Replit Observability

Overview

Monitor Replit deployment health, track cold starts, measure resource usage, and set up alerting. Covers Replit's built-in monitoring, external health checking, structured logging, and integration with monitoring services.

Prerequisites

• Replit app deployed (Autoscale or Reserved VM)
• Health endpoint implemented (

  /health

  )
• External monitoring service (UptimeRobot, Better Stack, or Prometheus)

Instructions

Step 1: Health Endpoint with Detailed Metrics

// src/routes/health.ts — comprehensive health check
import { Router } from 'express';
import { pool } from '../services/postgres';

const router = Router();
const startTime = Date.now();

router.get('/health', async (req, res) => {
  const checks: Record<string, any> = {
    status: 'ok',
    uptime: process.uptime(),
    bootTime: ((Date.now() - startTime) / 1000).toFixed(1) + 's ago',
    timestamp: new Date().toISOString(),
    repl: process.env.REPL_SLUG,
    region: process.env.REPLIT_DEPLOYMENT_REGION,
    env: process.env.NODE_ENV,
  };

  // Database check
  if (process.env.DATABASE_URL) {
    const dbStart = Date.now();
    try {
      await pool.query('SELECT 1');
      checks.database = {
        status: 'connected',
        latencyMs: Date.now() - dbStart,
        pool: { total: pool.totalCount, idle: pool.idleCount },
      };
    } catch (err: any) {
      checks.database = { status: 'disconnected', error: err.message };
      checks.status = 'degraded';
    }
  }

  // Memory metrics
  const mem = process.memoryUsage();
  checks.memory = {
    heapMB: Math.round(mem.heapUsed / 1024 / 1024),
    totalMB: Math.round(mem.heapTotal / 1024 / 1024),
    rssMB: Math.round(mem.rss / 1024 / 1024),
    percent: ((mem.heapUsed / mem.heapTotal) * 100).toFixed(1),
  };

  // Node.js info
  checks.runtime = {
    node: process.version,
    platform: process.platform,
    pid: process.pid,
  };

  res.status(checks.status === 'ok' ? 200 : 503).json(checks);
});

// Lightweight ping for uptime monitors
router.get('/ping', (req, res) => res.send('pong'));

export default router;

Step 2: Structured Logging

// src/utils/logger.ts — structured JSON logging
const IS_PROD = process.env.NODE_ENV === 'production';

type LogLevel = 'debug' | 'info' | 'warn' | 'error';

function log(level: LogLevel, message: string, data?: Record<string, any>) {
  if (level === 'debug' && IS_PROD) return;

  const entry = {
    timestamp: new Date().toISOString(),
    level,
    message,
    repl: process.env.REPL_SLUG,
    ...data,
  };

  // JSON format for machine parsing, human-readable in dev
  if (IS_PROD) {
    console[level === 'error' ? 'error' : 'log'](JSON.stringify(entry));
  } else {
    console[level === 'error' ? 'error' : 'log'](
      `[${level.toUpperCase()}] ${message}`,
      data || ''
    );
  }
}

export const logger = {
  debug: (msg: string, data?: any) => log('debug', msg, data),
  info: (msg: string, data?: any) => log('info', msg, data),
  warn: (msg: string, data?: any) => log('warn', msg, data),
  error: (msg: string, data?: any) => log('error', msg, data),
};

// Request logging middleware
export function requestLogger(req: any, res: any, next: any) {
  const start = Date.now();
  res.on('finish', () => {
    logger.info('request', {
      method: req.method,
      path: req.path,
      status: res.statusCode,
      durationMs: Date.now() - start,
      userId: req.headers['x-replit-user-id'] || 'anonymous',
    });
  });
  next();
}

Step 3: External Uptime Monitoring

Set up external monitors to detect Autoscale cold starts and outages:

UptimeRobot (free tier: 50 monitors):
1. Create new monitor: HTTP(s)
2. URL: https://your-app.replit.app/ping
3. Interval: 5 minutes
4. Alert contacts: email, Slack webhook

Better Stack / Datadog / Grafana Cloud:
- Same setup, more features
- Track response time trends
- Detect cold start patterns
- Set up PagerDuty integration

Key metrics to monitor externally:
- Uptime percentage (target: 99.9%)
- Response time P95 (target: < 2s)
- Cold start frequency (Autoscale only)
- SSL certificate expiry

Step 4: Cold Start Detection

// Track cold starts for Autoscale deployments
const COLD_START_THRESHOLD_MS = 5000;
let firstRequestTime: number | null = null;

app.use((req, res, next) => {
  if (!firstRequestTime) {
    firstRequestTime = Date.now();
    const bootTime = process.uptime();
    if (bootTime < 30) { // Just started
      logger.info('cold_start_detected', {
        bootTimeMs: Math.round(bootTime * 1000),
        path: req.path,
      });
    }
  }
  next();
});

Step 5: Alerting Rules

// src/utils/alerts.ts — send alerts to Slack on issues
async function alertSlack(message: string, severity: 'info' | 'warning' | 'critical') {
  const webhookUrl = process.env.SLACK_WEBHOOK_URL;
  if (!webhookUrl) return;

  const emoji = { info: 'information_source', warning: 'warning', critical: 'rotating_light' };
  await fetch(webhookUrl, {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({
      text: `:${emoji[severity]}: [${severity.toUpperCase()}] ${process.env.REPL_SLUG}\n${message}`,
    }),
  });
}

// Monitor memory usage
setInterval(async () => {
  const mem = process.memoryUsage();
  const heapPercent = (mem.heapUsed / mem.heapTotal) * 100;

  if (heapPercent > 90) {
    await alertSlack(`Memory critical: ${heapPercent.toFixed(1)}% heap used`, 'critical');
  } else if (heapPercent > 75) {
    await alertSlack(`Memory warning: ${heapPercent.toFixed(1)}% heap used`, 'warning');
  }
}, 60000);

// Monitor error rate
let errorCount = 0;
let requestCount = 0;

app.use((req, res, next) => {
  requestCount++;
  res.on('finish', () => {
    if (res.statusCode >= 500) errorCount++;
  });
  next();
});

setInterval(async () => {
  if (requestCount > 0) {
    const errorRate = (errorCount / requestCount) * 100;
    if (errorRate > 5) {
      await alertSlack(`Error rate: ${errorRate.toFixed(1)}% (${errorCount}/${requestCount})`, 'critical');
    }
  }
  errorCount = 0;
  requestCount = 0;
}, 300000); // Check every 5 minutes

Step 6: Replit Dashboard Monitoring

Built-in monitoring in Replit:
1. Deployment Settings > Logs: real-time stdout/stderr
2. Deployment Settings > History: deploy timeline + rollbacks
3. Database pane > Settings: storage usage + connection info
4. Billing > Usage: compute, egress, and storage costs

Check deployment logs:
- Click on active deployment
- View real-time log stream
- Filter by error/warning
- Logs persist across container restarts

Error Handling

Issue	Cause	Solution
Cold starts undetected	No external monitor	Set up UptimeRobot or similar
Deployment logs missing	Container restarted	Use external log aggregator
Memory leak unnoticed	No memory monitoring	Add heap tracking + alerts
DB pool exhaustion	Too many connections	Monitor pool.totalCount in health

Resources

• Monitoring Deployments
• Replit Status Page
• UptimeRobot

Next Steps

For incident response, see

replit-incident-runbook

.