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

    return handleRequest(headers, body, ui, Request.Type.GET,
        createExtensionVersionResource(extensionName, null));
  }
Service for extensions 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/ExtensionsService.java