Two new features were announced recently to help performance issues. The first is provisioned concurrency, which allows you to set a number of lambda instances that are kept warm. This helps alleviated some of the issues with cold-start penalties and helps with long-tail latency. Secondly, HTTP API's were announced for API Gateway - this mode strips back some of the features of the existing API Gateway service and offers better performance for web frameworks like flask and express.
In most circumstances it's a cost optimization. Re-homing from EC2 or a container will likely result in less management and lower cost - though it does depend on the traffic to the application in question. For example, if you experience spiky traffic load, and/or demand that follows a certain pattern (e.g. significantly fewer users at certain periods), lambda's scale-to-zero model results in lower costs over other alternatives.
There are more advanced ways to architect an application use API Gateway, with things like VTL transformations/service integrations etc, and these offer better cost and performance, but asks for greater investment from a development team to learn and understand. You can often get a quicker return on your investment sticking with existing frameworks that your team is already familiar with.
As such, using light frameworks such as express and flask are viable ways to serve content from lambda and is a good way to start getting to grips with serverless without needing to throw out all your existing knowledge. It's a good example of AWS meeting people where they are.
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