Downloading an Object - Resumable (SDK for Go)_Object Storage Service

Function

Downloading large files often fails due to poor network conditions or program breakdowns. It is a waste of resources to restart the download process upon a download failure, and the restarted download process may still suffer from the unstable network. To resolve such issues, the resumable download API splits the file to be downloaded into multiple parts and downloads them separately. The download result of each part is recorded in a checkpoint file in real time. Only when all parts are successfully downloaded, the result indicating a successful download is returned. Otherwise, an error message is returned to remind you of calling the API again for re-downloading. Based on the download status of each part recorded in the checkpoint file, the re-downloading will download the parts failed to be downloaded previously, instead of downloading all parts. By virtue of this, resources are saved and efficiency is improved.

This API calls ObsClient.DownloadFile to perform a resumable download.

Restrictions

To download an object, you must be the bucket owner or have the required permission (obs:object:GetObject in IAM or GetObject in a bucket policy). For details, see Introduction to OBS Access Control, IAM Custom Policies, and Configuring an Object Policy.
The resumable download API, which is implemented based on partial download, is an encapsulated and enhanced version of partial download.
This API saves resources and improves efficiency upon the re-download, and speeds up the download process by concurrently downloading parts. Because this API is invisible to users, users are unaware of internal service details, such as the creation and deletion of checkpoint files, division of objects, and concurrent download of parts.

EnableCheckpoint: The default value is False, indicating that resumable download is disabled. In this case, the resumable download API is a simple encapsulation of the partial download API, and no checkpoint file will be generated.
CheckpointFile: This parameter is effective only when EnableCheckpoint is True.

Method

func (obsClient ObsClient) DownloadFile(input *DownloadFileInput) (output *GetObjectMetadataOutput, err error)

Request Parameters

**Table 1** List of request parameters
Parameter	Type	Mandatory (Yes/No)	Description
input	*DownloadFileInput	Yes	Explanation: Input parameters for a resumable download. For details, see Table 2.

**Table 2** DownloadFileInput
Parameter	Type	Mandatory (Yes/No)	Description
Bucket	string	Yes	Explanation: Bucket name Restrictions: A bucket name must be unique across all accounts and regions. A bucket name: Must be 3 to 63 characters long and start with a digit or letter. Lowercase letters, digits, hyphens (-), and periods (.) are allowed. Cannot be formatted as an IP address. Cannot start or end with a hyphen (-) or period (.). Cannot contain two consecutive periods (..), for example, my..bucket. Cannot contain a period (.) and a hyphen (-) adjacent to each other, for example, my-.bucket or my.-bucket. If you repeatedly create buckets of the same name in the same region, no error will be reported and the bucket attributes comply with those set in the first creation request. Default value: None
Key	string	Yes	Explanation: Object name. An object is uniquely identified by an object name in a bucket. An object name is a complete path that does not contain the bucket name. For example, if the address for accessing the object is examplebucket.obs.eu-west-101.myhuaweicloud.com/folder/test.txt, the object name is folder/test.txt. Value range: The value must contain 1 to 1,024 characters. Default value: None
VersionId	string	No	Explanation: Object version ID, for example, G001117FCE89978B0000401205D5DC9 Value range: The value must contain 32 characters. Default value: None. If this parameter is left blank, the latest version of the object is obtained.
DownloadFile	string	No	Explanation: Full path of the local directory to which the object is downloaded. Default value: If the parameter is left blank, the downloaded object is saved in the directory where the program is executed.
PartSize	int64	No	Explanation: Part size Value range: The value must be greater than 0 but less than the object size, in bytes. Default value: 9437184, indicating 9 MB for each part
TaskNum	int	No	Explanation: Maximum number of parts that can be downloaded concurrently Value range: The value must be greater than 0 but not exceed the result of the file size divided by the part size (rounded up). Default value: 1, indicating concurrent downloads are not used.
EnableCheckpoint	bool	No	Explanation: Whether to enable the resumable download Value range: true: The resumable download mode is enabled. false: The resumable download mode is disabled. Default value: false
CheckpointFile	string	No	Explanation: File used to record the download progress. This parameter is valid only in a resumable download. Default value: If the parameter is left blank, the file is in the same local directory as the downloaded object.
SseHeader	SseCHeader	No	Explanation: Server-side decryption headers. For details, see Table 3.
IfMatch	string	No	Explanation: Preset ETag. If the ETag of the object to be downloaded is the same as the preset ETag, the object is returned. Otherwise, an error is returned. Value range: The value must contain 32 characters. Default value: None
IfNoneMatch	string	No	Explanation: Preset ETag. If the ETag of the object to be downloaded is different from the preset ETag, the object is returned. Otherwise, an error is returned. Value range: The value must contain 32 characters. Default value: None
IfModifiedSince	time.Time	No	Explanation: The object is returned if it has been modified since the specified time; otherwise, an error is returned. Restrictions: The time must be in the ISO8601 format, for example, 2018-01-01T00:00:00.000Z. Sample code: *time.Now().Add(time.Duration(24) time.Hour) Default value**: None
IfUnmodifiedSince	time.Time	No	Explanation: The object is returned if it has not been modified since the specified time; otherwise, an error is returned. Restrictions: The time must be in the ISO8601 format, for example, 2018-01-01T00:00:00.000Z. Sample code: *time.Now().Add(time.Duration(24) time.Hour) Default value**: None

