Organizing objects using prefixes

You can use prefixes to organize the data that you store in Amazon S3 buckets. A prefix is a string of characters at the beginning of the object key name. A prefix can be any length, subject to the maximum length of the object key name (1,024 bytes). You can think of prefixes as a way to organize your data in a similar way to directories. However, prefixes are not directories.

Searching by prefix limits the results to only those keys that begin with the specified prefix. The delimiter causes a list operation to roll up all the keys that share a common prefix into a single summary list result.

The purpose of the prefix and delimiter parameters is to help you organize and then browse your keys hierarchically. To do this, first pick a delimiter for your bucket, such as slash (/), that doesn't occur in any of your anticipated key names. You can use another character as a delimiter. There is nothing unique about the slash (/) character, but it is a very common prefix delimiter. Next, construct your key names by concatenating all containing levels of the hierarchy, separating each level with the delimiter.

For example, if you were storing information about cities, you might naturally organize them by continent, then by country, then by province or state. Because these names don't usually contain punctuation, you might use slash (/) as the delimiter. The following examples use a slash (/) delimiter.

If you stored data for every city in the world in this manner, it would become awkward to manage a flat key namespace. By using Prefix and Delimiter with the list operation, you can use the hierarchy that you've created to list your data. For example, to list all the states in USA, set Delimiter='/' and Prefix='North America/USA/'. To list all the provinces in Canada for which you have data, set Delimiter='/' and Prefix='North America/Canada/'.

For more information about delimiters, prefixes, and nested folders, see Difference between prefixes and nested folders.

Listing objects using prefixes and delimiters

If you issue a list request with a delimiter, you can browse your hierarchy at only one level, skipping over and summarizing the (possibly millions of) keys nested at deeper levels. For example, assume that you have a bucket (amzn-s3-demo-bucket) with the following keys:

sample.jpg

photos/2006/January/sample.jpg

photos/2006/February/sample2.jpg

photos/2006/February/sample3.jpg

photos/2006/February/sample4.jpg

The sample bucket has only the sample.jpg object at the root level. To list only the root level objects in the bucket, you send a GET request on the bucket with the slash (/) delimiter character. In response, Amazon S3 returns the sample.jpg object key because it does not contain the / delimiter character. All other keys contain the delimiter character. Amazon S3 groups these keys and returns a single CommonPrefixes element with the prefix value photos/, which is a substring from the beginning of these keys to the first occurrence of the specified delimiter.

<ListBucketResult xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
  <Name>amzn-s3-demo-bucket</Name>
  <Prefix></Prefix>
  <Marker></Marker>
  <MaxKeys>1000</MaxKeys>
  <Delimiter>/</Delimiter>
  <IsTruncated>false</IsTruncated>
  <Contents>
    <Key>sample.jpg</Key>
    <LastModified>2011-07-24T19:39:30.000Z</LastModified>
    <ETag>&quot;d1a7fb5eab1c16cb4f7cf341cf188c3d&quot;</ETag>
    <Size>6</Size>
    <Owner>
      <ID>75cc57f09aa0c8caeab4f8c24e99d10f8e7faeebf76c078efc7c6caea54ba06a</ID>
      <DisplayName>displayname</DisplayName>
    </Owner>
    <StorageClass>STANDARD</StorageClass>
  </Contents>
  <CommonPrefixes>
    <Prefix>photos/</Prefix>
  </CommonPrefixes>
</ListBucketResult>

For more information about listing object keys programmatically, see Listing object keys programmatically.