Elastic Load Balancing (ELB) health checks are crucial for ensuring that traffic is only routed to healthy instances, and they typically rely on HTTP status codes to determine health.
The default health check path for an Application Load Balancer (ALB) is often set to "/".
If your application endpoint is different, this could lead to failing health checks.
Health check success codes can vary; while 200 OK is common, you may need to adjust this to match your application’s expected responses, such as 302 for redirects.
The Initial Health Check period for targets in an ELB is usually 30 seconds; during this time, the ELB will not route traffic to the target until it passes the health checks.
If a target fails health checks, the ELB will deregister it after a specified number of retries, which can lead to downtime if not monitored properly.
Reason codes returned by ELB health checks provide insight into failure causes; codes starting with "ELB" indicate issues on the load balancer side, while those with "Target" indicate problems with the target instance itself.
Security group settings can impact health checks; if your ECS task's security group does not allow inbound traffic from the ELB, health checks will fail.
AWS ECS has a feature called "Health Check Grace Period," which allows you to set a time where the scheduler ignores health check failures, giving your application enough time to start up.
Misconfigured target groups can lead to health check failures; ensure that the health check path and success codes match those of your application.
If your container cannot reach the internet or external services required for its health check, it may fail, even if the application is functioning correctly.
The health check interval is configurable; by default, it is set to 30 seconds, but adjusting this can help with applications that take longer to start.
Multiple Availability Zones (AZs) can affect ELB health checks; if a task starts in an AZ that the load balancer isn't configured to use, it will not pass health checks.
AWS provides a monitoring tool called CloudWatch that can track the health check status of your ELB, allowing for proactive troubleshooting.
The ALB supports both HTTP and HTTPS health checks; ensure that if you're using HTTPS, your SSL certificates are valid to prevent health check failures.
Container orchestration tools like ECS may require specific configurations to interact properly with ELBs, including ensuring the task is in the target group and configured correctly.
Using AWS Fargate for ECS tasks requires careful attention to networking configurations, as improperly configured VPC settings can lead to health check failures.
When deploying updates, new tasks may initially fail health checks until the application is fully running, which is where the Health Check Grace Period becomes critical.
The ELB can perform TCP health checks as well, which may be useful for applications that do not respond to HTTP requests or are not web-based services.
Continuous health check failures can lead to auto-scaling actions being triggered, which may inadvertently scale down your application if not monitored.
Debugging health check failures often involves looking into application logs to ensure that the application is responding correctly to the health check requests and that there are no underlying issues affecting performance.