I’m a big fan of following the Single Responsibility Principle when creating Lambda functions in my serverless applications. The idea of each function doing “one thing well” allows you to easily separate discrete pieces of business logic into reusable components. In addition, the Lambda concurrency model, along with the ability to add fine-grained IAM permissions per function, gives you a tremendous amount of control over the security, scalability, and cost of each part of your application.
However, there are several drawbacks with this approach that often attract criticism. These include things like increased complexity, higher likelihood of cold starts, separation of log files, and the inability to easily compose functions. I think there is merit to these criticisms, but I have personally found the benefits to far outweigh any of the negatives. A little bit of googling should help you find ways to mitigate many of these concerns, but I want to focus on the one that seems to trip most people up: function composition.