Errors may occur when data is transferred between the client and the server. COS can verify data integrity using MD5 and custom attributes, or the CRC64 check code.
COS will calculate the CRC64 of the newly uploaded object and store the result as object attributes. It will carry x-cos-hash-crc64ecma in the returned response header, which indicates the CRC64 value of the uploaded object calculated according to ECMA-182 standard. If an object already has a CRC64 value stored before this feature is activated, COS will not calculate its CRC64 value, nor will it be returned when the object is obtained.
APIs that currently support CRC64 are:
The following is an example of the response obtained after a user sends an Upload Part request. The x-cos-hash-crc64ecma header indicates the CRC64 value of the part, and the user can compare the value with the locally calculated CRC64 value to verify the integrity of the part.
HTTP/1.1 200 OK
content-length: 0
connection: close
date: Thu, 05 Dec 2019 01:58:03 GMT
etag: "358e8c8b1bfa35ee3bd44cb3d2cc416b"
server: tencent-cos
x-cos-hash-crc64ecma: 15060521397700495958
x-cos-request-id: NWRlODY0MmJfMjBiNDU4NjRfNjkyZl80ZjZi****The following is an example of the response obtained after a user sends a Complete Multipart Upload request. The x-cos-hash-crc64ecma header indicates the CRC64 value of the part, and the user can compare the value with the locally calculated CRC64 value to verify the integrity of the part.
HTTP/1.1 200 OK
content-type: application/xml
transfer-encoding: chunked
connection: close
date: Thu, 05 Dec 2019 02:01:17 GMT
server: tencent-cos
x-cos-hash-crc64ecma: 15060521397700495958
x-cos-request-id: NWRlODY0ZWRfMjNiMjU4NjRfOGQ4Ml81MDEw****
[Object Content]The following uses the Python SDK as an example to demonstrate how to verify an object. The complete sample code is as follows.
The code is based on Python 2.7. For more information on how to use the Python SDK, see Object Operations for Python SDK..
Configure user attributes, including SecretId, SecretKey, and region, and create a client object.
# -*- coding=utf-8
from qcloud_cos import CosConfig
from qcloud_cos import CosS3Client
from qcloud_cos import CosServiceError
from qcloud_cos import CosClientError
import sys
import logging
import hashlib
logging.basicConfig(level=logging.INFO, stream=sys.stdout)
# Configure user attributes, including SecretId, SecretKey, and region
# APPID has been removed from the configuration. Please add the APPID to the `Bucket` parameter in the format of `BucketName-APPID`.
secret_id = COS_SECRETID # Replace with your own SecretId
secret_key = COS_SECRETKEY # Replace with your own SecretKey
region = 'ap-beijing' # Replace with your own region (which is Beijing in this sample)
token = None # If a temporary key is used, token needs to be passed in. This is optional and is null by default
config = CosConfig(Region=region, SecretId=secret_id, SecretKey=secret_key, Token=token) # Get the configured object
client = CosS3Client(config)Simulate object parts and calculate the checksum of the entire object.
OBJECT_PART_SIZE = 1024 * 1024 # Size of each simulated part
OBJECT_TOTAL_SIZE = OBJECT_PART_SIZE * 1 + 123 # Total size of the object
object_body = '1' * OBJECT_TOTAL_SIZE # Object content
#Calculate the checksum of the entire object.
c64 = crcmod.mkCrcFun(0x142F0E1EBA9EA3693L, initCrc=0L, xorOut=0xffffffffffffffffL, rev=True)
local_crc64 =str(c64(object_body))# Initialize the multipart upload
response = client.create_multipart_upload(
Bucket='examplebucket-1250000000', #Replaced with your own bucket name. 'examplebucket' is a sample bucket, and '1250000000' is a sample APPID.
Key='exampleobject', # Replaced with the key value of your uploaded object
StorageClass='STANDARD', # Storage class of the object
)
# Get the UploadId of the multipart upload
upload_id = response['UploadId']During a multipart upload, an object is divided into multiple (up to 10,000) parts for upload. The size of each part can range from 1 MB to 5 GB, and the last part can be less than 1 MB. When uploading parts, configure the PartNumber of each part and its corresponding CRC64 value. After the parts are successfully uploaded, check the returned CRC64 value with the locally calculated value to verify the integrity of each part.
# Upload an object in parts where the size of each part is OBJECT_PART_SIZE except the last part which may be smaller
part_list = list()
position = 0
left_size = OBJECT_TOTAL_SIZE
part_number = 0
while left_size > 0:
part_number += 1
if left_size >= OBJECT_PART_SIZE:
body = object_body[position:position+OBJECT_PART_SIZE]
else:
body = object_body[position:]
position += OBJECT_PART_SIZE
left_size -= OBJECT_PART_SIZE
local_part_crc_64 = c64(body)#Calculate CRC64 locally
response = client.upload_part(
Bucket='examplebucket-1250000000',
Key=‘exampleobject’,
Body=body,
PartNumber=part_number,
UploadId=upload_id,
)
part_crc_64 = response['x-cos-hash-crc64ecma']# CRC64 returned by the server
if local_part_crc64 != part_crc_64:# Data Check
print 'crc64 check FAIL'
exit(-1)
etag = response['ETag']
part_list.append({'ETag' : etag, 'PartNumber' : part_number})After all parts are successfully uploaded, you need to complete the multipart upload. The CRC64 returned by COS and the CRC64 of the local object can be used for comparison and verification.
# Complete the multipart upload
response = client.complete_multipart_upload(
Bucket='examplebucket-1250000000', # Replace with your own bucket name, examplebucket is a sample bucket, and 1250000000 is a sample APPID
Key=‘exampleobject’, #Key value of the object
UploadId=upload_id,
MultipartUpload={ #Requires one-to-one correspondence between ETag and PartNumber for each part
'Part' : part_list
},
)
crc64ecma = response['x-cos-hash-crc64ecma']
if crc64ecma != local_crc64:# Data Check
print 'check crc64 Failed'
exit(-1)
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