Unhealthy, "SMTP refused"));

    // Define composite aggregation nodes.
    health.AddComposite(ServiceNames.NotificationSystem, n =>
    {
        n.DependsOn(nameof(MessageQueueService), Importance.Required);
        n.DependsOn("EmailProvider", Importance.Optional);
    });

    health.AddComposite(ServiceNames.Application, app =>
    {
        app.DependsOn<AuthService>(Importance.Required);
        app.DependsOn(ServiceNames.NotificationSystem, Importance.Important);
    });

    // Designate the root of the graph. When only one root is declared,
    // a plain HealthGraph singleton is registered.
    health.MarkAsRoot(ServiceNames.Application);

    health.UseMonitor(TimeSpan.FromSeconds(30));
});

Multiple roots (shared nodes, separate graphs)

Call MarkAsRoot more than once to materialize several graphs from a single shared node pool. Each graph is registered as a keyed HealthGraph (keyed by the root name). Use the generic MarkAsRoot<T>() overload to also register a strongly-typed HealthGraph<T> for consumers that don't have keyed service support:

builder.Services.AddPrognosis(health =>
{
    health.ScanForServices(typeof(Program).Assembly);

    health.AddComposite(ServiceNames.OpsDashboard, ops =>
    {
        ops.DependsOn<DatabaseService>(Importance.Required);
        ops.DependsOn<CacheService>(Importance.Required);
    });

    health.AddComposite(ServiceNames.CustomerView, cust =>
    {
        cust.DependsOn<AuthService>(Importance.Required);
    });

    // Each MarkAsRoot call produces a separate HealthGraph.
    // Nodes (e.g. DatabaseService) are shared across graphs.
    health.MarkAsRoot<OpsDashboard>();      // registers keyed + HealthGraph<OpsDashboard>
    health.MarkAsRoot<CustomerView>();      // registers keyed + HealthGraph<CustomerView>
});

// Keyed resolution (requires Microsoft.Extensions.DependencyInjection 8+):
var opsGraph    = sp.GetRequiredKeyedService<HealthGraph>("OpsDashboard");
var custGraph   = sp.GetRequiredKeyedService<HealthGraph>("CustomerView");

// Generic resolution (works on any DI container):
var opsGraph    = sp.GetRequiredService<HealthGraph<OpsDashboard>>().Graph;
var custGraph   = sp.GetRequiredService<HealthGraph<CustomerView>>().Graph;

Declare dependency edges on classes you own with attributes:

[DependsOn<DatabaseService>(Importance.Required)]
[DependsOn<CacheService>(Importance.Important)]
class AuthService : IHealthAware
{
    public HealthNode HealthNode { get; } = new DelegateHealthNode("AuthService");
}

Inject HealthGraph to access the materialized graph at runtime:

var graph = serviceProvider.GetRequiredService<HealthGraph>();
var report = graph.CreateReport();

// Type-safe lookup — uses typeof(AuthService).Name as key.
if (graph.TryGetNode<AuthService>(out var auth))
    Console.WriteLine($"AuthService has {auth.Dependencies.Count} deps");

// String-based lookup still available.
HealthNode dbService = graph["Database"];

Reactive extensions

The Prognosis.Reactive package provides Rx-based alternatives to polling. All extensions work on both HealthGraph (whole graph) and HealthNode (single subtree):

var graph = HealthGraph.Create(app);

// Timer-driven polling — emits HealthReport on change.
graph.PollHealthReport(TimeSpan.FromSeconds(30))
    .Subscribe(report => Console.WriteLine(report.OverallStatus));

// Push-triggered — reacts to StatusChanged events from the root, no polling delay.
graph.ObserveHealthReport()
    .Subscribe(report => Console.WriteLine(report.OverallStatus));

// Diff-based change stream — composable with any report source.
graph.PollHealthReport(TimeSpan.FromSeconds(30))
    .SelectServiceChanges()
    .Subscribe(change =>
        Console.WriteLine($"{change.Name}: {change.Previous} → {change.Current}"));

// Per-node evaluation stream.
database.HealthNode.ObserveStatus()
    .Subscribe(eval => Console.WriteLine($"Database: {eval.Status}"));

Sharing streams across subscribers

The Rx helpers produce cold observables — each subscription runs its own pipeline. To share a single evaluation across multiple subscribers:

// Auto-stop when last subscriber unsubscribes.
var shared = graph.CreateSharedReportStream(TimeSpan.FromSeconds(30));

// Replay latest report to late subscribers.
var shared = graph.CreateSharedReportStream(TimeSpan.FromSeconds(30),
    ShareStrategy.ReplayLatest);

// Push-triggered variant.
var shared = graph.CreateSharedObserveStream();

Or use standard Rx multicast operators directly: Publish().RefCount() or Replay(1).RefCount().

Serialization

Both enums use [JsonStringEnumConverter] so they serialize as "Healthy" / "Degraded" / etc. The HealthReport and HealthSnapshot records are designed as wire-friendly DTOs:

{
  "Timestamp": "2026-02-13T18:30:00+00:00",
  "OverallStatus": "Healthy",
  "Services": [
    { "Name": "Database.Connection", "Status": "Healthy" },
    { "Name": "Database.Latency", "Status": "Healthy" },
    { "Name": "Database.ConnectionPool", "Status": "Healthy" },
    { "Name": "Database", "Status": "Healthy" },
    { "Name": "AuthService", "Status": "Healthy" }
  ]
}

Project structure

Core (Prognosis)

Reactive extensions (Prognosis.Reactive)

Dependency injection (Prognosis.DependencyInjection)

Requirements

• .NET Standard 2.0 or .NET Standard 2.1 compatible runtime (.NET Framework 4.6.1+, .NET Core 2.0+, .NET 5+)
• System.Text.Json (bundled as a dependency)
• Microsoft.Bcl.AsyncInterfaces (netstandard2.0 only, bundled as a dependency)