Guide to AWS ELB Connection Draining

ELB Connection Draining is a feature provided by Amazon Web Services (AWS) as a part of Elastic Load Balancing (ELB). Its purpose is to ensure smooth execution of services hosted on AWS when an instance is de-registered or becomes unhealthy. Here is a comprehensive guide on ELB connection draining.

What is ELB Connection Draining?
When enabled, the ELB Connection Draining feature prevents new connections from being sent to the instance which is being de-registered or has become unhealthy while ensuring that already existing connections are served to their completion. This is achieved by redirecting the traffic to other healthy instances.

Importance of ELB Connection Draining
Without connection draining, in-progress connections could be abruptly terminated leading to a degraded user experience. By allowing in-flight requests to complete, it helps ensure that your application remains available to users during administrative activities or in the event of a failure.

How ELB Connection Draining Works?
When an instance is being de-registered, ELB waits for on-going connections to complete for a specified period, known as the draining timeout. If a timeout is not specified, the default value is 300 seconds. If connections do not complete within this period, they are forcibly closed.

Exam Tips: Answering Questions on ELB Connection Draining
- Always remember that ELB Connection Draining is used to redirect traffic from unhealthy instances.
- Understand that by allowing requests to complete, it ensures application availability.
- Know the default timeouts and how to modify them.
- Understand what happens if connections don't complete within the timeout period.
- Be aware that connection draining can be disabled if required by your use case.