@GET @ApiIgnore // until documented
  @Path("{linkId}")
  @Produces("text/plain")
  public Response getExtensionLink(String body, @Context HttpHeaders headers,
                                  @Context UriInfo ui, @PathParam("linkId") String linkId) {

    return handleRequest(headers, body, ui, Request.Type.GET, createExtensionLinkResource(linkId));
  }
Service for extension link management.

An extension version is like a stack version but it contains custom services.  Linking an extension
version to the current stack version allows the cluster to install the custom services contained in
the extension version.
ambari-server/src/main/java/org/apache/ambari/server/api/services/ExtensionLinksService.java