Reading files from AWS S3 in Golang - go

Question

I am trying to deploy a golang code on Heroku. My code needs a text file as input and I need to fetch this text file from S3 bucket. My go-code takes the filename as input, Can someone provide a code snippet for reading a file from S3 and storing its contents into a file?
My GOlang code-
func getDomains(path string) (lines []string, Error error) {
file, err := os.Open(path)
if err != nil {
log.Fatalln(err)
}
defer file.Close()
scanner := bufio.NewScanner(file)
for scanner.Scan() {
lines = append(lines, scanner.Text())
}
return lines, scanner.Err()
}
func Process(w http.ResponseWriter, r *http.Request) {
urls := make(chan *Http, Threads*10)
list, err := getDomains("**NEED A TEXT FILE FROM S3 HERE as an argument**")
if err != nil {
log.Fatalln(err)
}
var wg sync.WaitGroup
for i := 0; i < Threads; i++ {
wg.Add(1)
go func() {
for url := range urls {
url.DNS()
}
wg.Done()
}()
}
for i := 0; i < len(list); i++ {
Progress := fmt.Sprintln(w, len(list))
urls <- &Http{Url: list[i], Num: Progress}
}
close(urls)
wg.Wait()
fmt.Printf("\r%s", strings.Repeat(" ", 100))
fmt.Fprintln(w, "\rTask completed.\n")
}
Can someone suggest a good library for reading the file from S3 into a text file? I cannot download the file from S3 because I have to deploy it on Heroku.
A code snippet for example will be highly appreciated!

Answer 1

The code snippet below should work (given that you have installed the proper dependencies):
package main
import (
"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"
"fmt"
"log"
"os"
)
func main() {
// NOTE: you need to store your AWS credentials in ~/.aws/credentials
// 1) Define your bucket and item names
bucket := "<YOUR_BUCKET_NAME>"
item := "<YOUR_ITEM_NAME>"
// 2) Create an AWS session
sess, _ := session.NewSession(&aws.Config{
Region: aws.String("us-west-2")},
)
// 3) Create a new AWS S3 downloader
downloader := s3manager.NewDownloader(sess)
// 4) Download the item from the bucket. If an error occurs, log it and exit. Otherwise, notify the user that the download succeeded.
file, err := os.Create(item)
numBytes, err := downloader.Download(file,
&s3.GetObjectInput{
Bucket: aws.String(bucket),
Key: aws.String(item),
})
if err != nil {
log.Fatalf("Unable to download item %q, %v", item, err)
}
fmt.Println("Downloaded", file.Name(), numBytes, "bytes")
}
For more details you can check the AWS Go SDK and the Github Example

Answer 2

Using current stable AWS lib for go:
sess := session.Must(session.NewSession(&aws.Config{
....
}))
svc := s3.New(sess)
rawObject, err := svc.GetObject(
&s3.GetObjectInput{
Bucket: aws.String("toto"),
Key: aws.String("toto.txt"),
})
buf := new(bytes.Buffer)
buf.ReadFrom(rawObject.Body)
myFileContentAsString := buf.String()

Answer 3

Here is a function for getting an object using V2 of the SDK (adapted from examples in https://github.com/aws/aws-sdk-go-v2):
Note: No Error handling - demo code only.
package s3demo
import (
"os"
"context"
"fmt"
"io/ioutil"
"github.com/aws/aws-sdk-go-v2/aws"
"github.com/aws/aws-sdk-go-v2/aws/awserr"
"github.com/aws/aws-sdk-go-v2/aws/external"
"github.com/aws/aws-sdk-go-v2/service/s3"
)
func GetObjectWithV2SDKDemo() {
bucket := "YOUR_BUCKET"
key := "YOUR_OBJECT_KEY"
fileName := "YOUR_FILE_PATH"
// may need AWS_PROFILE and AWS_REGION populated as environment variables
cfg, err := external.LoadDefaultAWSConfig()
if err != nil {
panic("failed to load config, " + err.Error())
}
svc := s3.New(cfg)
ctx := context.Background()
req := svc.GetObjectRequest(&s3.GetObjectInput{
Bucket: aws.String(bucket),
Key: aws.String(key),
})
resp, err := req.Send(ctx)
if err != nil {
panic(err)
}
s3objectBytes, err := ioutil.ReadAll(resp.Body)
if err != nil {
panic(err)
}
// create file
f, err := os.Create(fileName)
defer f.Close()
if err != nil {
panic(err)
}
bytesWritten, err := f.Write(s3objectBytes)
if err != nil {
panic(err)
}
fmt.Printf("Fetched %d bytes for S3Object\n", bytesWritten)
fmt.Printf("successfully downloaded data from %s/%s\n to file %s", bucket, key, fileName)
}