How to Route New Features to AWS Lambda Without Touching Legacy Code

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Developers maintaining legacy applications often face a dilemma when new requirements arrive: modify risky old code or slow down rewrite teams. A third approach involves using an AWS Application Load Balancer (ALB) to intercept specific requests and route them to a Lambda function, leaving the existing EC2 application completely untouched. The ALB evaluates listener rules in priority order, directing traffic matching a defined URL path to a Lambda target group while all other requests flow to the legacy instance target group. Since the routing decision happens at the load balancer level, neither the Lambda function nor the EC2 application is aware of the other. This method allows teams to deliver new functionality without any code changes or deployment risk to the legacy system.

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