GoでAmazon S3をKVSとして使う

これはなに

永続化が必要なバイナリKVSとしてAmazon S3を使いたい場合があります。
セッションの取り回しや、aws.WriteAtBufferやエラーがトリッキーなのでスニペットです。

2020-12追記:
S3に強い一貫性がサポートされましたね！

package s3store

import (
        "bytes"

        "github.com/aws/aws-sdk-go/aws"
        "github.com/aws/aws-sdk-go/aws/awserr"
        "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"
)

type S3Store struct {
        Bucket     string
        Uploader   *s3manager.Uploader
        Downloader *s3manager.Downloader
}

func NewS3Store(bucket, region string) (*S3Store, error) {
        s := new(S3Store)
        s.Bucket = bucket

        sess, err := session.NewSession(&aws.Config{
                Region: aws.String(region),
        })
        if err != nil {
                return nil, err
        }

        s.Uploader = s3manager.NewUploader(sess)
        s.Downloader = s3manager.NewDownloader(sess)

        return s, nil
}

func (s *S3Store) Set(key string, body []byte) (err error) {
        params := &s3manager.UploadInput{
                Bucket: aws.String(s.Bucket),
                Key:    aws.String(key),
                Body:   bytes.NewReader(body),
        }

        _, err = s.Uploader.Upload(params)
        return
}

func (s *S3Store) Get(key string) ([]byte, error) {
        buffer := aws.NewWriteAtBuffer([]byte{})

        _, err := s.Downloader.Download(buffer, &s3.GetObjectInput{
                Bucket: aws.String(s.Bucket),
                Key:    aws.String(key),
        })
        if err != nil {
                if aerr, ok := err.(awserr.Error); ok && aerr.Code() == "NoSuchKey" {
                        return nil, nil
                }
                return nil, err
        }

        return buffer.Bytes(), nil
}