Out-of-line links

It is often neater to separate an extended link from all of the resources it identifies. An out-of-line link is simply a link that has no local resource. While an out-of-line link may physically appear in-line, in the sense that it can be placed in the flow of text, this has no significance, and only makes the linking mechanism more difficult to find and maintain. A more obvious place to put out-of-line links is at the top of the document:

It is important to note that some documents may not contain any information on the resources they hold, as in 'document 2' in the example above, because their linked resources are identified in another document. The possible absence of linking markup in a participating document has interesting benefits. As already noted, a document that cannot be edited, perhaps because it resides on a remote system and is not owned by the link creator, can nevertheless be remotely provided with links, both to other parts of the same document, and to other documents.

However, there is also a disadvantage to this approach. A processor given 'document 2' would be unaware of the existence of the first document, and therefore of any of the links defined there. A mechanism called a link database (or 'linkbase') may be used to overcome this problem. In this scenario, all or most links reside in a separate 'linking file' (a 'database of links').

When a large number of internal or third-party arcs are needed, general management of the link markup is greatly assisted if all of this markup is placed in a single, separate data file, placed in a well-known location accessible to all potential users of the links defined within this file:

But there needs to be some mechanism for finding and selecting a linkbase file, as soon as a user decides to open one of the documents that contains some of the starting point resources. Although the draft standard does not constrain XLink-compatible applications in this respect, it does include a single technique that will often be appropriate. Each document that contains participating resources (in particular, starting-point resources) could include a link that targets the linkbase.

<simpleLink xlink:href="file://myLinkBase.xml" ... />

But the XLink application needs to be aware of the special purpose of this link, and not attempt to display the linkbase document to the user. The reserved Arcrole attribute value 'http://www.w3.org/1999/xlink/properties/linkbase' can be used to ensure that the link is interpreted correctly:

<simpleLink xlink:href="file://myLinkBase.xml"
    arcrole =
      "http://www.w3.org/1999/xlink/properties/linkbase" />

A linkbase file could also include further arcs to other linkbase files to achieve effective management of huge numbers of links that form natural sub-groupings, though an XLink-sensitive application may wish to set a limit to how many such arcs it will follow. The application must also watch for and avoid cyclical arcs between link bases, and ignore duplicate links.

Note that linking using this technique cannot start from a read-only document, as it would not contain the link to the link group file, unless the browser is also given the link group file previously.