lakesuperior_legacy/doc/notes/fcrepo4_deltas.md

Divergencies between lakesuperior and FCREPO4

Endpoints

The FCREPO root endpoint is /rest. The LAKEsuperior root endpoint is /ldp.

This should not pose a problem if a client does not have rest hard-coded in its code, but in any event, the /rest endpoint is provided for backwards compatibility.

LAKEsuperior adds the (currently stub) query endpoint. Other endpoints for non-LDP services may be opened in the future.

Automatic pairtree generation

A POST request without a slug in FCREPO4 results in a pairtree consisting of several intermediate nodes leading to the automatically minted identifier. E.g.

POST /rest

results in /rest/8c/9a/07/4e/8c9a074e-dda3-5256-ea30-eec2dd4fcf61 being created.

The same request in LAKEsuperior would create /rest/8c9a074e-dda3-5256-ea30-eec2dd4fcf61 (obviously the identifiers will be different).

Explicit intermediate paths

In FCREPO4, a PUT request to /rest/a/b/c, given /rest/a and rest/a/b not previously existing, results in the creation of Pairtree resources that are retrievable. In LAKEsuperior the same operation results only in the creation of containment triple in the graph store, which are not exposed in the LDP API. Therefore, a GET to rest/a in FCREPO4 will result in a 200, a GET to rest/a in LAKEsuperior in a 404.

In both above cases, PUTting into rest/a yields a 409, POSTing to it results in a 201.

Lenient handling

FCREPO4 requires server-managed triples to be expressly indicated in a PUT request, unless the Prefer heeader is set to handling=lenient; received="minimal", in which case the RDF payload must not have any server-managed triples.

LAKEsuperior works under the assumption that client should never provide server-managed triples. It automatically handles PUT requests sent to existing resources by returning a 412 if any server managed triples are included in the payload. This is the same as setting Prefer to handling=strict, which is the default.

If Prefer is set to handling=lenient, all server-managed triples sent with the payload are ignored.

"Include" and "Omit" options for children

LAKEsuperior offers an additional Prefer header option to exclude all references to child resources (i.e. by removing all the ldp:contains triples) while leaving the other server-managed triples when retrieving a resource:

Prefer: return=representation; [include | omit]="http://fedora.info/definitions/v4/repository#Children"

The default behavior is including all children URIs.

Automatic LDP class assignment

Since LAKEsuperior rejects client-provided server-managed triples, and since the LDP types are among them, the LDP container type is inferred from the provided properties: if the ldp:hasMemberRelation and ldp:membershipResource properties are provided, the resource is a Direct Container. If in addition to these the ldp:insertedContentRelation property is present, the resource is an Indirect Container. If any of the first two are missing, the resource is a Container (@TODO discuss: shall it be a Basic Container?)

LDP-NR metadata by content negotiation

FCREPO4 relies on the /fcr:metadata identifier to retrieve RDF metadata about an LDP-NR. LAKEsuperior supports this as a legacy option, but encourages the use of content negotiation to do the same. Any request to an LDP-NR with an Accept header set to one of the supported RDF serialization formats will yield the RDF metadata of the resource instead of the binary contents.

Tombstone methods

If a client requests a tombstone resource in FCREPO4 with a method other than DELETE, the server will return 405 Method Not Allowed regardless of whether the tombstone exists or not.

LAKEsuperior will return 405 only if the tombstone actually exists, 404 otherwise.

Atomicity

FCREPO4 supports batch atomic operations whereas a transaction can be opened and a number of operations (i.e. multiple R/W requests to the repository) can be performed. The operations are persisted in the repository only if and when the transaction is committed.

LAKesuperior only supports atomicity for a single LDP request. I.e. a single HTTTP request that should reult in multiple write operations to the storage layer is only persisted if no exception is thrown. Otherwise, the operation is rolled back in order to prevent resources to be left in an inconsistent state.

Web UI

FCREPO4 includes a web UI for simple CRUD operations.

Such a UI is not foreseen to be built in LAKEsuperior any time soon since the API interaction leaves a greater degree of flexibility. In addition, the underlying triplestore layer may provide a UI for complex RDF queries.