API Monitoring

API Monitoring is the continuous process of tracking the availability, performance, and correctness of your API endpoints in real-time or at scheduled intervals.

Goals of API Monitoring:

1. Ensure uptime — the API is available for consumers.
2. Track performance metrics — response time, latency, throughput.
3. Detect errors and failures — server errors, authentication failures, invalid responses.
4. Enable alerts and notifications for operational issues.
5. Provide insights for scaling — detect load patterns, bottlenecks.

Key Metrics in API Monitoring

Metric	Description
Uptime / Availability	Percentage of time API is operational (e.g., 99.9%)
Response Time / Latency	Time taken to process a request
Error Rate	Percentage of failed requests (HTTP 4xx, 5xx)
Throughput / Requests per Second	How many requests API handles per time unit
SLA Compliance	Whether API meets Service Level Agreements
Resource Usage	CPU, memory, database connections

What Is API Logging

API Logging involves capturing detailed information about API requests, responses, and system events to facilitate debugging, auditing, and monitoring.

Logs Typically Include:

• Timestamp of request
• HTTP method & URL
• Headers (Authorization, Content-Type)
• Request body / parameters
• Response status and body
• Execution time
• Errors or exceptions
• User or service identity (optional)

Logging Levels

Level	Purpose
INFO	General API usage (request received, successful response)
DEBUG	Detailed internal info for troubleshooting (DB queries, variables)
WARN	Non-critical issues (deprecated endpoints, slow responses)
ERROR	Failures or exceptions (server errors, crashes)
FATAL	Critical failures causing downtime

Logger Implementation

Logger Middleware

// logger.js
const fs = require('fs');
const path = require('path');

const logFile = path.join(__dirname, 'api.log');

function logger(req, res, next) {
  const start = Date.now();
  const { method, url, body } = req;

  res.on('finish', () => {
    const duration = Date.now() - start;
    const logEntry = {
      timestamp: new Date().toISOString(),
      method,
      url,
      status: res.statusCode,
      duration: `${duration}ms`,
      requestBody: body,
    };
    fs.appendFileSync(logFile, JSON.stringify(logEntry) + '\n');
  });

  next();
}

module.exports = logger;

Using Logger in App

const express = require('express');
const logger = require('./logger');
const app = express();

app.use(express.json());
app.use(logger);

app.post('/api/users', (req, res) => {
  if (!req.body.name || !req.body.email) {
    return res.status(400).json({ error: "Name and email required" });
  }
  res.status(201).json({ id: 1, ...req.body });
});

app.listen(3000, () => console.log("API running on port 3000"));

Integrating Monitoring Tools

Logging data can be sent to monitoring tools like:

• ELK Stack (Elasticsearch, Logstash, Kibana)
• Prometheus + Grafana (metrics collection)
• Datadog, New Relic, Splunk (full observability)

Example Metrics Extraction

• Count all 500 status codes → triggers alert
• Measure average response time → visualize slow endpoints
• Track POST /api/users errors → SLA compliance