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Getting to know the S3 Event Object

In this post i am going to show you how you can get into the content of the file that trigged your lambda.

As we have seen in the previous post, the event object is passed as an argument to our lambda function, like this: 

exports.handler = (event, context, callback) => {

};

When you place a trigger on a Lambda function based on a S3 event, S3 will pass all the context information that you need inside of the event object. In this case, our lambda will be trigged by an S3 event and the event object that will be passed to it will have a Record property with an array of objects inside of it like is shown below: 

{
   "event":{
      "Records":[
         {
            "eventVersion":"2.0",
            "eventSource":"aws:s3",
            "awsRegion":"us-east-1",
            "eventTime":"2018-09-22T14:25:20.411Z",
            "eventName":"ObjectCreated:Put",
            "userIdentity":{
               "principalId":""
            },
            "requestParameters":{
               "sourceIPAddress":""
            },
            "responseElements":{
               "x-amz-request-id":"",
               "x-amz-id-2":""
            },
            "s3":{
               "s3SchemaVersion":"1.0",
               "configurationId":"",
               "bucket":{
                  "name":"",
                  "ownerIdentity":{
                     "principalId":""
                  },
                  "arn":""
               },
               "object":{
                  "key":"",
                  "size":226,
                  "eTag":"",
                  "versionId":"",
                  "sequencer":""
               }
            }
         }
      ]
   }
}

You will want to pay a special attention to the S3 property here. This property contains an object that have two important properties for us, the bucket and the key. Using those information, that is, the bucket name and the file name, that we can get into the content of our S3 file like this: 

var bucket = event.Records[0].s3.bucket.name;
var file = event.Records[0].s3.object.key;

var params = {
  Bucket: bucket, 
  Key: file
 };
 s3.getObject(params, function(err, data) {
   if (err) console.log(err, err.stack);
   else     console.log(JSON.parse(data.Body.toString('utf-8'))); 
 });

This will return a data object containing your object properties, assuming you hava a json object on it. The JSON.parse() here is used to convert it back to an object, instead of having it as string.

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