Conditional Sharing – Virtuoso ACL Groups Revisited


Previously we saw how ACLs can be used in Virtuoso to protect different types of resources. Today we will look into conditional groups which allow to share resources or grant permissions to a dynamic group of individuals. This means that we do not maintain a list of group members but instead define a set of conditions which an individual needs to fulfill in order to be part of the group in question.

That does sound very dry. Let’s just jump to an example:

@prefix oplacl: <http://www.openlinksw.com/ontology/acl#> .
[] a oplacl:ConditionalGroup ;
  foaf:name "People I know" ;
  oplacl:hasCondition [
    a oplacl:QueryCondition ;
    oplacl:hasQuery """ask where { graph <urn:my> { <urn:me> foaf:knows ^{uri}^ } }"""
  ] .

This group is based on a single condition which uses a simple SPARQL ASK query. The ask query contains a variable ^{uri}^ which the ACL engine will replace with the URI of the authenticated user. The group contains anyone who is in a foaf:knows relationship to urn:me in named graph urn:my. (Ideally the latter graph should be write-protected using ACLs as described before.)

Now we use this group in ACL rules. That means we first create it:

$ curl -X POST \
    --data-binary @group.ttl \
    -H"Content-Type: text/turtle" \
    -u dba:dba \
    http://localhost:8890/acl/groups

As a result we get a description of the newly created group which also contains its URI. Let’s imagine this URI is http://localhost:8890/acl/groups/1.

To mix things up we will use the group for sharing permission to access a service instead of files or named graphs. Like many of the Virtuoso-hosted services the URI Shortener is ACL controlled. We can restrict access to it using ACLs.

As always the URI Shortener has its own ACL scope which we need to enable for the ACL system to kick in:

sparql
prefix oplacl: <http://www.openlinksw.com/ontology/acl#>
with <urn:virtuoso:val:config>
delete {
  oplacl:DefaultRealm oplacl:hasDisabledAclScope <urn:virtuoso:val:scopes:curi> .
}
insert {
  oplacl:DefaultRealm oplacl:hasEnabledAclScope <urn:virtuoso:val:scopes:curi> .
};

Now we can go ahead and create our new ACL rule which allows anyone in our conditional group to shorten URLs:

[] a acl:Authorization ;
  oplacl:hasAccessMode oplacl:Write ;
  acl:accessTo <http://localhost:8890/c> ;
  acl:agent <http://localhost:8890/acl/groups/1> ;
  oplacl:hasScope <urn:virtuoso:val:scopes:curi> ;
  oplacl:hasRealm oplacl:DefaultRealm .

Finally we add one URI to the conditional group as follows:

sparql
insert into <urn:my> {
  <urn:me> foaf:knows <http://www.facebook.com/sebastian.trug> .
};

As a result my facebook account has access to the URL Shortener:
Virtuoso URI Shortener

The example we saw here uses a simple query to determine the members of the conditional group. These queries could get much more complex and multiple query conditions could be combined. In addition Virtuoso handles a set of non-query conditions (see also oplacl:GenericCondition). The most basic one being the following which matches any authenticated person:

[] a oplacl:ConditionalGroup ;
  foaf:name "Valid Identifiers" ;
  oplacl:hasCondition [
    a oplacl:GroupCondition, oplacl:GenericCondition ;
    oplacl:hasCriteria oplacl:NetID ;
    oplacl:hasComparator oplacl:IsNotNull ;
    oplacl:hasValue 1
  ] .

This shall be enough on conditional groups for today. There will be more playing around with ACLs in the future…

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