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When a resource is put under version control, it becomes a "versioned resource". Many servers protect versioned resources from modifications by considering them "checked in", and by requiring a "checkout" operation before modification, and a "checkin" operation to get back to the "checked-in" state. Other servers allow modification, in which case the checkout/checkin operation may happen implicitly.
When a versioned resource is checked out and then subsequently checked in, the version that was checked out becomes a "predecessor" of the version created by the checkin. A client can specify multiple predecessors for a new version if the new version is logically a merge of those predecessors. The inverse of the predecessor relation is the "successor" relation. Therefore, if X is a predecessor of Y, then Y is a successor of X.
When included on a versioned resource, this link points to a resource containing the latest (e.g., current) version.¶
The latest version is defined by the system. For linear versioning systems, this is probably the latest version by timestamp. For systems that support branching, there will be multiple latest versions, one for each branch in the version history.¶
Some systems may allow multiple of these link relations.¶
In general this mechanism seems sound but I'm not sure that
the security considerations are entirely adequate. This
mechanism lets you learn information about other versions
of a resource even if you potentially don't have permission
to view them directly. Consider a limiting case where each
version of the resource had a name that contained the
change set for that resource. E.g.,
In this case, seeing other parts of the version tree leaks
information about those versions. I don't think that this
is a problem for the draft, but it might be useful to
mention that this feature has implications for name
Automated agents should take care when these relations cross administrative domains (e.g., the URI has a different authority than the current document). Such agents should also take care to detect circular references.¶
WebDAV: for version-controlled resources, DAV:checked-in ([RFC3253], Section 3.2.1) or DAV:checked-out ([RFC3253], Section 3.3.1), depending on checkin state. For version resources, a successor version that itself does not have any successors.
JCR: the version node identified by the jcr:baseVersion property ([JSR-283], Section 188.8.131.52) for versionable nodes; for version nodes, a successor version that itself does not have any successors.
WebDAV: for version-controlled resources that are checked-out in place: the resource itself. For version resources: each resource identified by a member of the DAV:checkout-set property (see [RFC3253], Section 3.4.3).
JCR: for checked-out versionable nodes: the node itself.
The "Web Linking" specification ([draft-nottingham-http-link-header]) generalizes Atom link relations, and also re-introduces the HTTP "Link" header as a way to expose link relations in HTTP responses. This will make it possible to expose version links independently from a specific vocabulary, be it the Atom Feed Format ([RFC4287]) or WebDAV properties ([RFC3253]).¶
For instance, a response to an VERSION-CONTROL request ([RFC3253], Section 3.5) could expose newly created version-history and checked-in version as link relations:¶