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<h1 id="a-users-roles-privileges-scheme-using-graphs">A Users, Roles & Privileges Scheme Using Graphs<a class="headerlink" href="#a-users-roles-privileges-scheme-using-graphs" title="Permanent link">¶</a></h1>
<p>The basic elements:</p>
<ul>
<li>Every agent that can interact with a system is represented by a <strong>user</strong>.</li>
<li>Every capability the system has is authorized by a distinct <strong>privilege</strong>.</li>
<li>Each user has a list of zero or more <strong>roles</strong>.<ul>
<li>Roles can <strong>imply</strong> further roles. This relationship is transitive: if role A implies role B, then a member of role A is a member of role B; if role B also implies role C, then a member of role A is also a member of role C. It helps if the resulting role graph is acyclic, but it's not necessary.</li>
<li>Roles can <strong>grant</strong> privileges.</li>
</ul>
</li>
</ul>
<p>A user's privileges are the union of the privileges granted by the transitive closure of their roles.</p>
<pre><code class="sql">create table "user" (
username varchar
primary key
-- credentials &c
);
create table role (
name varchar
primary key
);
create table role_member (
role varchar
not null
references role,
member varchar
not null
references "user",
primary key (role, member)
);
create table role_implies (
role varchar
not null
references role,
implied_role varchar
not null
);
create table privilege (
privilege varchar
primary key
);
create table role_grants (
role varchar
not null
references role,
privilege varchar
not null
references privilege,
primary key (role, privilege)
);
</code></pre>
<p>If your database supports recursive CTEs, this schema can be queried in one shot, since we can have the database do all the graph-walking along roles:</p>
<pre><code class="sql">with recursive user_roles (role) AS (
select
role
from
role_member
where
member = 'SOME USERNAME'
union
select
implied_role as role
from
user_roles
join role_implies on
user_roles.role = role_implies.role
)
select distinct
role_grants.privilege as privilege
from
user_roles
join role_grants on
user_roles.role = role_grants.role
order by privilege;
</code></pre>
<p>If not, you'll need to pull the entire graph into memory and manipulate it there: this schema doesn't give you any easy handles to identify only the roles transitively included in the role of interest, and repeatedly querying for each step of the graph requires an IO roundtrip at each step, burning whole milliseconds along the way.</p>
<p>Realistic use cases should have fairly simple graphs: elemental privileges are grouped into concrete roles, which are in turn grouped into abstracted roles (by department, for example), which are in turn granted to users. If the average user is in tens of roles and has hundreds of privileges, the entire dataset fits in memory, and PostgreSQL performs well. In PostgreSQL, the above schema handles ~10k privileges and ~10k roles with randomly-generated graph relationships in around 100ms on my laptop, which is pretty slow but not intolerable. Perverse cases (interconnected total subgraphs, deeply-nested linear graphs) can take absurd time but do not reflect any likely permissions scheme.</p></div>
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