@accounter/server/dist/server/src/observability/pool-monitor.js

Accounter GraphQL server

import { monitorEventLoopDelay } from 'node:perf_hooks';
import { getTenantDbClientStats } from '../modules/app-providers/tenant-db-client.js';
const NS_PER_MS = 1e6;
/**
 * Periodic health heartbeat for the Postgres pool.
 *
 * The failure this exists for is silent by construction: an exhausted pool
 * produces no error, no CPU, and no log line — requests simply queue in
 * `pool.connect()` forever. `waiting > 0` alongside `idle === 0` is that state,
 * visible in a single line, and the heartbeat continuing to print at all is
 * proof the event loop is still turning.
 */
export function startPoolMonitor({ pool, intervalMs, max }) {
    const loopDelay = monitorEventLoopDelay({ resolution: 20 });
    loopDelay.enable();
    const timer = setInterval(() => {
        const clients = getTenantDbClientStats();
        const saturated = pool.waitingCount > 0 || pool.totalCount - pool.idleCount >= max;
        const snapshot = {
            msg: 'db-pool-heartbeat',
            pool: {
                total: pool.totalCount,
                idle: pool.idleCount,
                waiting: pool.waitingCount,
                max,
            },
            sessions: {
                holdingConnection: clients.holdingConnection,
                maxIdleMs: clients.maxIdleMs,
            },
            eventLoopDelayP99Ms: Math.round(loopDelay.percentile(99) / NS_PER_MS),
        };
        // A saturated pool is one step from wedging every request — say so loudly.
        if (saturated) {
            console.error(JSON.stringify({ ...snapshot, level: 'error', saturated: true }));
        }
        else {
            console.log(JSON.stringify(snapshot));
        }
        loopDelay.reset();
    }, intervalMs);
    // The heartbeat must never be the reason the process stays alive.
    timer.unref();
    return {
        stop: () => {
            clearInterval(timer);
            loopDelay.disable();
        },
    };
}
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