This page discusses the metadata fields that are stored along with objects in Cloud Storage. To learn how to view and set metadata on objects, see Viewing and Editing Object Metadata.
Introduction
Objects stored in Cloud Storage have metadata associated with them. Metadata
identifies properties of the object, as well as specifies how the
object should be handled when it's accessed. Metadata exists as key:value
pairs. For example, the storage class of an object is represented
by the metadata entry storageClass:REGIONAL. storageClass is the key
for the metadata, and all objects have such a key associated with them.
REGIONAL specifies the value this specific object has, and the value varies
from object to object.
The mutability of metadata varies: some metadata you can edit at any time, some
metadata you can only set at the time the object is created, and some metadata
you can only view. For example, you can edit the value of the Cache-Control
metadata at any time, but you can only assign the storageClass metadata when
the object is created or rewritten, and you cannot directly edit the value
for the generation metadata, though the generation value changes when
the object is overwritten.
Editable metadata
There are two categories of metadata that users can change for objects:
Fixed-key metadata: Metadata whose keys are set, but for which you can specify a value.
Custom metadata: Metadata that you add by specifying both a key and a value associated with the key.
When editing metadata, you should generally avoid non-ascii characters, because they are not permitted in HTTP headers, which the XML API uses. When using the XML API, there is also a 16KB limit to the combined size of the request URL and HTTP headers, so the total size of your metadata should take this limit into account.
Fixed-key metadata
You can edit the following metadata for objects, though you must have sufficient permission to do so:
- Access control metadata
- Cache-Control
- Content-Disposition
- Content-Encoding
- Content-Language
- Content-Type
Access control metadata
Cloud Storage uses Identity and Access Management (IAM) and Access Control Lists (ACLs) to control access to objects. Use these links to learn about these access control methods and associated metadata.
Cache-Control
The Cache-Control metadata can specify two different aspects
of how data is served from Cloud Storage: whether the data can be cached
and whether the data can be transformed.
Caching data
The Cache-Control metadata allows you to control whether and for how long
browser and Internet caches are allowed to cache your objects. Setting the
value of Cache-Control to no-cache means the object should not be cached,
while setting the value to max-age=[TIME_IN_SECONDS] means the object can
be cached for up to specified length of time.
Cache-Control only applies to objects that are publicly accessible,
because non-public data are not cacheable. Unless otherwise specified, the
Cache-Control setting for publicly accessible objects is 3600 seconds.
If you allow caching, at download time you might see older versions of
objects, even after uploading a newer replacement object, because the older
objects remain in the cache for a period of time. Additionally, because objects
can be cached at various places on the Internet, there is no way to force a
cached object to expire globally. If you want to prevent caching of publicly
readable objects, you should set Cache-Control:private on the object.
Transforming data
Cache-Control metadata also allows you to serve objects as they are stored,
without applying any transformations to the data, such as removing gzip
content-encoding for incompatible clients. To serve an object as-is, set
Cache-Control:no-transform.
Content-Disposition
The Content-Disposition metadata specifies presentation
information about the data being transmitted. Setting Content-Disposition
allows you to control presentation style of the content, for example
determining whether an attachment should be automatically displayed or whether
some form of action from the user should be required to open it. See
https://tools.ietf.org/html/rfc6266 for the Content-Disposition
specification.
Content-Encoding
The Content-Encoding metadata can be used to indicate that an object is
compressed, while still maintaining the object's underlying Content-Type.
For example, a text file that is gzip compressed can have the fact that
it's a text file indicated in Content-Type and the fact that it's
gzip compressed indicated in Content-Encoding. You should ensure that
files are, in fact, compressed using the specified Content-Encoding before
uploading them, or else unexpected behavior can occur when attempting to
download the objects. For more information, see the Transcoding page.
For compressible content, such as text, using Content-Encoding: gzip saves
network and storage costs and improves content serving performance. However,
for content that is already inherently compressed, such as archives and many
media formats, applying another level of compression and marking it in the
Content-Encoding metadata is typically detrimental to both object size and
performance and should be avoided.
Content-Language
The Content-Language metadata indicates the language(s) that
the object is intended for. Refer to ISO 639-1 language codes for the
supported values of this metadata.
Content-Type
The most commonly set metadata is Content-Type (also known as media type),
which allows browsers to render the object properly. All objects have
a value specified in their Content-Type metadata, but this value does not
have to match the underlying type of the object. For example, if the
Content-Type is not specified by the uploader and cannot be determined,
it is set to application/octet-stream or
application/x-www-form-urlencoded, depending on how you uploaded the object.
Refer to the IANA Media Types page for a list of valid content types.
Custom metadata
Custom metadata is metadata that you can add and remove. To create custom
metadata, you specify both a value and a key. Once you have created a
custom metadata key:value pair, you can delete the key or change the value.
The Viewing and Editing Metadata page includes information on setting custom metadata. Note that using custom metadata incurs storage and network costs.
Non-editable metadata
Some metadata cannot be edited directly. This metadata is set at the time of object creation or rewrite. As part of the object creation or rewrite, you can set some such metadata, such as the storage class of the object or customer-supplied encryption keys. Other metadata is automatically added and can only be viewed, such the generation number of the object or the time of creation.
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