The Federation SAIL allows multiple datasets to be virtually combined into a single dataset. The Federation SAIL combines multiple RDF stores that may exist on a remote server or are embedded in the same JVM. The Federation distributes sections of the query to different members based on the data contained in each of the members. These results are then joined together within the federation to provide the same result as if all the data was co-located within a single store.
The federation has a complex configuration, when compared to other repositories, and requires its own configuration template. A configuration template, like the one shown in Figure 10, should be placed in a "templates" directory within the console's data directory (~/.aduna/openrdf-sesame-console/templates/bsbm-federation.ttl). A federation can then be created using the create command and the file name without the .ttl suffix.
Figure 10. BSBM Federation
#
# Sesame configuration template for three federated repositories
#
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>.
@prefix rep: <http://www.openrdf.org/config/repository#>.
@prefix sr: <http://www.openrdf.org/config/repository/sail#>.
@prefix sail: <http://www.openrdf.org/config/sail#>.
@prefix sparql: <http://www.openrdf.org/config/repository/sparql#>.
@prefix fed: <http://www.openrdf.org/config/sail/federation#>.
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix foaf: <http://xmlns.com/foaf/0.1/> .
@prefix dc: <http://purl.org/dc/elements/1.1/> .
@prefix rev: <http://purl.org/stuff/rev#> .
@prefix bsbm: <http://www4.wiwiss.fu-berlin.de/bizer/bsbm/v01/vocabulary/> .
@prefix bsbm-inst: <http://www4.wiwiss.fu-berlin.de/bizer/bsbm/v01/instances/> .
[] a rep:Repository ;
rep:repositoryID "{%Repository ID|bsbm-federation%}" ;
rdfs:label "{%Repository title|BSBM Federation%}" ;
rep:repositoryImpl [
rep:repositoryType "openrdf:SailRepository" ;
sr:sailImpl [
sail:sailType "openrdf:Federation" ;
fed:distinct true ;
fed:localPropertySpace rdf: ;
fed:localPropertySpace rdfs: ;
fed:localPropertySpace foaf: ;
fed:localPropertySpace dc: ;
fed:localPropertySpace rev: ;
fed:localPropertySpace bsbm: ;
fed:member [
rep:repositoryType "openrdf:SPARQLRepository" ;
sparql:endpoint <http://localhost:8080/openrdf-sesame/repositories/producers> ;
sparql:subjectSpace bsbm: ;
sparql:subjectSpace bsbm-inst:Product ;
sparql:subjectSpace bsbm-inst:dataFromProducer
];
fed:member [
rep:repositoryType "openrdf:SPARQLRepository" ;
sparql:endpoint <http://localhost:8080/openrdf-sesame/repositories/vendors> ;
sparql:subjectSpace bsbm-inst:dataFromVendor
];
fed:member [
rep:repositoryType "openrdf:SPARQLRepository" ;
sparql:endpoint <http://localhost:8080/openrdf-sesame/repositories/ratings> ;
sparql:subjectSpace bsbm-inst:dataFromRatingSite
]
]
].
Perhaps the most significant option is the fed:distinct option. When absent, the federation passes the results from multiple members through a distinct filter. By setting this option to true, this step is avoid and can greatly improve performance, but may result in duplicate results if some member include duplicate quads (the same subject, predicate, object, and context combination appears in multiple members).
The fed:localPropertySpace option is the namespaces of predicates used in statements that are stored in the same member for the same subject. For the BSBM datasets, this is all predicates, because each subject is only described in one member. The effect of this option is that if multiple basic graph patterns use predicates that start with one of these values and have the same subject variable, they will only be joined within the same member. If the data is partitioned by predicates (every predicate appears in at most one member) this option has no effect.
The sparql:subjectSpace is an SPARQLRepository optimization that is used by the client to indicate what subject URI prefixes are located on the server. This option reduces bandwidth by allowing the client to predetermine if a statement might exist on the server without connecting to it remotely.
The Federation configuration above, uses the SPARQL protocol exclusively to communicate with its members. It would work with any SPARQL compliant endpoint and therefore is also read only.