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Row-level security in Azure Cosmos DB for PostgreSQL

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APPLIES TO: Azure Cosmos DB for PostgreSQL (powered by the Citus database extension to PostgreSQL)

PostgreSQL row-level security policies restrict which users can modify or access which table rows. Row-level security can be especially useful in a multi-tenant cluster. It allows individual tenants to have full SQL access to the database while hiding each tenant’s information from other tenants.

Implementing for multi-tenant apps

We can implement the separation of tenant data by using a naming convention for database roles that ties into table row-level security policies. We’ll assign each tenant a database role in a numbered sequence: tenant1, tenant2, etc. Tenants will connect to Azure Cosmos DB for PostgreSQL using these separate roles. Row-level security policies can compare the role name to values in the tenant_id distribution column to decide whether to allow access.

Here's how to apply the approach on a simplified events table distributed by tenant_id. First create the roles tenant1 and tenant2. Then run the following SQL commands as the citus administrator user:

CREATE TABLE events(
  tenant_id int,
  id int,
  type text
);

SELECT create_distributed_table('events','tenant_id');

INSERT INTO events VALUES (1,1,'foo'), (2,2,'bar');

-- assumes that roles tenant1 and tenant2 exist
GRANT select, update, insert, delete
  ON events TO tenant1, tenant2;

As it stands, anyone with select permissions for this table can see both rows. Users from either tenant can see and update the row of the other tenant. We can solve the data leak with row-level table security policies.

Each policy consists of two clauses: USING and WITH CHECK. When a user tries to read or write rows, the database evaluates each row against these clauses. PostgreSQL checks existing table rows against the expression specified in the USING clause, and rows that would be created via INSERT or UPDATE against the WITH CHECK clause.

-- first a policy for the system admin "citus" user
CREATE POLICY admin_all ON events
  TO citus           -- apply to this role
  USING (true)       -- read any existing row
  WITH CHECK (true); -- insert or update any row

-- next a policy which allows role "tenant<n>" to
-- access rows where tenant_id = <n>
CREATE POLICY user_mod ON events
  USING (current_user = 'tenant' || tenant_id::text);
  -- lack of CHECK means same condition as USING

-- enforce the policies
ALTER TABLE events ENABLE ROW LEVEL SECURITY;

Now roles tenant1 and tenant2 get different results for their queries:

Connected as tenant1:

SELECT * FROM events;

┌───────────┬────┬──────┐
│ tenant_id │ id │ type │
├───────────┼────┼──────┤
│         1 │  1 │ foo  │
└───────────┴────┴──────┘

Connected as tenant2:

SELECT * FROM events;

┌───────────┬────┬──────┐
│ tenant_id │ id │ type │
├───────────┼────┼──────┤
│         2 │  2 │ bar  │
└───────────┴────┴──────┘

INSERT INTO events VALUES (3,3,'surprise');
/*
ERROR:  new row violates row-level security policy for table "events_102055"
*/

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