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 exmaple, 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.

In addition to the key, each Amazon S3 object has metadata. It is a set of name-value pairs. 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.

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.

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.

Each name, value pair must conform to US-ASCII when using REST and UTF-8 when using SOAP or browser-based uploads via POST.

	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. 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.