LAKEsuperior Content Model Rationale
====================================

Internal and Public URIs; Identifiers
-------------------------------------

Resource URIs are stored internally in LAKEsuperior as domain-agnostic
URIs with the scheme ``info:fcres<resource UID>``. This allows resources
to be portable across systems. E.g. a resource with an internal URI of
``info:fcres/a/b/c``, when accessed via the
``http://localhost:8000/ldp`` endpoint, will be found at
``http://localhost:8000/ldp/a/b/c``.

The resource UID making up the looks like a UNIX filesystem path,
i.e. it always starts with a forward slash and can be made up of
multiple segments separated by slashes. E.g. ``/`` is the root node UID,
``/a`` is a resource UID just below root. their internal URIs are
``info:fcres/`` and ``info:fcres/a`` respectively.

In the Python API, the UID and internal URI of an LDP resource can be
accessed via the ``uid`` and ``uri`` properties respectively:

::

    >>> import lakesuperior.env_setup
    >>> from lakesuperior.api import resource
    >>> rsrc = resource.get('/a/b/c')
    >>> rsrc.uid
    /a/b/c
    >>> rsrc.uri
    rdflib.terms.URIRef('info:fcres/a/b/c')

Store Layout
------------

One of the key concepts in LAKEsuperior is the store layout. This is a
module built with a specific purpose in mind, i.e. allowing fine-grained
recording of provenance metadata while providing reasonable performance.

Store layout modules could be replaceable (work needs to be done to
develop an interface to allow that). The default (and only at the
moment) layout shipped with LAKEsuperior is the :mod:`resource-centric
layout <lakesuperior.store.ldp_rs.rsrc_centric_layout>`. This
layout implements a so-called `graph-per-aspect
pattern <http://patterns.dataincubator.org/book/graph-per-aspect.html>`__
which stores different sets of statements about a resource in separate
named graphs.

The named graphs used for each resource are:

-  An admin graph (``info:fcsystem/graph/admin<resource UID>``) which
   stores administrative metadata, mostly server-managed triples such as
   LDP types, system create/update timestamps and agents, etc.
-  A structure graph (``info:fcsystem/graph/structure<resource UID>``)
   reserved for containment triples. The reason for this separation is
   purely convenience, since it makes it easy to retrieve all the
   properties of a large container without its child references.
-  One (and, possibly, in the future, more user\-defined) named graph for
   user-provided data (``info:fcsystem/graph/userdata/_main<resource UID>``).

Each of these graphs can be annotated with provenance metadata. The
layout decides which triples go in which graph based on the predicate or
RDF type contained in the triple. Adding logic to support arbitrary
named graphs based e.g. on user agent, or to add more provenance
information, should be relatively simple.
