Working with object metadata - Amazon Simple Storage Service

Working with object metadata

You can set object metadata in Amazon S3 at the time you upload the object. Object metadata is a set of name-value pairs. After you upload the object, you cannot modify object metadata. The only way to modify object metadata is to make a copy of the object and set the metadata.

When you create an object, you also specify the key name, which uniquely identifies the object in the bucket. The object key (or key name) uniquely identifies the object in an Amazon S3 bucket. For more information, see Creating object key names.

There are two kinds of metadata in Amazon S3: system-defined metadata and user-defined metadata. The sections below provide more information about system-defined and user-defined metadata. For more information about editing metadata using the Amazon S3 console, see Editing object metadata in the Amazon S3 console.

System-defined object metadata

For each object stored in a bucket, Amazon S3 maintains a set of system metadata. Amazon S3 processes this system metadata as needed. For example, Amazon S3 maintains object creation date and size metadata and uses this information as part of object management.

There are two categories of system metadata:

  1. Metadata such as object creation date is system controlled, where only Amazon S3 can modify the value.

  2. Other system metadata, such as the storage class configured for the object and whether the object has server-side encryption enabled, are examples of system metadata whose values you control. If your bucket is configured as a website, sometimes you might want to redirect a page request to another page or an external URL. In this case, a webpage is an object in your bucket. Amazon S3 stores the page redirect value as system metadata whose value you control.

    When you create objects, you can configure values of these system metadata items or update the values when you need to. For more information about storage classes, see Using Amazon S3 storage classes.

    Amazon S3 uses AWS KMS keys to encrypt your Amazon S3 objects. AWS KMS encrypts only the object data. The checksum, along with the specified algorithm, are stored as part of the object's metadata. If server-side encryption is requested for the object, then the checksum is stored in encrypted form. For more information about server-side encryption, see Protecting data using encryption.

Note

The PUT request header is limited to 8 KB in size. Within the PUT request header, the system-defined metadata is limited to 2 KB in size. The size of system-defined metadata is measured by taking the sum of the number of bytes in the US-ASCII encoding of each key and value.

The following table provides a list of system-defined metadata and whether you can update it.

Name Description Can user modify the value?
Date Current date and time. No
Cache-Control A general header field used to specify caching policies. Yes
Content-Disposition Object presentational information. Yes
Content-Length Object size in bytes. No
Content-Type Object type. Yes
Last-Modified

Object creation date or the last modified date, whichever is the latest.

No
ETag Represents a specific version of an object. For objects that are not uploaded as a multipart upload and are either unencrypted or encrypted by server-side encryption with Amazon S3 managed keys (SSE-S3), the ETag is an MD5 digest of the data. No
x-amz-server-side-encryption Indicates whether server-side encryption is enabled for the object, and whether that encryption is from the AWS Key Management Service (AWS KMS) or from Amazon S3 managed encryption (SSE-S3). For more information, see Protecting data using server-side encryption. Yes
x-amz-checksum-crc32, x-amz-checksum-crc32c, x-amz-checksum-sha1, x-amz-checksum-sha256 Contains the checksum or digest of the object. At most, one of these headers will be set at a time, depending on the checksum algorithm that you instruct Amazon S3 to use. For more information about choosing the checksum algorithm, see Checking object integrity. No
x-amz-version-id Object version. When you enable versioning on a bucket, Amazon S3 assigns a version number to objects added to the bucket. For more information, see Using versioning in S3 buckets. No
x-amz-delete-marker In a bucket that has versioning enabled, this Boolean marker indicates whether the object is a delete marker. No
x-amz-storage-class Storage class used for storing the object. For more information, see Using Amazon S3 storage classes. Yes
x-amz-website-redirect-location Redirects requests for the associated object to another object in the same bucket or an external URL. For more information, see (Optional) Configuring a webpage redirect. This value is unique to each individual object and is not copied by default. Yes
x-amz-server-side-encryption-aws-kms-key-id If x-amz-server-side-encryption is present and has the value of aws:kms, this indicates the ID of the AWS KMS symmetric encryption KMS key that was used for the object. Yes
x-amz-server-side-encryption-customer-algorithm Indicates whether server-side encryption with customer-provided encryption keys (SSE-C) is enabled. For more information, see Protecting data using server-side encryption with customer-provided encryption keys (SSE-C). Yes
x-amz-tagging The tag-set for the object. The tag-set must be encoded as URL Query parameters. Yes

User-defined object metadata

When uploading an object, you can also assign metadata to the object. You provide this optional information as a name-value (key-value) pair when you send a PUT or POST request to create the object. When you upload objects using the REST API, the optional user-defined metadata names must begin with "x-amz-meta-" to distinguish them from other HTTP headers. When you retrieve the object using the REST API, this prefix is returned. When you upload objects using the SOAP API, the prefix is not required. When you retrieve the object using the SOAP API, the prefix is removed, regardless of which API you used to upload the object.

Note

SOAP support over HTTP is deprecated, but SOAP is still available over HTTPS. New Amazon S3 features are not supported for SOAP. Instead of using SOAP, we recommend that you use either the REST API or the AWS SDKs.

When metadata is retrieved through the REST API, Amazon S3 combines headers that have the same name (ignoring case) into a comma-delimited list. If some metadata contains unprintable characters, it is not returned. Instead, the x-amz-missing-meta header is returned with a value of the number of unprintable metadata entries. The HeadObject action retrieves metadata from an object without returning the object itself. This operation is useful if you're only interested in an object's metadata. To use HEAD, you must have READ access to the object. For more information, see HeadObject in the Amazon Simple Storage Service API Reference.

User-defined metadata is a set of key-value pairs. Amazon S3 stores user-defined metadata keys in lowercase.

Amazon S3 allows arbitrary Unicode characters in your metadata values.

To avoid issues around the presentation of these metadata values, you should conform to using US-ASCII characters when using REST and UTF-8 when using SOAP or browser-based uploads via POST.

When using non US-ASCII characters in your metadata values, the provided unicode string is examined for non US-ASCII characters. Values of such headers are character decoded as per RFC 2047 before storing and encoded as per RFC 2047 to make them mail-safe before returning. If the string contains only US-ASCII characters, it is presented as is.

The following is an example.

PUT /Key HTTP/1.1 Host: awsexamplebucket1.s3.amazonaws.com x-amz-meta-nonascii: ÄMÄZÕÑ S3 HEAD /Key HTTP/1.1 Host: awsexamplebucket1.s3.amazonaws.com x-amz-meta-nonascii: =?UTF-8?B?w4PChE3Dg8KEWsODwpXDg8KRIFMz?= PUT /Key HTTP/1.1 Host: awsexamplebucket1.s3.amazonaws.com x-amz-meta-ascii: AMAZONS3 HEAD /Key HTTP/1.1 Host: awsexamplebucket1.s3.amazonaws.com x-amz-meta-ascii: AMAZONS3
Note

The PUT request header is limited to 8 KB in size. Within the PUT request header, the user-defined metadata is limited to 2 KB in size. The size of user-defined metadata is measured by taking the sum of the number of bytes in the UTF-8 encoding of each key and value.

For information about changing the metadata of your object after it’s been uploaded by creating a copy of the object, modifying it, and replacing the old object, or creating a new version, see Editing object metadata in the Amazon S3 console.