Health checks

Kevlar.Extensions.Diagnostics.HealthChecks reports circuit breaker state through .NET health checks. Add the package to an application that uses AddHealthChecks, then register the check:

using System;
using Kevlar;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Diagnostics.HealthChecks;

var services = new ServiceCollection();
services.AddLogging(); // ASP.NET Core hosts already register logging.
services.AddShield("catalog", Shield.CircuitBreaker(
    consecutiveFailures: 3, breakDuration: TimeSpan.FromSeconds(30)));
services.AddHealthChecks()
    .AddKevlar(name: "kevlar", failureStatus: HealthStatus.Degraded);

The check resolves registered shields through IKevlarRegistry, including typed shields and registrations added dynamically. Reloading shields report only their current publication. Inspection reads state without executing an operation, admitting a half-open probe, or resetting a breaker. A selected lazy registration is built on its first inspection; factories should finish promptly.

Status and selection

Closed circuits are healthy. Open and half-open circuits use failureStatus (degraded by default). Isolated circuits are unhealthy by default. The check returns the worst configured health status across all selected circuits. Configure isolated status, filter by shield name, or supply monitors:

using System;
using Kevlar;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Diagnostics.HealthChecks;

var services = new ServiceCollection();
services.AddLogging();
var monitor = new CircuitBreakerMonitor();
var externalShield = Shield.CircuitBreaker(options => options.Monitor = monitor);

services.AddHealthChecks().AddKevlar(
    name: "dependencies",
    failureStatus: HealthStatus.Unhealthy,
    configure: options =>
    {
        options.IsolatedStatus = HealthStatus.Degraded;
        options.ShieldFilter = name => name.StartsWith("critical-", StringComparison.Ordinal);
        options.Monitors.Add("external", monitor);
    },
    tags: ["ready"]);

The filter runs before resolving registry factories and also filters named partition providers. Explicit monitors are always included; a monitor can report multiple bound breakers. Options and the monitor collection are copied at registration. The filter delegate and monitor instances remain shared; callers must make their own mutable state safe for concurrent inspections.

Set IncludeRegistry = false for a monitor-only check. Automatic enumeration requires the built-in registry; custom IKevlarRegistry implementations can supply explicit monitors with enumeration disabled. An unbound monitor, failed factory, or unsupported registry produces the registration's failure status and exposes the exception through the health-check service. An empty selection is healthy.

Retained partitions and report data

Both typed and untyped providers registered with AddPartitionedShield are included automatically. Inspection snapshots retained partitions; it never creates partitions, refreshes their idle expiration, or exposes partition keys. Removal and eviction affect subsequent reports. Each report is an observational snapshot, not a transaction across concurrent executions or registry changes.

HealthReportEntry.Data contains these values:

Key	Value
shields	Array of records containing name, resultType (empty for untyped), and breakers (state strings in pipeline order).
monitors	Dictionary from configured monitor name to an array of bound circuit state strings.
partitions	Array of records containing name, serviceType, partitionCount, openPartitionCount, halfOpenPartitionCount, isolatedPartitionCount, and breakers (an array of state arrays).

A partition with multiple open breakers counts once in openPartitionCount. Half-open and isolated partitions have separate counts; a partition containing different breaker states may appear in more than one count. Partition array positions are temporary and must not be treated as stable identities. Type fields use assembly-qualified names to distinguish typed registrations.

Use the health-check service or a custom ASP.NET Core response writer to serialize report data; the default endpoint response does not include this dictionary. Select this check for readiness when circuit state should affect traffic routing. An open dependency circuit alone usually does not require restarting the process through a liveness failure.