Object Storage Service:Conditional downloads (Go SDK V1)

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 and endpoints.

• 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 Configure OSSClient instances.

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

Conditions

OSS supports the following conditions:

Sample code

The following code shows how to perform a 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.
	// Set yourEndpoint to the Endpoint of the bucket. For example, for a bucket in the China (Hangzhou) region, set the Endpoint to https://oss-cn-hangzhou.aliyuncs.com. For other regions, set the Endpoint as needed.
	// Set yourRegion to the region where the bucket is located. For example, for a bucket in the China (Hangzhou) region, set the region to cn-hangzhou. For other regions, set the region as needed.
	clientOptions := []oss.ClientOption{oss.SetCredentialsProvider(&provider)}
	clientOptions = append(clientOptions, oss.Region("yourRegion"))
	// Set the signature version.
	clientOptions = append(clientOptions, oss.AuthVersion(oss.AuthV4))
	client, err := oss.New("yourEndpoint", "", "", clientOptions...)
	if err != nil {
		fmt.Println("Error:", err)
		os.Exit(-1)
	}

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

	// Assume that the object was last modified at 18:43:02 on November 21, 2023. If the specified UTC time is earlier than this time, the IfModifiedSince condition is met and the download is triggered.
	date := time.Date(2023, time.November, 21, 10, 40, 02, 0, time.UTC)

	// The condition is not met. The file is not downloaded.
	// Set yourObjectName to the full path of the object. The full path does not include the bucket name.
	err = bucket.GetObjectToFile("yourObjectName", "LocalFile", oss.IfUnmodifiedSince(date))
	if err == nil {
		fmt.Println("Error:", err)
		os.Exit(-1)
	}

	// The condition is met. The file is downloaded.
	err = bucket.GetObjectToFile("yourObjectName", "LocalFile", oss.IfModifiedSince(date))
	if err != nil {
		fmt.Println("Error:", err)
		os.Exit(-1)
	}
}

References

• For the complete sample code for conditional downloads, see the GitHub example.

• For more information about the API operation for conditional downloads, see GetObject.