Certain resources used in a Revit model are stored outside of the .rvt file. For example, the data used for keynotes, images used as decals during rendering, CAD links, and Revit links are all stored outside the model. Creating a new implementation of this server allows the server to supply one or more types of such resources from an arbitrary source. For example, a server could provide the keynote data from a database or from a file format that Revit does not support.

If a model references resources supplied by this server, Revit will request the resource from the server when it is required. Most external resources are loaded into memory at the time the model is loaded. The server will also be invoked if the resource is explicitly reloaded.

IExternalResourceServer can declare that a resource is already up-to-date via GetResourceVersionStatus(ExternalResourceReference) If the resource is up-to-date, Revit will skip loading to improve performance.

Each resource load request will be associated with a GUID, so that server implementers can uniquely identify a given load request. This may be useful to, for example, store server-side errors associated with an attempt to load a particular resource.

If your server handles Revit links, you must take special care with link paths. When a Revit document is uploaded to your server, any locally-pathed linked documents should be brought to the server along with the host document. In addition, the ExternalResourceReferences for these links will need to be modified in the host document. The host document should reference the Revit links at their server locations, not their local file locations. Revit may not be able to find links if the paths are not set up correctly. See [!:Autodesk::Revit::DB::TransmissionData::ReadTransmissionData] to inspect the set of links contained within a Revit model. See [!:Autodesk::Revit::DB::RevitLinkType::LoadFrom] to reload a link from a server version.

Here is an example of a case which uses nested links: A user has a Revit model containing one link, Link.rvt, which contains one nested link, Nest.rvt. The user uploads Link.rvt to a server, using an add-in provided by that server. The server provider must also take Nest.rvt. Further, the server provider must open Link.rvt and modify the reference to Nest.rvt so that it references the version on the server. Otherwise, Revit will not be able to find Nest.rvt when another user tries to load Link.rvt from the server.

The external resource framework has been designed to allow server authors to display UI related to the resource load operation and UI browse operation. No UI should be displayed directly from an IExternalResourceServer. Instead, developers should create an IExternalResourceUIServer which will handle UI tasks on behalf of the IExternalResourceServer. For more information, see the documentation for the LoadResource(Guid, ExternalResourceType, ExternalResourceReference, ExternalResourceLoadContext, ExternalResourceLoadContent) and SetupBrowserData(ExternalResourceBrowserData) methods.