How do I configure, modify, and query object metadata? - Object Storage Service

Objects that are stored in Object Storage Service (OSS) consist of keys, data, and object metadata. Object metadata describes the object. Object metadata includes standard HTTP headers and user metadata. You can create custom HTTP request policies such as object cache policies and forced object download policies by configuring standard HTTP headers. You can configure user metadata for an object to identify the purposes or attributes of the object.

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 configure object metadata, you must have the oss:PutObject permission. To query object metadata, you must have the oss:GetObject permission. For more information, see Attach a custom policy to a RAM user.

Sample code

Configure object metadata when you upload an object

Warning

When you upload objects to OSS, if an object stored in OSS has the same name as the uploaded object, OSS overwrites the existing object with the uploaded object. Before you configure object metadata by uploading an object, make sure that the destination bucket does not contain an object that has the same name to avoid accidentally overwriting existing objects. For more information about how to prevent objects with the same names from being overwritten, see Prevent objects with the same names from being overwritten.

package main

import (
	"log"
	"strings"
	"time"

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

func main() {
	// Obtain access credentials from environment variables. Before you run the 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 {
		log.Fatalf("Failed to create credentials provider: %v", err)
	}

	// 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 {
		log.Fatalf("Failed to create OSS client: %v", err)
	}

	// Specify the bucket name. Example: examplebucket. 
	bucketName := "examplebucket" // Replace this parameter with the actual name of your bucket.
	bucket, err := client.Bucket(bucketName)
	if err != nil {
		log.Fatalf("Failed to get bucket: %v", err)
	}

	// Configure object metadata. Set the expiration time to 23:00:00 January 10, 2049 in GMT, access control list (ACL) to public-read, and MyProp to MyPropVal as user metadata.
	expires := time.Date(2049, time.January, 10, 23, 0, 0, 0, time.UTC)
	options := []oss.Option{
		oss.Expires(expires),
		oss.ObjectACL(oss.ACLPublicRead),
		oss.Meta("MyProp", "MyPropVal"),
	}

	// Use a data stream to upload the object. 
	// Specify the full path of the object. The full path of the object cannot contain the bucket name. Example: exampledir/exampleobject.txt. 
	objectName := "exampledir/exampleobject.txt" // Replace this parameter with the actual path of the object.
	err = bucket.PutObject(objectName, strings.NewReader("MyObjectValue"), options...)
	if err != nil {
		log.Fatalf("Failed to upload object: %v", err)
	}

	// Query object metadata. 
	props, err := bucket.GetObjectDetailedMeta(objectName)
	if err != nil {
		log.Fatalf("Failed to get object detailed metadata: %v", err)
	}
	log.Printf("Object Meta: %+v\n", props)
}

Modify object metadata

...

// Modify one object metadata parameter at a time. 
err = bucket.SetObjectMeta(objectName, oss.Meta("MyMeta", "MyMetaValue1"))
if err != nil {
	log.Fatalf("Failed to set single metadata: %v", err)
}

// Modify multiple object metadata parameters at a time. 
options := []oss.Option{
	oss.Meta("MyMeta", "MyMetaValue2"),
	oss.Meta("MyObjectLocation", "HangZhou"),
}
err = bucket.SetObjectMeta(objectName, options...)
if err != nil {
	log.Fatalf("Failed to set multiple metadatas: %v", err)
}

// Query object metadata. 
props, err := bucket.GetObjectDetailedMeta(objectName)
if err != nil {
	log.Fatalf("Failed to get object detailed metadata: %v", err)
}
log.Printf("Object Meta: %+v\n", props)

Query object metadata

...

// Query object metadata. 
props, err := bucket.GetObjectDetailedMeta(objectName)
if err != nil {
	log.Fatalf("Failed to get object detailed metadata: %v", err)
}
log.Printf("Object Meta: %+v\n", props)

FAQ

When I upload an object to a bucket and set metadata for the object simultaneously, how do I prevent an existing object with the same name in the bucket from being overwritten?

You can use one of the following methods to prevent objects with the same name in the bucket from being overwritten:

Enable versioning
If you enable versioning for the bucket, OSS keeps the existing object with the same name as a previous version and stores the object that you upload as the current version. You can restore the previous versions at any time. For more information, see Enable version control.
Include the x-oss-forbid-overwrite header in the upload request
You can include the x-oss-forbid-overwrite header in the upload request and set the header to true. If you upload an object that has the same name as an existing object, the upload fails and OSS returns the FileAlreadyExists error. For more information, see PutObject.

References

For the complete sample code that is used to query object metadata, visit GitHub.
For more information about the API operation that you can call to configure object metadata during simple upload, see PutObject.
For more information about the API operation that you can call to modify object metadata, see CopyObject.
For more information about the API operation that you can call to query object metadata, see GetObjectMeta and HeadObject.