Social networking applications on the web handle the personal data of a large number of concurrently active users. These applications must comply with complex
privacy requirements, while achieving scalability and high performance. Applying constraints to the flow of data through such applications to enforce privacy policy is challenging because individual components process data belonging to many
different users.

We introduce a practical approach for uniformly enforcing privacy requirements in such applications using the actor-based Erlang programming language. To isolate the personal data of users, we exploit Erlang’s inexpensive process model and
use Erlang’s message passing mechanism to add policy checks. We illustrate this approach by describing the architecture of a privacy-preserving message dispatcher in a micro-blogging service. Our performance evaluation of a prototype imple
mentation shows that this approach can enforce finegrainedprivacy guarantees with a low performance overhead.