This page discusses the conversion of files to and from a gzip-compressed state. The page includes an overview of transcoding, best practices for working with associated metadata, and compressed file behavior in Cloud Storage.
Transcoding and gzip
gzip is a form of data compression: it typically reduces the size of a file. This allows the file to be transferred faster and stored using less space than if it were not compressed. Compressing a file can reduce both cost and transfer time. Transcoding, in Cloud Storage, is the automatic changing of a file's compression before it's served to a requester. When transcoding results in a file becoming gzip-compressed, it can be considered compressive, whereas when the result is a file that is no longer gzip-compressed, it can be considered decompressive. Cloud Storage supports the decompressive form of transcoding.
If files are stored as gzip-compressed objects on Cloud Storage, they can be automatically decompressed before being sent to a requester, resulting in a file that is identity encoded (that is, not compressed). This allows for reduced storage costs for the object within Cloud Storage, but gives the requester the file itself, without any compression. This is useful, for example, when serving files to customers.
In order to be eligible for decompressive transcoding, an object must meet two criteria:
The file is gzip-compressed when stored in Cloud Storage.
The associated metadata includes
When an object meets these two criteria, it undergoes decompressive transcoding
when requested, in which case it is also served without a
header. If you want an object that meets both criteria to be
served in its compressed state (for example, to reduce egress cost or time),
there are two ways to prevent decompressive transcoding from occurring:
If the request for the object includes an
Accept-Encoding: gzipheader, the object is served as-is in that specific request, along with a
Content-Encoding: gzipresponse header.
Cache-Controlmetadata field for the object is set to
no-transform, the object is served as a compressed object in all subsequent requests, regardless of any
Content-Type vs. Content-Encoding
There are several behaviors that you should be aware of concerning how
Content-Encoding relate to transcoding. Both are metadata
stored along with an object. See Viewing and Editing Object Metadata for
step-by-step instructions on how to add metadata to objects.
Content-Type should be included in all uploads and indicates the type
of object being uploaded. For example:
indicates that the uploaded object is a plain-text file. While there is no check
to guarantee the specified
Content-Type matches the true nature of an
uploaded object, incorrectly specifying its type will at best cause requesters
to receive something other than what they were expecting and could lead to
Content-Encoding is optional and can, if desired, be included in
the upload of files that are compressed. For example:
indicates that the uploaded object is gzip-compressed. As with
Content-Type, there is no check to guarantee the specified
is actually applied to the uploaded object, and incorrectly specifying an
object's encoding could lead to unintended behavior on subsequent download
When uploading a gzip-compressed object, the recommended way to set your metadata is to specify both the
Content-Encoding. For example, for a compressed, plain-text file:
Content-Type: text/plain Content-Encoding: gzip
This gives the most information about the state of the object to anyone accessing it. Doing so also makes the object eligible for decompressive transcoding when it is later downloaded, allowing client applications to handle the semantics of the
Alternatively, you can upload the object with the
Content-Typeset to indicate compression and NO
Content-Encodingat all. For example:
However, in this case the only thing immediately known about the object is that it is gzip-compressed, with no information regarding the underlying object type. Moreover, the object is not eligible for decompressive transcoding.
While it is possible to do so, a file that is gzip-compressed should not be uploaded with the compressed nature of the file omitted. For example, for a gzip-compressed plain-text file, you should avoid only setting
Content-Type: text/plain. Doing so misrepresents the state of the object as it will be delivered to a requester.
Similarly, objects should not be uploaded with an omitted
Content-Type, even if a
Content-Encodingis included. Doing so may result in
Content-Typebeing set to a default value, but may result in the request being rejected, depending on how the upload is made.
You should not set your metadata to redundantly report the compression of the object:
Content-Type: application/gzip Content-Encoding: gzip
This implies you are uploading a gzip-compressed object that has been gzip-compressed a second time, when that is not usually the case (if you actually plan to doubly compress a file, please see the using gzip on compressed objects section below). When decompressive transcoding occurs on such an incorrectly reported object, the object is served identity encoded, but requesters think that they have received an object which still has a layer of compression associated with it. Attempts to decompress the object will fail.
Similarly, a file that is not gzip-compressed should not be uploaded with the
Content-Encoding: gzip. Doing so makes the object appear to be eligible for transcoding, but when requests for the object are made, attempts at transcoding fail.
Using gzip on compressed objects
Some objects, such as many video, audio, and image files, not to mention gzip files themselves, are already compressed. Using gzip on such objects offers virtually no benefit: in almost all cases, doing so makes the object larger due to gzip overhead. For this reason, using gzip on compressed content is generally discouraged and may cause undesired behaviors.
For example, while Cloud Storage allows "doubly compressed" objects (that is,
objects that are gzip-compressed but also have an underlying Content-Type that is
itself compressed) to be uploaded and stored, it does not allow objects to be
served in a doubly compressed state unless their
Cache-Control metadata includes
no-transform. Instead, it removes the outer, gzip, level of compression,
Content-Encoding response header, and serves the
resulting object. This occurs even for requests with
Using the Range header
When transcoding occurs, if the request for the object includes a
header, that header is silently ignored. This means that requests for partial
content are not fulfilled, and the response instead serves the entire
requested object. This behavior arises because it is not possible to select
a range from a compressed file without first decompressing the file in its
entirety: each request for part of a file would be accompanied
by the decompression of the entire, potentially large, file, which would
poorly utilize resources. You should be aware of this behavior and avoid
Range header when using transcoding, as charges are incurred
for the transmission of the entire object and not just the range requested.
For more information on allowed response behavior to requests with
headers, see the specification.
If requests with
Range headers are needed, you should ensure that transcoding
does not occur for the requested object. You can achieve this by choosing
the appropriate properties when uploading objects to begin with. For example,
range requests for objects with
Content-Type: application/gzip and no
Content-Encoding are performed as requested.