A DevOps team deploys an ECS application behind an ALB using CodeDeploy with blue/green all-at-once. A recent release caused a large increase in response times and required a rollback. The team wants a deployment approach that lets them observe a new version before shifting all traffic and to roll back quickly if response times rise. Which combination of steps meets this?

The problem describes a need to observe a new version before shifting all traffic and to roll back quickly if response times rise. Changing the CodeDeploy deployment configuration to CodeDeployDefault.ECSCanary10Percent5Minutes directly addresses the need to observe a new version before shifting all traffic. A canary deployment shifts a small percentage of traffic (10% in this case) to the new version for a short period (5 minutes) before shifting the rest, allowing for observation. Creating a CloudWatch alarm on the ALB TargetResponseTime metric that triggers when response time exceeds a threshold, and associating it with the CodeDeploy deployment group to roll back, directly addresses the need to roll back quickly if response times rise. This provides an automated mechanism to detect performance degradation and initiate a rollback. CodeDeployDefault.ECSLinear10PercentEvery3Minutes is a linear deployment, not a canary, and doesn't provide the initial observation period as effectively. UnHealthyHostCount and TargetConnectionErrorCount are important metrics, but TargetResponseTime is the most direct indicator of the specific problem described (increased response times).