AWS Lambda function in Go to resize an image uploaded to S3. Uses https://github.com/apex/apex

main.go

package main

import (
	"bytes"
	"image/jpeg"
	"log"
	"path"
	"strings"

	apex "github.com/apex/go-apex"
	apexS3 "github.com/apex/go-apex/s3"
	"github.com/aws/aws-sdk-go/aws"
	"github.com/aws/aws-sdk-go/aws/session"
	"github.com/aws/aws-sdk-go/service/s3"
	"github.com/aws/aws-sdk-go/service/s3/s3manager"
	"github.com/nfnt/resize"
)

func main() {
	apexS3.HandleFunc(func(event *apexS3.Event, ctx *apex.Context) error {
		for _, record := range event.Records {

			sess := session.New(&aws.Config{Region: aws.String(record.AWSRegion),
				DisableRestProtocolURICleaning: aws.Bool(true)})

			bucket := record.S3.Bucket.Name
			key := record.S3.Object.Key

			log.Printf("Bucket: %s", bucket)
			log.Printf("Key: %s", key)

			if strings.HasSuffix(key, "thumb.jpg") {
				continue
			}

			log.Printf("Fetching s3://%v/%v", bucket, key)

			buff := &aws.WriteAtBuffer{}
			s3dl := s3manager.NewDownloader(sess)
			_, err := s3dl.Download(buff, &s3.GetObjectInput{
				Bucket: aws.String(bucket),
				Key:    aws.String(key),
			})

			if err != nil {
				log.Printf("Could not download from S3: %v", err)
			}

			log.Printf("Decoding image: %v bytes", len(buff.Bytes()))

			imageBytes := buff.Bytes()
			reader := bytes.NewReader(imageBytes)

			img, err := jpeg.Decode(reader)
			if err != nil {
				log.Printf("bad response: %s", err)
			}

			log.Printf("Generating thumbnail")
			thumbnail := resize.Thumbnail(150, 150, img, resize.Lanczos3)

			log.Printf("Encoding image for upload to S3")
			buf := new(bytes.Buffer)
			err = jpeg.Encode(buf, thumbnail, nil)

			if err != nil {
				log.Printf("JPEG encoding error: %v", err)
			}

			// Set filename "filename-thumb.jpg"
			ext := path.Ext(key)
			thumbkey := key[0:len(key)-len(ext)] + "-thumb.jpg"

			log.Printf("Preparing S3 object: %s", thumbkey)

			uploader := s3manager.NewUploader(sess)
			result, err := uploader.Upload(&s3manager.UploadInput{
				Body:   bytes.NewReader(buf.Bytes()),
				Bucket: aws.String(bucket),
				Key:    aws.String(thumbkey),
			})

			if err != nil {
				log.Printf("Failed to upload: %v", err)
			}

			log.Printf("Successfully uploaded to: %v", result.Location)

		}
		return nil
	})
}

It's very cool, thanks, man.