Amazon Simple Storage Service
Developer Guide (API Version 2006-03-01)
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Object Key and Metadata

Each Amazon S3 object has data, a key, and metadata. When you create an object you specify the key name. This key name uniquely identifies the object in the bucket. For example, in Amazon S3 console (see AWS Management Console), when you highlight a bucket, a list of objects in your bucket appear. These names are the object keys. The name for a key is a sequence of Unicode characters whose UTF-8 encoding is at most 1024 bytes long.


If you anticipate that your workload against Amazon S3 will exceed 100 requests per second, follow the Amazon S3 key naming guidelines for best performance. For information, see Request Rate and Performance Considerations.

In addition to the key, each Amazon S3 object has metadata. It is a set of name-value pairs. You can set object metadata at the time you upload it. After you upload the object, you cannot modify object metadata. The only way to modify object metadata is to make copy of the object and set the metadata. For more information, go to PUT Object - Copy in the Amazon Simple Storage Service API Reference. You can use the Amazon S3 management console to update the object metadata but internally it makes an object copy replacing the existing object to set the metadata.

There are two kinds of metadata: system metadata and user-defined metadata.

System-Defined 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:

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

  • Other system metadata such as the storage class configured for the object, whether object has server-side encryption enabled are examples of system metadata whose values you control. If you have your bucket configured as a website, sometimes you might want to redirect a page request to another page or an external URL. In this case a web page 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. For more information about storage class and server-side encryption, see Using Data Encryption.

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

NameDescriptionCan User Modify the Value?
DateObject creation date.No
Content-LengthObject size in bytes.No
Last-ModifiedDate the object was last modified.No
Content-MD5The base64 encoded 128-bit MD5 digest of the object.No
x-amz-server-side-encryptionIndicates whether server-side encryption is enabled for the object, that is, the object data is encrypted at rest. For more information, see Using Server-Side Encryption. Yes
x-amz-version-idObject version. When you enable versioning on a bucket, Amazon S3 assigns version number to objects added to the bucket. For more information, see Using Versioning.No
x-amz-delete-markerIn a bucket that has version enabled, this boolean marker that indicates whether the object is a delete marker. No
x-amz-storage-classStorage class used for storing the object. 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 Configuring a Web Page Redirect .Yes

Storage Classes

Each Amazon S3 object is associated with a storage class. Amazon S3 supports the following storage classes:

  • Standard — The Standard storage class provides 99.999999999% durability and 99.99% availability of objects over a given year. It is designed to sustain the concurrent loss of data in two facilities.

  • Reduced Redundancy Storage (RRS) — The RRS storage class reduces costs by storing noncritical, reproducible data at lower levels of redundancy than the Standard storage class. It provides 99.99% durability and 99.99% availability of objects over a given year. This durability level corresponds to an average annual expected loss of 0.01% of objects

  • Glacier — The Glacier storage class is suitable for archiving data, where data access is infrequent and a retrieval time of several hours is acceptable. The Glacier storage class uses the very low-cost Amazon Glacier storage service, but you still manage objects in this storage class through Amazon S3.


You cannot associate an object with the Glacier storage class as you upload it. You transition existing Amazon S3 objects to the Glacier storage class by using lifecycle management. For more information, see Object Lifecycle Management.

User-Defined Metadata

When uploading an object, you can also assign metadata to the object. You provide this optional information as a name, value pair when you send a PUT or POST request to create the object. When uploading 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 uploading 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.


SOAP support over HTTP is deprecated, but it is still available over HTTPS. New Amazon S3 features will not be supported for 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 the unprintable metadata entries.

Amazon S3 stores user-defined metadata in lowercase. Each name, value pair must conform to US-ASCII when using REST and UTF-8 when using SOAP or browser-based uploads via POST.


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. User-defined metadata is a set of key-value pairs. 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.