For this question, refer to the HipLocal case study.

Include unit tests in their code, and prevent deployments to QA until all tests have a passing status.

Include performance tests in their code, and prevent deployments to QA until all tests have a passing status.

Create health checks for the QA environment, and redeploy the APIs at a later time if the environment is unhealthy.

Redeploy the APIs to App Engine using Traffic Splitting. Do not move QA traffic to the new versions if errors are found.