Execute Lambda Functions on S3 Event Triggers

Several events are being fired in S3 on a regular basis. They can be anything from new files that are uploaded to buckets to files being moved around, deleted, among others. All these activities fire events of various types in S3 on a real-time basis.

Setting up the Lambda S3 Role

Lambda needs to have permission to access your S3 bucket upon getting executed. Besides, it is optional to CloudWatch if you intend to log Lambda activity. Before starting to build your Lambda function, it is essential for you to create an IAM role which Lambda will use to work with S3 and to write logs to CloudWatch. When you set up this role, you must adhere to appropriate S3 and CloudWatch policies. We have shown an example below. As you can see, the policy grants the role of access to my CloudWatch logs and gives full authority to S3.
"Version": "2012-10-17",
"Statement": [
{
"Effect": "Allow",
"Action": [
"logs:*"
],
"Resource": "arn:aws:logs:*:*:*"
},
{
"Effect": "Allow",
"Action": [
"s3:GetObject",
"s3:PutObject"
],
"Resource": "arn:aws:s3:::*"
}
],
"Statement": [
{
"Effect": "Allow",
"Action": "s3:*",
"Resource": "*"
}
]
}

You will need to create this role and give a name to it.  It could be either LambdaS3 or whatever you’d like.

Creating the Lambda Function – Adding Code

After setting up the role, the next step is to create the function. For this, you'll have to click Create Function after browsing to Lambda. After clicking, a screen will come up showing that you are using the role you just created. 

Also Read: AWS Identity and Access Management and Features of IAM User

Creating the Lambda Function – Adding the Trigger

Once you have the code to be used inside the function, the next step is to create the S3 trigger by selecting it on the left-hand side. As my will is to trigger off of new uploads, the event I create will trigger off of the PUT event to do it.

I have the option to choose a prefix or suffix if I decide to narrow down the filter criteria. However, for this example, I want to execute this Lambda function on every file that’s uploaded into my adbtestbucket bucket.

Also Read: How To Automate AWS Cloud Using Terraform

Here's what your lambda function will look like after getting created.

Summary

The Lambda function we created here consists of a role with appropriate rights to both the S3 service and CloudWatch. It also has the code to execute and finally the event trigger. Once you combine all these objects, you have an immense potential to perform even the most complicated of automation tasks in AWS.

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