Your error stems from the AWS ECS service, where Prefect attempts to register a new task definition. This operation can fail if there are too many concurrent attempts to create a new revision of a task definition family. This typically happens when multiple flow runs are initiated simultaneously, leading to simultaneous task definition registrations. ### Why RegisterTaskDefinition is Triggered 1. Task Definition Changes: Prefect registers a new task definition if it detects changes in the task configuration compared to existing definitions. This can include changes in environment variables, resource allocations, or container images. 2. Lack of Predefined Task Definition: If a predefined task definition ARN is not specified, Prefect will attempt to register a new one. 3. Concurrent Executions: When multiple flows are started at the same time, each may attempt to register a task definition concurrently, leading to rate limit issues. ### Mitigation Strategies 1. Use Existing Task Definitions: Specify an existing task definition ARN when configuring your ECS task to avoid unnecessary registrations. This can be done by setting the

task_definition_arn

parameter in the

ECSTask

block. 2. Rate Limiting: Implement rate limiting or staggering of flow run submissions to avoid concurrent task definition registrations. 3. Task Definition Management: Regularly clean up old task definitions and consider using a versioning strategy to manage task definitions effectively. For further details, you might find these GitHub issues informative: - Rate limiting on task registration using ECS Workpools - Documentation/example on how to use predefined Task Definitions If you need to explore more about ECS task configurations, refer to the Prefect ECS documentation.