**Table 3** SseCHeader
Parameter	Type	Mandatory (Yes/No)	Description
Encryption	string	Yes if used as a request parameter	Explanation: SSE-C used for encrypting objects Value range: AES256, indicating objects are encrypted using SSE-C Default value: None
Key	string	Yes if used as a request parameter	Explanation: Key for encrypting the object when SSE-C is used Restrictions: The value is a Base64-encoded 256-bit key, for example, K7QkYpBkM5+hca27fsNkUnNVaobncnLht/rCB2o/9Cw=. Default value: None
KeyMD5	string	No if used as a request parameter	Explanation: MD5 value of the key for encrypting objects when SSE-C is used. This value is used to check whether any error occurs during the transmission of the key. Restrictions: The value is encrypted by MD5 and then encoded by Base64, for example, 4XvB3tbNTN+tIEVa0/fGaQ==. Default value: None

Responses

**Table 4** List of returned results
Parameter	Type	Description
output	*GetObjectMetadataOutput	Explanation: Returned results. For details, see Table 5.
err	error	Explanation: Error messages returned by the API

**Table 5** GetObjectMetadataOutput
Parameter	Type	Description
StatusCode	int	Explanation: HTTP status code Value range: A status code is a group of digits that can be 2xx (indicating successes) or 4xx or 5xx (indicating errors). It indicates the status of a response. For more information, see Status Code. Default value: None
RequestId	string	Explanation: Request ID returned by the OBS server Default value: None
ResponseHeaders	map[string][]string	Explanation: HTTP response headers Default value: None
StorageClass	StorageClassType	Explanation: Object storage class Value range: See Table 6. Default value: None
AllowOrigin	string	Explanation: If Origin in the request meets the CORS rules of the bucket, AllowedOrigin specified in the CORS rules is returned. AllowedOrigin indicates the origin from which the requests can access the bucket. Restrictions: Domain name of the origin. Each origin can contain only one wildcard character (), for example, https://.vbs.example.com. Default value**: None
AllowHeader	string	Explanation: If RequestHeader in the request meets the CORS rules of the bucket, AllowedHeader specified in the CORS rules is returned. AllowedHeader indicates the allowed headers for cross-origin requests. Only CORS requests matching the allowed headers are valid. Restrictions: Each header can contain only one wildcard character (). Spaces, ampersands (&), colons (:), and less-than signs (<) are not allowed. Default value*: None
AllowMethod	string	Explanation: AllowedMethod in the CORS rules of the bucket. It specifies the HTTP method of cross-origin requests, that is, the operation type of buckets and objects. Value range: The following HTTP methods are supported: GET PUT HEAD POST DELETE Default value: None
ExposeHeader	string	Explanation: ExposeHeader in the CORS rules of the bucket. It specifies the CORS-allowed additional headers in the response. These headers provide additional information to clients. By default, your browser can only access headers Content-Length and Content-Type. If your browser needs to access other headers, add them to a list of the allowed additional headers. Restrictions: Spaces, wildcard characters (), ampersands (&), colons (:), and less-than signs (<) are not allowed. Default value*: None
MaxAgeSeconds	int	Explanation: MaxAgeSeconds in the CORS rules of the bucket. It specifies the time your client can cache the response for a cross-origin request. Restrictions: Each CORS rule can specify only one value for MaxAgeSeconds. Value range: 0 to (2³¹ – 1), in seconds Default value: 100
ContentLength	int64	Explanation: Object size in bytes Value range: The value ranges from 0 TB to 48.8 TB, in bytes. Default value: None
ContentType	string	Explanation: MIME type of the file to be downloaded. MIME type is a standard way of describing a data type and is used by the browser to decide how to display data. Value range: See What Is Content-Type (MIME)? Default value: If you do not specify this parameter when uploading an object, the SDK determines the object type based on the suffix of the specified object name and automatically assigns a value to this parameter.
