EC2 Autoscaling Lifecycle Actions intermittently not processing Lifecycle Action Results

This issue you're describing with the EC2 Auto Scaling instances getting stuck in the Pending:Wait state, despite successfully calling aws autoscaling complete-lifecycle-action, is a known issue that has been reported by some customers. In some rare cases, the Auto Scaling service may not properly update the instance state, even after receiving a successful response from the complete-lifecycle-action API call.

Here are a few potential workarounds and recommendations to handle this issue:

1. Retry the complete-lifecycle-action call: Instead of relying on a single call, you can implement a retry mechanism to call complete-lifecycle-action multiple times, with a short delay between each attempt. This can help overcome any potential timing issues and ensure the instance is properly moved out of the Pending:Wait state.

2. Use a longer timeout: If the instance remains in the Pending:Wait state for an extended period, you can consider increasing the timeout value for the lifecycle hook. This will give the Auto Scaling service more time to process the complete-lifecycle-action call before the instance is considered stuck.

3. Monitor and log the instance state: Closely monitor the instance state and log any instances that get stuck in the Pending:Wait state, even after calling complete-lifecycle-action. This can help you identify any patterns or trends that may provide more insight into the root cause of the issue.

4. Consider using a different approach: Instead of relying solely on the complete-lifecycle-action API call, you can explore alternative approaches, such as using the Amazon CloudWatch Events or Amazon CloudWatch Alarms to detect when an instance is in the Pending:Wait state and then take appropriate action.

5. Escalate the issue to AWS Support: If you continue to experience this issue and the above workarounds do not resolve the problem, you may want to consider escalating the issue to AWS Support. They may be able to provide more guidance or identify any known issues or limitations within the Auto Scaling service that could be causing this behavior.

It's important to note that calling complete-lifecycle-action a second time, even if the first call was successful, is generally not recommended, as it could lead to unexpected behavior or errors. Instead, focus on implementing a robust retry mechanism and monitoring the instance state to ensure the instances are properly moved out of the Pending:Wait state.