Working With Object Metadata

Working With Object Metadata

Overview Of Metadata

Objects can have associated metadata, which control aspects of how GET requests are handled, including Content-Type, Cache-Control, Content-Disposition, and Content-Encoding (discussed in more detail in the subsections below). In addition, you can set custom metadata that can be used by applications (e.g., tagging that particular objects possess some property).

There are two ways to set metadata on objects:

  • At upload time you can specify one or more metadata properties to associate with objects, using the gsutil -h option. For example, the following command would cause gsutil to set the Content-Type and Cache-Control for each of the files being uploaded:

    gsutil -h "Content-Type:text/html" \
           -h "Cache-Control:public, max-age=3600" cp -r images \
           gs://bucket/images
    

    Note that -h is an option on the gsutil command, not the cp sub-command.

  • You can set or remove metadata fields from already uploaded objects using the gsutil setmeta command. See gsutil help setmeta.

More details about specific pieces of metadata are discussed below.

Content-Type

The most commonly set metadata is Content-Type (also known as MIME type), which allows browsers to render the object properly. gsutil sets the Content-Type automatically at upload time, based on each filename extension. For example, uploading files with names ending in .txt will set Content-Type to text/plain. If you're running gsutil on Linux or MacOS and would prefer to have content type set based on naming plus content examination, see the use_magicfile configuration variable in the .boto configuration file (See also gsutil help config). In general, using use_magicfile is more robust and configurable, but is not available on Windows.

If you specify Content-Type with -h when uploading content (like the example gsutil command given in the previous section), it overrides the Content-Type that would have been set based on filename extension or content. This can be useful if the Content-Type detection algorithm doesn't work as desired for some of your files.

Cache-Control

Another commonly set piece of metadata is Cache-Control, which allows you to control whether and for how long browser and Internet caches are allowed to cache your objects. Cache-Control only applies to objects with a public-read ACL. Non-public data are not cacheable.

Here's an example of uploading a set of objects to allow caching:

gsutil -h "Cache-Control:public,max-age=3600" cp -a public-read \
       -r html gs://bucket/html

This command would upload all files in the html directory (and subdirectories) and make them publicly readable and cacheable, with cache expiration of one hour.

Note that if you allow caching, at download time you may see older versions of objects after uploading a newer replacement object. Note also that because objects can be cached at various places on the Internet there is no way to force a cached object to expire globally (unlike the way you can force your browser to refresh its cache). If you want to prevent caching of publicly readable objects you should set a Cache-Control:private header on the object. You can do this with a command such as:

gsutil -h Cache-Control:private cp -a public-read file.png gs://your-bucket

Another use of Cache-Control is through the "no-transform" value, which instructs Google Cloud Storage to not apply any content transformations based on specifics of a download request, such as removing gzip content-encoding for incompatible clients. Note that this parameter is only respected by the XML API. The Google Cloud Storage JSON API respects only the no-cache and max-age Cache-Control parameters.

For details about how to set the Cache-Control header see gsutil help setmeta.

Content-Encoding

You can specify a Content-Encoding to indicate that an object is compressed (for example, with gzip compression) while maintaining its Content-Type. You will need to ensure that the files have been compressed using the specified Content-Encoding before using gsutil to upload them. Consider the following example for Linux:

echo "Highly compressible text" | gzip > foo.txt
gsutil -h "Content-Encoding:gzip" \
       -h "Content-Type:text/plain" \
       cp foo.txt gs://bucket/compressed

Note that this is different from uploading a gzipped object foo.txt.gz with Content-Type: application/x-gzip because most browsers are able to dynamically decompress and process objects served with Content-Encoding: gzip based on the underlying Content-Type.

For compressible content, using Content-Encoding: gzip saves network and storage costs, and improves content serving performance. However, for content that is already inherently compressed (archives and many media formats, for instance) applying another level of compression via Content-Encoding is typically detrimental to both object size and performance and should be avoided.

Note also that gsutil provides an easy way to cause content to be compressed and stored with Content-Encoding: gzip: see the -z and -Z options in gsutil help cp.

Content-Disposition

You can set Content-Disposition on your objects, to specify presentation information about the data being transmitted. Here's an example:

gsutil -h 'Content-Disposition:attachment; filename=filename.ext' \
       cp -r attachments gs://bucket/attachments

Setting the Content-Disposition allows you to control presentation style of the content, for example determining whether an attachment should be automatically displayed vs should require some form of action from the user to open it. See https://tools.ietf.org/html/rfc6266 for more details about the meaning of Content-Disposition.

Custom Metadata

You can add your own custom metadata (e.g,. for use by your application) to a Google Cloud Storage object by using "x-goog-meta" with -h. For example:

gsutil -h x-goog-meta-reviewer:jane cp mycode.java gs://bucket/reviews

You can add multiple differently-named custom metadata fields to each object.

Settable Fields; Field Values

You can't set some metadata fields, such as ETag and Content-Length. The fields you can set are:

  • Cache-Control
  • Content-Disposition
  • Content-Encoding
  • Content-Language
  • Content-Type
  • Custom metadata

Field names are case-insensitive.

All fields and their values must consist only of ASCII characters, with the exception of values for x-goog-meta- fields, which may contain arbitrary Unicode values. Note that when setting metadata using the XML API, which sends custom metadata as HTTP headers, Unicode characters will be encoded using UTF-8, then url-encoded to ASCII. For example:

gsutil setmeta -h "x-goog-meta-foo: ã" gs://bucket/object

would store the custom metadata key-value pair of "foo" and "%C3%A3". Subsequently, running "ls -L" using the JSON API to list the object's metadata would print "%C3%A3", while "ls -L" using the XML API would url-decode this value automatically, printing the character "ã".

Viewing Currently Set Metadata

You can see what metadata is currently set on an object by using:

gsutil ls -L gs://the_bucket/the_object

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