LastModified	time.Time	Explanation: Time when the last modification was made to the object Restrictions: The time must be in the ISO8601 format, for example, 2018-01-01T00:00:00.000Z. Default value: None
ETag	string	Explanation: Base64-encoded, 128-bit MD5 value of an object. ETag is the unique identifier of the object content. It can be used to determine whether the object content is changed. For example, if the ETag value is A when an object is uploaded, but changes to B when the object is downloaded, it indicates that the object content has been changed. The ETag reflects changes to the object content, rather than the object metadata. An uploaded or copied object has a unique ETag after being encrypted using MD5. Restrictions: If an object is encrypted using server-side encryption, the ETag is not the MD5 value of the object. Value range: The value must contain 32 characters. Default value: None
VersionId	string	Explanation: Object version ID Value range: The value must contain 32 characters. Default value: None
Restore	string	Explanation: Restore status of an object. For an Archive object that is being restored or has been restored, this header is returned. For example, ongoing-request="true" indicates that the object is being restored. ongoing-request="false", expiry-date="Wed, 7 Nov 2012 00:00:00 GMT" indicates that the object has been restored. expiry-date indicates when the restored object expires. Restrictions: This parameter is only available for Archive objects. Default value: None
Expiration	string	Explanation: Expiration details of the object. Example: "expiry-date=\"Mon, 11 Sep 2023 00:00:00 GMT\"" Default value: None
SseHeader	SseCHeader or SseKmsHeader	Explanation: Server-side encryption header information. If SSE-C is used, see Table 7. If SSE-KMS is used, see Table 8.
WebsiteRedirectLocation	string	Explanation: If the bucket is configured with website hosting, the request for obtaining the object can be redirected to another object in the bucket or an external URL. This parameter specifies the address the request for the object is redirected to. The request is redirected to object anotherPage.html in the same bucket: WebsiteRedirectLocation:/anotherPage.html The request is redirected to an external URL http://www.example.com/: WebsiteRedirectLocation:http://www.example.com/ OBS obtains the specified value from the header and stores it in the object metadata WebsiteRedirectLocation. Restrictions: The value must start with a slash (/), http://, or https:// and cannot exceed 2 KB. OBS only supports redirection for objects in the root directory of a bucket. Default value: None
Metadata	map[string]string	Explanation: Custom object metadata. You can add a header starting with x-obs-meta- in the request to define metadata. The custom metadata will be returned in the response when you retrieve the object or query the object metadata. Restrictions: The custom metadata cannot exceed 8 KB. To measure the custom metadata, sum the number of bytes in the UTF-8 encoding of each key and value. The custom metadata keys are case insensitive, but are stored in lowercase in OBS. The key values are case sensitive. Both custom metadata keys and their values must conform to US-ASCII standards. If non-ASCII or unrecognizable characters are required, they must be encoded or decoded in URL or Base64 on the client, because the server does not perform such operations. Default value: None

**Table 6** StorageClassType
Constant	Default Value	Description
StorageClassStandard	STANDARD	OBS Standard Features low access latency and high throughput and is used for storing massive, frequently accessed (multiple times a month) or small objects (< 1 MB) requiring quick response.
StorageClassWarm	WARM	OBS Infrequent Access Used for storing data that is semi-frequently accessed (fewer than 12 times a year) but is instantly available when needed.
StorageClassCold	COLD	OBS Archive Used for storing rarely accessed (once a year) data.

