In the previous chapters, we learned about various essential components for building a standard serverless web application such as functions, Rest API, datastore, user management, hosting, and domain registration. As discussed at the end of the previous chapter, a real-world application may not always be a web application, or may be extended with additional capabilities.

Until now, we were invoking Lambda from the API gateway. However, Lambdas may also be invoked reactively as a result of some triggers based on states of other services. In this chapter, we will first discuss adding messaging and notification support to serverless applications. Later, we will see how to trigger Lambdas based on state changes in various other services.

The following are the prerequisites for this chapter:

• A working AWS account
• Configuring AWS CLI as discussed in the Your first Lambda with AWS CLI recipe of Chapter 1, Getting Started with Serverless Computing on AWS