The aws-cli documentation and command line help text have not been updated yet to include the syntax for subscribing an AWS Lambda function to an SNS topic, but it does work!
Here’s the format:
aws sns subscribe \
--topic-arn arn:aws:sns:REGION:ACCOUNT:SNSTOPIC \
--protocol lambda \
--notification-endpoint arn:aws:lambda:REGION:ACCOUNT:function:LAMBDAFUNCTION
where REGION, ACCOUNT, SNSTOPIC, and LAMBDAFUNCTION are substituted with appropriate values for your account.
For example:
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.
Unfortunately…
A fantastic new and oft-requested AWS feature was released during AWS re:Invent, but has gotten lost in all the hype about AWS Lambda functions being triggered when objects are added to S3 buckets. AWS Lambda is currently in limited Preview mode and you have to request access, but this related feature is already available and ready to use.
I’m talking about automatic S3 bucket notifications to SNS topics and SQS queues when new S3 objects are added.
Unlike AWS Lambda, with S3 bucket notifications you do need to maintain the infrastructure to run your code, but you’re already running EC2 instances for application servers and job processing, so this will fit right in.
To detect and respond to S3 object creation in the past, you needed to either have every process that uploaded to S3 subsequently trigger your back end code in some way, or you needed to poll the S3 bucket to see if new objects had been added. The former adds code complexity and tight coupling dependencies. The latter can be costly in performance and latency, especially as the number of objects in the bucket grows.
With the new S3 bucket notification configuration options, the addition of an object to a bucket can send a message to an SNS topic or to an SQS queue, triggering your code quickly and effortlessly.
Here’s a working example of how to set up and use S3 bucket notification configurations to send messages to SNS on object creation and update.