swift/doc/source/api/large_objects.rst

Large objects

By default, the content of an object cannot be greater than 5 GB. However, you can use a number of smaller objects to construct a large object. The large object is comprised of two types of objects:

• Segment objects store the object content. You can divide your content into segments, and upload each segment into its own segment object. Segment objects do not have any special features. You create, update, download, and delete segment objects just as you would normal objects.
• A manifest object links the segment objects into one logical large object. When you download a manifest object, Object Storage concatenates and returns the contents of the segment objects in the response body of the request. This behavior extends to the response headers returned by GET and HEAD requests. The Content-Length response header value is the total size of all segment objects. Object Storage calculates the ETag response header value by taking the ETag value of each segment, concatenating them together, and returning the MD5 checksum of the result. The manifest object types are:

  Static large objects

  The manifest object content is an ordered list of the names of the segment objects in JSON format.

  Dynamic large objects

  The manifest object has a X-Object-Manifest metadata header. The value of this header is {container}/{prefix}, where {container} is the name of the container where the segment objects are stored, and {prefix} is a string that all segment objects have in common. The manifest object should have no content. However, this is not enforced.

Note

If you make a COPY request by using a manifest object as the source, the new object is a normal, and not a segment, object. If the total size of the source segment objects exceeds 5 GB, the COPY request fails. However, you can make a duplicate of the manifest object and this new object can be larger than 5 GB.

Static large objects

To create a static large object, divide your content into pieces and create (upload) a segment object to contain each piece.

You must record the ETag response header that the PUT operation returns. Alternatively, you can calculate the MD5 checksum of the segment prior to uploading and include this in the ETag request header. This ensures that the upload cannot corrupt your data.

List the name of each segment object along with its size and MD5 checksum in order.

Create a manifest object. Include the ``?multipart-manifest=put`` query string at the end of the manifest object name to indicate that this is a manifest object.

The body of the PUT request on the manifest object comprises a json list, where each element contains the following attributes:

• path. The container and object name in the format: {container-name}/{object-name}
• etag. The MD5 checksum of the content of the segment object. This value must match the ETag of that object.
• size_bytes. The size of the segment object. This value must match the Content-Length of that object.

Example Static large object manifest list

This example shows three segment objects. You can use several containers and the object names do not have to conform to a specific pattern, in contrast to dynamic large objects.

[
    {
        "path": "mycontainer/objseg1",
        "etag": "0228c7926b8b642dfb29554cd1f00963",
        "size_bytes": 1468006
    },
    {
        "path": "mycontainer/pseudodir/seg-obj2",
        "etag": "5bfc9ea51a00b790717eeb934fb77b9b",
        "size_bytes": 1572864
    },
    {
        "path": "other-container/seg-final",
        "etag": "b9c3da507d2557c1ddc51f27c54bae51",
        "size_bytes": 256
    }
]

The Content-Length request header must contain the length of the json content—not the length of the segment objects. However, after the PUT operation completes, the Content-Length metadata is set to the total length of all the object segments. A similar situation applies to the ETag. If used in the PUT operation, it must contain the MD5 checksum of the json content. The ETag metadata value is then set to be the MD5 checksum of the concatenated ETag values of the object segments. You can also set the Content-Type request header and custom object metadata.

When the PUT operation sees the ``?multipart-manifest=put`` query parameter, it reads the request body and verifies that each segment object exists and that the sizes and ETags match. If there is a mismatch, the PUToperation fails.

If everything matches, the manifest object is created. The X-Static-Large-Object metadata is set to true indicating that this is a static object manifest.

Normally when you perform a GET operation on the manifest object, the response body contains the concatenated content of the segment objects. To download the manifest list, use the ``?multipart-manifest=get`` query parameter. The resulting list is not formatted the same as the manifest you originally used in the PUT operation.

If you use the DELETE operation on a manifest object, the manifest object is deleted. The segment objects are not affected. However, if you add the ``?multipart-manifest=delete`` query parameter, the segment objects are deleted and if all are successfully deleted, the manifest object is also deleted.

To change the manifest, use a PUT operation with the ``?multipart-manifest=put`` query parameter. This request creates a manifest object. You can also update the object metadata in the usual way.

Dynamic large objects

You must segment objects that are larger than 5 GB before you can upload them. You then upload the segment objects like you would any other object and create a dynamic large manifest object. The manifest object tells Object Storage how to find the segment objects that comprise the large object. The segments remain individually addressable, but retrieving the manifest object streams all the segments concatenated. There is no limit to the number of segments that can be a part of a single large object.

