A company publishes new AWS Lambda function versions regularly and uses aliases for production. They want to roll out a new version to a fixed percentage of production traffic initially (traffic shifting) to improve releases. Which configuration achieves this behavior?

Alias routing with a weighted alias is the correct solution because it directly supports traffic shifting for Lambda functions. You can configure an alias to point to two different Lambda function versions, assigning a specific percentage of invocations to each. This allows you to gradually shift traffic to a new version, enabling canary deployments or A/B testing. Canary and linear deployment types are features of AWS CodeDeploy, which can be used to deploy Lambda functions, but the question specifically asks about Lambda's native capabilities for traffic shifting with aliases. Environment variables are for configuration within a function and cannot control traffic routing between different versions.