Object Storage Service:Conditional download

Usage notes

• In this topic, the public endpoint of the China (Hangzhou) region is used. If you want to access OSS from other Alibaba Cloud services in the same region as OSS, use an internal endpoint. For more information about OSS regions and endpoints, see Regions, endpoints and open ports.

• In this topic, access credentials are obtained from environment variables. For more information about how to configure access credentials, see Configure access credentials.

• In this topic, an OSSClient instance is created by using an OSS endpoint. If you want to create an OSSClient instance by using custom domain names or Security Token Service (STS), see Initialization.

• To perform conditional download, you must have the oss:GetObject permission. For more information, see Attach a custom policy to a RAM user.

Conditions

The following table describes the available object download conditions.

Examples

The following sample code provides an example on how to perform conditional download:

package main

import (
	"fmt"
	"os"
	"time"

	"github.com/aliyun/aliyun-oss-go-sdk/oss"
)

func main() {
	// Obtain access credentials from environment variables. Before you run the sample code, make sure that the OSS_ACCESS_KEY_ID and OSS_ACCESS_KEY_SECRET environment variables are configured. 
	provider, err := oss.NewEnvironmentVariableCredentialsProvider()
	if err != nil {
		fmt.Println("Error:", err)
		os.Exit(-1)
	}

	// Create an OSSClient instance. 
        // Specify the endpoint of the region in which the bucket is located. For example, if the bucket is located in the China (Hangzhou) region, set the endpoint to https://oss-cn-hangzhou.aliyuncs.com. Specify your actual endpoint. 
	// Specify the region in which the bucket is located. For example, if the bucket is located in the China (Hangzhou) region, set the region to cn-hangzhou. Specify the actual region.
	clientOptions := []oss.ClientOption{oss.SetCredentialsProvider(&provider)}
	clientOptions = append(clientOptions, oss.Region("yourRegion"))
	// Specify the version of the signature algorithm.
	clientOptions = append(clientOptions, oss.AuthVersion(oss.AuthV4))
	client, err := oss.New("yourEndpoint", "", "", clientOptions...)
	if err != nil {
		fmt.Println("Error:", err)
		os.Exit(-1)
	}

	// Specify the name of your bucket.
	bucket, err := client.Bucket("yourBucketName")
	if err != nil {
		fmt.Println("Error:", err)
		os.Exit(-1)
	}

	// For example, an object was last modified at 18:43:02, on November 21, 2023. If the specified time is earlier than the last modified time, the object meets the If-Modified-Since condition and the object is downloaded. 
	date := time.Date(2023, time.November, 21, 10, 40, 02, 0, time.UTC)

	// The object is not downloaded if it does not meet the specified conditions. 
        // Specify the full path of the object. Do not include the bucket name in the full path. 
	err = bucket.GetObjectToFile("yourObjectName", "LocalFile", oss.IfUnmodifiedSince(date))
	if err == nil {
		fmt.Println("Error:", err)
		os.Exit(-1)
	}

	// The object is downloaded if it meets the specified conditions. 
	err = bucket.GetObjectToFile("yourObjectName", "LocalFile", oss.IfModifiedSince(date))
	if err != nil {
		fmt.Println("Error:", err)
		os.Exit(-1)
	}
}

References

• For the complete sample code that is used to perform conditional download, visit GitHub.

• For more information about the API operation that you can call to perform conditional download, see GetObject.