# Divergencies between lakesuperior and FCREPO4 This is a (vastly incomplete) list of discrepancies between the current FCREPO4 implementation and LAKEsuperior. More will be added as more clients will use it. ## Not yet implemented (but in the plans) See [TODO](TODO) - Various header handling - Versioning - AuthN/Z - Fixity check - Blank nodes - Hash URIs ## Potentially breaking changes The following divergences may lead into incompatibilities with some clients. ### 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. ### 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. ### Web UI FCREPO4 includes a web UI for simple CRUD operations. Such a UI is not in the immediate LAKEsuperior development plans. However, a very basic UI may at some point be built for read-only interaction (i.e. requesting resources and clicking through relationship links). A more complete UI should be built for simple and/or SPARQL queries. ## Non-standard client breaking changes The following changes may be incompatible with clients relying on some FCREPO4 behavior not endorsed by LDP or other specifications. ### 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. ### Non-mandatory, non-authoritative slug in version POST FCREPO requires a `Slug` header to POST to `fcr:versions` to create a new version. LAKEsuperior adheres to the more general FCREPO POST rule and if no slug is provided, an automatic ID is generated instead. The ID is a UUID4. Note that internally this ID is not called "label" but "uid" since it is treated as a fully qualified identifier. The `fcrepo:hasVersionLabel` predicate, however ambiguous in this context, will be kept until the adoption of Memento, which will change the retrieval mechanisms. Also, if a POST is issued on the same resource `fcr:versions` location using a version ID that already exists, LAKEsuperior will just mint a random identifier rather than returning an error. ## Deprecation track LAKEsuperior offers some "legacy" options to replicate the FCREPO4 behavior, however encourages new development to use a different approach for some types of interaction. ### 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 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?) Clients are encouraged to omit LDP types in PUT, POST and PATCH requests. ### Lenient handling FCREPO4 requires server-managed triples to be expressly indicated in a PUT request, unless the `Prefer` header 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. Clients using the `Prefer` header to control PUT behavior as advertised by the specs should not notice any difference. ### 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. ## Optional improvements The following are improvements in performance or usability that can only taken advantage of if client code is adjusted. ### "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. ### Soft-delete and purge In FCREPO4 a deleted resource leaves a tombstone deleting all traces of the previous resource. In LAKEsuperior, a normal DELETE creates a new version snapshot of the resource and puts a tombstone in its place. The resource versions are still available in the `fcr:versions` location. The resource can be "resurrected" by issuing a POST to its tombstone. This will result in a `201`. If a tombstone is deleted, the resource and its versions are completely deleted (purged). Moreover, setting the `Prefer:no-tombstone` header option on DELETE allows to delete a resource and its versions directly without leaving a tombstone.