# All files / src/services HealthService.ts

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     /**
     * Health Check Service
     *
     * Provides a lightweight, synchronous-friendly health snapshot for the
     * MCP server.  The snapshot is designed to be consumed by:
     * - Monitoring integrations (CloudWatch, Prometheus, etc.)
     * - Future CLI health commands
     *
     * **Intelligence Perspective:** Operational health metrics are a
     * prerequisite for reliable intelligence product delivery—degraded
     * connectivity or exhausted rate limits must surface immediately.
     *
     * **Business Perspective:** Health endpoints support SLA dashboards and
     * enable proactive incident response before customers are impacted.
     *
     * ISMS Policy: MO-001 (Monitoring and Alerting), PE-001 (Performance Standards)
     *
     * @module services/HealthService
     */

    import type { RateLimiter, RateLimiterStatus } from '../utils/rateLimiter.js';
    import type { MetricsService } from './MetricsService.js';
    import { MetricName } from './MetricsService.js';

    /**
     * Overall server health verdict.
     *
     * | Value       | Meaning |
     * |-------------|---------|
     * | `healthy`   | All subsystems operating normally |
     * | `degraded`  | One or more subsystems impaired but functional |
     * | `unhealthy` | Critical subsystem failure |
     */
    export type HealthStatusLevel = 'healthy' | 'degraded' | 'unhealthy';

    /**
     * Rate-limiter status snapshot for health reporting.
     * Type alias of {@link RateLimiterStatus} to prevent the two from drifting.
     */
    export type RateLimiterHealthStatus = RateLimiterStatus;

    /**
     * Cache status snapshot for health reporting.
     */
    export interface CacheHealthStatus {
      /** Whether the cache is populated (has at least one entry) */
      populated: boolean;
      /** Human-readable description of cache state */
      description: string;
    }

    /**
     * Full health status returned by {@link HealthService.checkHealth}.
     */
    export interface HealthStatus {
      /** Overall health verdict */
      status: HealthStatusLevel;
      /**
       * Whether the EP API base URL is reachable (based on recent metrics).
       * `null` means no API calls have been recorded yet — reachability is unknown.
       */
      epApiReachable: boolean | null;
      /** Cache subsystem status */
      cache: CacheHealthStatus;
      /** Rate-limiter subsystem status */
      rateLimiter: RateLimiterHealthStatus;
      /** ISO-8601 timestamp of this health snapshot */
      timestamp: string;
      /** Server uptime in milliseconds */
      uptimeMs: number;
    }

    /**
     * Health Check Service
     *
     * Aggregates status from the rate limiter and metrics service to
     * produce a structured {@link HealthStatus} snapshot.
     *
     * The check is intentionally **synchronous-safe** – it never makes
     * network calls.  Network reachability is inferred from recent
     * metrics (EP API error rate).
     *
     * @example
     * ```typescript
     * const healthService = new HealthService(rateLimiter, metricsService);
     * const status = healthService.checkHealth();
     * console.log(status.status); // 'healthy' | 'degraded' | 'unhealthy'
     * ```
     */
    export class HealthService {
      private readonly startTime: number;

      /**
       * Creates a new {@link HealthService} instance.
       *
       * @param rateLimiter - Rate limiter whose token availability is checked as
       *   part of the degraded-state heuristic
       * @param metricsService - Metrics service providing EP API call and error
       *   counters used to infer reachability
       *
       * @example
       * ```typescript
       * const healthService = new HealthService(
       *   createStandardRateLimiter(),
       *   new MetricsService()
       * );
       * ```
       *
       * @since 0.8.0
       */
      constructor(
        private readonly rateLimiter: RateLimiter,
        private readonly metricsService: MetricsService
      ) {
        this.startTime = Date.now();
      }

      /**
       * Produces a health status snapshot.
       *
       * Checks:
       * 1. Rate-limiter token availability (degraded if < 10 %)
       * 2. EP API error counter (unhealthy if recent errors > 0 with no successes)
       *
       * The check is **synchronous-safe** — it never makes network calls.
       * Reachability is inferred from cached metric counters.
       *
       * @returns Structured {@link HealthStatus} object with `status`,
       *   `epApiReachable`, `cache`, `rateLimiter`, `timestamp`, and `uptimeMs`
       * @throws {Error} If the underlying metrics or rate-limiter service throws
       *   unexpectedly (should not occur under normal operating conditions)
       *
       * @example
       * ```typescript
       * const healthService = new HealthService(rateLimiter, metricsService);
       * const status = healthService.checkHealth();
       * if (status.status !== 'healthy') {
       *   console.warn('Server degraded:', status);
       * }
       * ```
       *
       * @since 0.8.0
       */
      checkHealth(): HealthStatus {
        const rateLimiterStatus = this.buildRateLimiterStatus();
        const epApiReachable = this.isEpApiReachable();
        const cacheStatus = this.buildCacheStatus();
        const status = this.deriveOverallStatus(rateLimiterStatus, epApiReachable);

        return {
          status,
          epApiReachable,
          cache: cacheStatus,
          rateLimiter: rateLimiterStatus,
          timestamp: new Date().toISOString(),
          uptimeMs: Date.now() - this.startTime,
        };
      }

      /**
       * Build rate-limiter snapshot by delegating to RateLimiter.getStatus().
       * Cyclomatic complexity: 1
       */
      private buildRateLimiterStatus(): RateLimiterHealthStatus {
        return this.rateLimiter.getStatus();
      }

      /**
       * Determine EP API reachability from recorded metrics.
       * Cyclomatic complexity: 2
       *
       * Returns `null` when no API calls have been recorded yet (unknown state),
       * `false` when all calls failed (error count >= call count > 0),
       * `true` when at least some calls succeeded.
       */
      private isEpApiReachable(): boolean | null {
        const errorCount = this.metricsService.getMetric(MetricName.EP_API_ERROR_COUNT) ?? 0;
        const callCount = this.metricsService.getMetric(MetricName.EP_API_CALL_COUNT) ?? 0;

        if (callCount === 0) {
          return null;
        }

        return errorCount < callCount;
      }

      /**
       * Build a descriptive cache status object.
       * Cyclomatic complexity: 2
       */
      private buildCacheStatus(): CacheHealthStatus {
        const hits = this.metricsService.getMetric(MetricName.EP_CACHE_HIT_COUNT) ?? 0;
        const misses = this.metricsService.getMetric(MetricName.EP_CACHE_MISS_COUNT) ?? 0;
        const total = hits + misses;

        const populated = total > 0;
        const description = populated
          ? `${String(hits)} hits / ${String(misses)} misses (${String(total)} total)`
          : 'No cache activity yet';

        return { populated, description };
      }

      /**
       * Derive the overall health verdict from sub-system checks.
       * Cyclomatic complexity: 3
       */
      private deriveOverallStatus(
        rateLimiter: RateLimiterHealthStatus,
        epApiReachable: boolean | null
      ): HealthStatusLevel {
        if (epApiReachable === false) {
          return 'unhealthy';
        }

        const rateLimiterLow =
          rateLimiter.maxTokens > 0 &&
          rateLimiter.availableTokens / rateLimiter.maxTokens < 0.1;

        return rateLimiterLow ? 'degraded' : 'healthy';
      }
    }