AWS Load Balancer Error Codes and Monitoring

Load Balancer Error Codes

• 2XX Success Codes
  • 200: Successful request.
• 4XX Client Error Codes
  • 400: Bad Request.
  • 401: Unauthorized.
  • 403: Forbidden.
  • 460: Client Closed Connection.
  • 463: X-Forwarded-For Header Malformed.
• 5XX Server Error Codes
  • 500: Internal Server Error.
  • 502: Bad Gateway.
  • 503: Service Unavailable (EC2 instances not responding).
  • 504: Gateway Timeout.
  • 561: Unauthorized.

Key Points for Exam

• 4XX: Indicates client-side problems.
• 5XX: Indicates server-side problems.

Load Balancer Metrics in CloudWatch

• Backend Connection Errors: Monitor errors from EC2 instances.
• HealthyHostCount: Number of healthy instances.
• UnHealthyHostCount: Number of unhealthy instances.
• 2XX/3XX/4XX/5XX Counts: Indicate success, redirection, client errors, and server errors, respectively.
• Latency: Time taken to respond to clients.
• RequestCounts: Total requests for the ALB.
• RequestCountPerTarget: Average requests received by each EC2 instance.
• SurgeQueueLength: Pending requests routed to healthy instances (max value 1000).
• SpilloverCount: Requests rejected due to full queue.

Troubleshooting Using Metrics

• 400 Bad_Request: Malformed request from the client.