**Table 7** SseCHeader
Parameter	Type	Mandatory (Yes/No)	Description
Encryption	string	Yes if used as a request parameter	Explanation: SSE-C used for encrypting objects Value range: AES256, indicating objects are encrypted using SSE-C Default value: None
Key	string	Yes if used as a request parameter	Explanation: Key for encrypting the object when SSE-C is used Restrictions: The value is a Base64-encoded 256-bit key, for example, K7QkYpBkM5+hca27fsNkUnNVaobncnLht/rCB2o/9Cw=. Default value: None
KeyMD5	string	No if used as a request parameter	Explanation: MD5 value of the key for encrypting objects when SSE-C is used. This value is used to check whether any error occurs during the transmission of the key. Restrictions: The value is encrypted by MD5 and then encoded by Base64, for example, 4XvB3tbNTN+tIEVa0/fGaQ==. Default value: None

**Table 8** SseKmsHeader
Parameter	Type	Mandatory (Yes/No)	Description
Encryption	string	Yes if used as a request parameter	Explanation: SSE-KMS used for encrypting objects Value range: kms, indicating objects are encrypted using SSE-KMS Default value: None
Key	string	No if used as a request parameter	Explanation: ID of the KMS master key when SSE-KMS is used Value range: Valid value formats are as follows: regionID:domainID:key/key_id key_id In the preceding formats: regionID indicates the ID of the region where the key is used. domainID indicates the ID of the account where the key is used. To obtain it, see How Do I Get My Account ID and User ID? key_id indicates the ID of the key created on Data Encryption Workshop (DEW). Default value: If this parameter is not specified, the default master key will be used. If there is no such a default master key, OBS will create one and use it by default.

Code Examples

This example downloads example/objectname from examplebucket in a resumable download.

    
     
       
       
         package main
import (
    "fmt"
    "os"
    obs "github.com/huaweicloud/huaweicloud-sdk-go-obs/obs"
)
func main() {
    //Obtain an AK/SK pair using environment variables or import an AK/SK pair in other ways. Using hard coding may result in leakage.
    //Obtain an AK/SK pair on the management console. For details, see https://support.huaweicloud.com/eu/usermanual-ca/ca_01_0003.html.
    ak := os.Getenv("AccessKeyID")
    sk := os.Getenv("SecretAccessKey")
    // (Optional) If you use a temporary AK/SK pair and a security token to access OBS, you are advised not to use hard coding to reduce leakage risks. You can obtain an AK/SK pair using environment variables or import an AK/SK pair in other ways.
    // securityToken := os.Getenv("SecurityToken")
    // Enter the endpoint corresponding to the bucket. EU-Dublin is used here as an example. Replace it with the one currently in use.
    endPoint := "https://obs.eu-west-101.myhuaweicloud.eu" 
    // Create an obsClient instance.
    // If you use a temporary AK/SK pair and a security token to access OBS, use the obs.WithSecurityToken method to specify a security token when creating an instance.
    obsClient, err := obs.New(ak, sk, endPoint/*, obs.WithSecurityToken(securityToken)*/)
    if err != nil {
        fmt.Printf("Create obsClient error, errMsg: %s", err.Error())
    }
    input := &obs.DownloadFileInput{}
    // Specify a bucket name.
    input.Bucket = "examplebucket"
    // Specify the object (example/objectname as an example) to download.
    input.Key = "example/objectname"
    // Specify a local full path (/tmp/objectname as an example) for the specified object. If the path is left blank, the current working directory will be used by default.
    input.DownloadFile = "/tmp/objectname"
    // Specify whether to enable resumable download. Value true is used as an example. The default value is false, indicating that resumable download is disabled.
    input.EnableCheckpoint = true
    // Specify a part size, in bytes. This example sets each part to 9 MB.
    input.PartSize = 9 * 1024 * 1024
    // Specify the maximum number of parts that can be concurrently downloaded. 5 is used as an example.
    input.TaskNum = 5
    // Download the object using resumable download.
    output, err := obsClient.DownloadFile(input)
    if err == nil {
        fmt.Printf("Download file(%s) under the bucket(%s) successful!\n", input.Key, input.Bucket)
        fmt.Printf("StorageClass:%s, ETag:%s, ContentType:%s, ContentLength:%d, LastModified:%s\n",
            output.StorageClass, output.ETag, output.ContentType, output.ContentLength, output.LastModified)
        return
    }
    fmt.Printf("Download file(%s) under the bucket(%s) fail!\n", input.Key, input.Bucket)
    if obsError, ok := err.(obs.ObsError); ok {
        fmt.Println("An ObsError was found, which means your request sent to OBS was rejected with an error response.")
        fmt.Println(obsError.Error())
    } else {
        fmt.Println("An Exception was found, which means the client encountered an internal problem when attempting to communicate with OBS, for example, the client was unable to access the network.")
        fmt.Println(err)
    }
}

        

      

    
   

Downloading an Object - Resumable (SDK for Go)

Function

Restrictions

Method

Request Parameters

Responses

Code Examples

Helpful Links

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