Today, Amazon announced that AWS Lambda functions can be subscribed to Amazon SNS topics.
This means that any message posted to an SNS topic can trigger the execution of custom code you have written, but you don’t have to maintain any infrastructure to keep that code available to listen for those events and you don’t have to pay for any infrastructure when the code is not being run.
This is, in my opinion, the first time that Amazon can truly say that AWS Lambda is event-driven, as we now have a central, independent, event management system (SNS) where any authorized entity can trigger the event (post a message to a topic) and any authorized AWS Lambda function can listen for the event, and neither has to know about the other.
Making this instantly useful is the fact that there already are a number of AWS services and events that can post messages to Amazon SNS. This means there are a lot of application ideas that are ready to be implemented with nothing but a few commands to set up the SNS topic, and some snippets of nodejs code to upload as an AWS Lambda function.