/**
 * Singleton /health poller — one schedule per endpoint per browser profile.
 *
 * Best practices:
 * - Cross-tab leader via Web Locks (fallback: in-process mutex only)
 * - Pause automatic polls while document is hidden
 * - 429 = rate-limited (keep last known DB state; backoff via Retry-After)
 * - Real outages (5xx / unhealthy JSON / network) use exponential backoff
 * - No overlapping in-flight requests
 */
export interface HealthCoordinatorSnapshot {
    readonly isDatabaseAvailable: boolean;
    readonly isChecking: boolean;
    readonly hasResolvedInitialCheck: boolean;
    readonly retryCount: number;
    readonly nextRetryDelay: number;
    readonly error: Error | null;
}
export interface SubscribeDatabaseHealthInput {
    readonly endpoint: string;
    readonly enabled?: boolean;
    readonly pollInterval?: number;
    readonly maxRetries?: number;
}
export declare function subscribeDatabaseHealth(input: SubscribeDatabaseHealthInput, listener: (snapshot: HealthCoordinatorSnapshot) => void): () => void;
export declare function requestDatabaseHealthCheck(endpoint: string, options?: {
    readonly force?: boolean;
}): Promise<void>;
export declare function resolveDatabaseHealthEndpoint(): string;
/** Test-only reset */
export declare function resetDatabaseHealthCoordinatorForTests(): void;
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