asettle 384b91eb82 Moving DLO functionality doc to the middleware code

This change moves the RST DLO documentation from
statically inside overview_large_objects.rst and moves it
to middleware/dlo.py.
This is where all middleware RST documentation is defined.

The overview_large_objects.rst is still the main page
for information on large objects, so now dynamically
points to both the DLO and SLO middleware RST
documentation and the relevant middleware.rst page
simply points to it.

Change-Id: I40d918c8b7bc608ab945805d69fe359521df038a
Closes-bug: #1276852

2015-10-13 10:35:46 +01:00

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Large Object Support

Overview

Swift has a limit on the size of a single uploaded object; by default this is 5GB. However, the download size of a single object is virtually unlimited with the concept of segmentation. Segments of the larger object are uploaded and a special manifest file is created that, when downloaded, sends all the segments concatenated as a single object. This also offers much greater upload speed with the possibility of parallel uploads of the segments.

Dynamic Large Objects

swift.common.middleware.dlo

Static Large Objects

swift.common.middleware.slo

Direct API

SLO support centers around the user generated manifest file. After the user has uploaded the segments into their account a manifest file needs to be built and uploaded. All object segments, except the last, must be above 1 MB (by default) in size. Please see the SLO docs for slo-doc further details.

Additional Notes

With a GET or HEAD of a manifest file, the X-Object-Manifest: <container>/<prefix> header will be returned with the concatenated object so you can tell where it's getting its segments from.
The response's Content-Length for a GET or HEAD on the manifest file will be the sum of all the segments in the <container>/<prefix> listing, dynamically. So, uploading additional segments after the manifest is created will cause the concatenated object to be that much larger; there's no need to recreate the manifest file.
The response's Content-Type for a GET or HEAD on the manifest will be 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.
The response's ETag for a GET or HEAD on the manifest file will be the MD5 sum of the concatenated string of ETags for each of the segments in the manifest (for DLO, from the listing <container>/<prefix>). Usually in Swift the ETag is the MD5 sum of the contents of the object, and that holds true for each segment independently. But it's not meaningful to generate such an ETag for the manifest itself so this method was chosen to at least offer change detection.

Note

If you are using the container sync feature you will need to ensure both your manifest file and your segment files are synced if they happen to be in different containers.

History

Dynamic large object support has gone through various iterations before settling on this implementation.

The primary factor driving the limitation of object size in swift is maintaining balance among the partitions of the ring. To maintain an even dispersion of disk usage throughout the cluster the obvious storage pattern was to simply split larger objects into smaller segments, which could then be glued together during a read.

Before the introduction of large object support some applications were already splitting their uploads into segments and re-assembling them on the client side after retrieving the individual pieces. This design allowed the client to support backup and archiving of large data sets, but was also frequently employed to improve performance or reduce errors due to network interruption. The major disadvantage of this method is that knowledge of the original partitioning scheme is required to properly reassemble the object, which is not practical for some use cases, such as CDN origination.

In order to eliminate any barrier to entry for clients wanting to store objects larger than 5GB, initially we also prototyped fully transparent support for large object uploads. A fully transparent implementation would support a larger max size by automatically splitting objects into segments during upload within the proxy without any changes to the client API. All segments were completely hidden from the client API.

This solution introduced a number of challenging failure conditions into the cluster, wouldn't provide the client with any option to do parallel uploads, and had no basis for a resume feature. The transparent implementation was deemed just too complex for the benefit.

The current "user manifest" design was chosen in order to provide a transparent download of large objects to the client and still provide the uploading client a clean API to support segmented uploads.

To meet an many use cases as possible swift supports two types of large object manifests. Dynamic and static large object manifests both support the same idea of allowing the user to upload many segments to be later downloaded as a single file.

Dynamic large objects rely on a container listing to provide the manifest. This has the advantage of allowing the user to add/removes segments from the manifest at any time. It has the disadvantage of relying on eventually consistent container listings. All three copies of the container dbs must be updated for a complete list to be guaranteed. Also, all segments must be in a single container, which can limit concurrent upload speed.

Static large objects rely on a user provided manifest file. A user can upload objects into multiple containers and then reference those objects (segments) in a self generated manifest file. Future GETs to that file will download the concatenation of the specified segments. This has the advantage of being able to immediately download the complete object once the manifest has been successfully PUT. Being able to upload segments into separate containers also improves concurrent upload speed. It has the disadvantage that the manifest is finalized once PUT. Any changes to it means it has to be replaced.

Between these two methods the user has great flexibility in how (s)he chooses to upload and retrieve large objects to swift. Swift does not, however, stop the user from harming themselves. In both cases the segments are deletable by the user at any time. If a segment was deleted by mistake, a dynamic large object, having no way of knowing it was ever there, would happily ignore the deleted file and the user will get an incomplete file. A static large object would, when failing to retrieve the object specified in the manifest, drop the connection and the user would receive partial results.

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