To ensure the download works correctly, you must upload all the object segments to the same container and ensure that each object name is prefixed in such a way that it sorts in the order in which it should be concatenated. You also create and upload a manifest file. The manifest file is a zero-byte file with the extra X-Object-Manifest {container}/{prefix} header, where {container} is the container the object segments are in and {prefix} is the common prefix for all the segments. You must UTF-8-encode and then URL-encode the container and common prefix in the X-Object-Manifest header.

It is best to upload all the segments first and then create or update the manifest. With this method, the full object is not available for downloading until the upload is complete. Also, you can upload a new set of segments to a second location and update the manifest to point to this new location. During the upload of the new segments, the original manifest is still available to download the first set of segments.

Example Upload segment of large object request: HTTP

PUT /{api_version}/{account}/{container}/{object} HTTP/1.1
Host: storage.clouddrive.com
X-Auth-Token: eaaafd18-0fed-4b3a-81b4-663c99ec1cbb
ETag: 8a964ee2a5e88be344f36c22562a6486
Content-Length: 1
X-Object-Meta-PIN: 1234

No response body is returned. A status code of 2``nn`` (between 200 and 299, inclusive) indicates a successful write; status 411 Length Required denotes a missing Content-Length or Content-Type header in the request. If the MD5 checksum of the data written to the storage system does NOT match the (optionally) supplied ETag value, a 422 Unprocessable Entity response is returned.

You can continue uploading segments like this example shows, prior to uploading the manifest.

Example Upload next segment of large object request: HTTP

PUT /{api_version}/{account}/{container}/{object} HTTP/1.1
Host: storage.clouddrive.com
X-Auth-Token: eaaafd18-0fed-4b3a-81b4-663c99ec1cbb
ETag: 8a964ee2a5e88be344f36c22562a6486
Content-Length: 1
X-Object-Meta-PIN: 1234

Next, upload the manifest you created that indicates the container the object segments reside within. Note that uploading additional segments after the manifest is created causes the concatenated object to be that much larger but you do not need to recreate the manifest file for subsequent additional segments.

Example Upload manifest request: HTTP

PUT /{api_version}/{account}/{container}/{object} HTTP/1.1
Host: storage.clouddrive.com
X-Auth-Token: eaaafd18-0fed-4b3a-81b4-663c99ec1cbb
Content-Length: 0
X-Object-Meta-PIN: 1234
X-Object-Manifest: {container}/{prefix}

Example Upload manifest response: HTTP

[...]

The Content-Type in the response for a GET or HEAD on the manifest is the same as the Content-Type set during the PUT request that created the manifest. You can easily change the Content-Type by reissuing the PUT request.

Comparison of static and dynamic large objects

While static and dynamic objects have similar behavior, here are their differences:

Comparing static and dynamic large objects

Static large object: Assured end-to-end integrity. The list of segments includes the MD5 checksum (ETag) of each segment. You cannot upload the manifest object if the ETag in the list differs from the uploaded segment object. If a segment is somehow lost, an attempt to download the manifest object results in an error. You must upload the segment objects before you upload the manifest object. You cannot add or remove segment objects from the manifest. However, you can create a completely new manifest object of the same name with a different manifest list.

With static large objects, you can upload new segment objects or remove existing segments. The names must simply match the {prefix} supplied in X-Object-Manifest. The segment objects must be at least 1 MB in size (by default). The final segment object can be any size. At most, 1000 segments are supported (by default). The manifest list includes the container name of each object. Segment objects can be in different containers.

Dynamic large object: End-to-end integrity is not guaranteed. The eventual consistency model means that although you have uploaded a segment object, it might not appear in the container listing until later. If you download the manifest before it appears in the container, it does not form part of the content returned in response to a GET request.

With dynamic large objects, you can upload manifest and segment objects in any order. In case a premature download of the manifest occurs, we recommend users upload the manifest object after the segments. However, the system does not enforce the order. Segment objects can be any size. All segment objects must be in the same container.

Manifest object metadata

For static large objects, the object has X-Static-Large-Object set to true. You do not set this metadata directly. Instead the system sets it when you PUT a static manifest object.

For dynamic object,s the X-Object-Manifest value is the {container}/{prefix}, which indicates where the segment objects are located. You supply this request header in the PUT operation.

Copying the manifest object

With static large objects, you include the ``?multipart-manifest=get`` query string in the COPY request. The new object contains the same manifest as the original. The segment objects are not copied. Instead, both the original and new manifest objects share the same set of segment objects.

When creating dynamic large objects, the COPY operation does not create a manifest object but a normal object with content same as what you would get on a GET request to original manifest object.

To duplicate a manifest object: * Use the GET operation to read the value of X-Object-Manifest and use this value in the X-Object-Manifest request header in a PUT operation. * Alternatively, you can include ``?multipart-manifest=get`` query string in the COPY request. This creates a new manifest object that shares the same set of segment objects as the original manifest object.