catapult/third_party/gsutil/gslib/addlhelp/metadata.py

# -*- coding: utf-8 -*-
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#     http://www.apache.org/licenses/LICENSE-2.0
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"""Additional help about object metadata."""

from __future__ import absolute_import

from gslib.help_provider import HelpProvider

_DETAILED_HELP_TEXT = ("""
<B>OVERVIEW OF METADATA</B>
  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 headers 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.


<B>CONTENT TYPE</B>
  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 gsutil/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 a Content-Type header 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.

  You can also completely suppress content type detection in gsutil, by
  specifying an empty string on the Content-Type header:

    gsutil -h 'Content-Type:' cp -r images gs://bucket/images

  In this case, the Google Cloud Storage service will not attempt to detect
  the content type. In general this approach will work better than using
  filename extension-based content detection in gsutil, because the list of
  filename extensions is kept more current in the server-side content detection
  system than in the Python library upon which gsutil content type detection
  depends. (For example, at the time of writing this, the filename extension
  ".webp" was recognized by the server-side content detection system, but
  not by gsutil.)


<B>CACHE-CONTROL</B>
  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 an object set 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).

  Another use of the Cache-Control header 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.

  Note that if you upload an object with a public-read ACL and don't include a
  Cache-Control header, it will be served with a Cache-Control header allowing
  the object to be cached for 3600 seconds. This will not happen if the object
  is uploaded with a non-public ACL and then changed to public. Moreover, if you
  upload an object with a public-read ACL and later change the ACL not to be
  public-read, the object will no longer be served with the default
  Cache-Control header noted above (so will be served as not cacheable).
  
  For details about how to set the Cache-Control header see
  "gsutil help setmeta".


<B>CONTENT-ENCODING</B>
  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 option in "gsutil help cp".


<B>CONTENT-DISPOSITION</B>
  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 http://www.w3.org/Protocols/rfc2616/rfc2616-sec19.html#sec19.5.1
  for more details about the meaning of Content-Disposition.


<B>CUSTOM METADATA</B>
  You can add your own custom metadata (e.g,. for use by your application)
  to an object by setting a header that starts with "x-goog-meta", 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.


<B>SETTABLE FIELDS; FIELD VALUES</B>
  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-MD5
  - Content-Type
  - Any field starting with a matching Cloud Storage Provider
    prefix, such as x-goog-meta- (i.e., custom metadata).

  Header names are case-insensitive.

  x-goog-meta- fields can have data set to arbitrary Unicode values. All
  other fields must have ASCII values.


<B>VIEWING CURRENTLY SET METADATA</B>
  You can see what metadata is currently set on an object by using:

    gsutil ls -L gs://the_bucket/the_object
""")


class CommandOptions(HelpProvider):
  """Additional help about object metadata."""

  # Help specification. See help_provider.py for documentation.
  help_spec = HelpProvider.HelpSpec(
      help_name='metadata',
      help_name_aliases=[
          'cache-control', 'caching', 'content type', 'mime type', 'mime',
          'type'],
      help_type='additional_help',
      help_one_line_summary='Working With Object Metadata',
      help_text=_DETAILED_HELP_TEXT,
      subcommand_help_text={},
  )