Checkly` uses AWS EC2 instances managed by Terraform to run browser checks, with the goal of achieving zero downtime deployments. The company's architecture is based on a fan-out/fan-in pattern, where workers subscribe to an SQS queue and process incoming requests in parallel. To achieve zero downtime deployments, `Checkly` uses Terraform's `create_before_destroy` flag and the `remote-exec` provisioner to ensure that new instances are created before old ones are deleted. The company also uses a user-data file to bootstrap the application and start Docker containers on new instances. Terraform's module hierarchy is used to deploy to multiple regions with one command, and the deployment workflow involves building a new container, updating the version in the `module.tf` file, and using the `terraform taint` command to force a create/destroy cycle. The company also uses monitoring tools like AppOptics to detect failures and alert the team when something goes wrong.