CODEWITHSUNDEEP
pythonamazon-web-servicesamazon-s3md5boto
kk1957
kk1957

Reputation: 8834

boto get md5 s3 file

I have a use case where I upload hundreds of file to my S3 bucket using multi part upload. After each upload I need to make sure that the uploaded file is not corrupt (basically check for data integrity). Currently, after uploading the file, I re-download it and compute the md5 on the content string and compare it with the md5 of local file. So something like:

conn = S3Connection('access key', 'secretkey')
bucket = conn.get_bucket('bucket_name')
source_path = 'file_to_upload'
source_size = os.stat(source_path).st_size

mp = bucket.initiate_multipart_upload(os.path.basename(source_path))
chunk_size = 52428800
chunk_count = int(math.ceil(source_size / chunk_size))

for i in range(chunk_count + 1):
   offset = chunk_size * i
   bytes = min(chunk_size, source_size - offset)
   with FileChunkIO(source_path, 'r', offset=offset, bytes=bytes) as fp:
       mp.upload_part_from_file(fp, part_num=i + 1, md5=k.compute_md5(fp, bytes))
mp.complete_upload()
    
obj_key = bucket.get_key('file_name')
print(obj_key.md5) #prints None
print(obj_key.base64md5) #prints None

content = bucket.get_key('file_name').get_contents_as_string()
# compute the md5 on content

This approach is wasteful as it doubles the bandwidth usage. I tried

bucket.get_key('file_name').md5 
bucket.get_key('file_name').base64md5

but both return None.

Is there any other way to achieve md5 without downloading the whole thing?

Upvotes: 18

Views: 23594

Answers (4)

veben
veben

Reputation: 22332

You can recover md5 without downloading the file, from e_tag attribute, like that:

boto3.resource('s3').Object(<BUCKET_NAME>, file_path).e_tag[1 :-1]

Then use this function to compare classic s3 files:

def md5_checksum(file_path):
    m = hashlib.md5()
    with open(file_path, 'rb') as f:
        for data in iter(lambda: f.read(1024 * 1024), b''):
            m.update(data)
    return m.hexdigest()

Or this function for multi-part files:

def etag_checksum(file_path, chunk_size=8 * 1024 * 1024):
    md5s = []
    with open(file_path, 'rb') as f:
        for data in iter(lambda: f.read(chunk_size), b''):
            md5s.append(hashlib.md5(data).digest())
    m = hashlib.md5("".join(md5s))
    return '{}-{}'.format(m.hexdigest(), len(md5s))

Finally use this function to choose between the two:

def md5_compare(file_path, s3_file_md5):
    if '-' in s3_file_md5 and s3_file_md5 == etag_checksum(file_path):
        return True
    if '-' not in s3_file_md5 and s3_file_md5 == md5_checksum(file_path):
        return True
    print("MD5 not equals for file " + file_path)
    return False

Credit to: https://zihao.me/post/calculating-etag-for-aws-s3-objects/

Upvotes: 3

bsplosion
bsplosion

Reputation: 2886

Since 2016, the best way to do this without any additional object retrievals is by presenting the --content-md5 argument during a PutObject request. AWS will then verify that the provided MD5 matches their calculated MD5. This also works for multipart uploads and objects >5GB.

An example call from the knowledge center:

aws s3api put-object --bucket awsexamplebucket --key awsexampleobject.txt --body awsexampleobjectpath --content-md5 examplemd5value1234567== --metadata md5checksum=examplemd5value1234567==

https://aws.amazon.com/premiumsupport/knowledge-center/data-integrity-s3/

Upvotes: 1

glefait
glefait

Reputation: 1691

With boto3, I use head_object to retrieve the ETag.

import boto3
import botocore

def s3_md5sum(bucket_name, resource_name):
    try:
        md5sum = boto3.client('s3').head_object(
            Bucket=bucket_name,
            Key=resource_name
        )['ETag'][1:-1]
    except botocore.exceptions.ClientError:
        md5sum = None
        pass
    return md5sum

Upvotes: 11

NoamG
NoamG

Reputation: 1161

yes
use bucket.get_key('file_name').etag[1 :-1]
this way get key's MD5 without downloading it's contents.

Upvotes: 26

Related Questions