The actor model is a message-passing concurrency model that avoids deadlocks and low-level data races by construction. This facilitates concurrent programming, especially in the context of complex interactive applications where modularity, security and fault-tolerance are required. The tradeoff is that the actor model sacrifices expressiveness and safety guarantees with respect to parallel access to shared state. In this paper we present domains as a set of novel language abstractions for safely encapsulating and sharing state within the actor model. We introduce four types of domains, namely immutable, isolated, observable and shared domains that each is tailored to a certain access pattern on that shared state. The domains are characterized with an operational semantics. For each we discuss how the actor model׳s safety guarantees are upheld even in the presence of conceptually shared state. Furthermore, the proposed language abstractions are evaluated with a case study in Scala comparing them to other synchronization mechanisms to demonstrate their benefits in deadlock freedom, parallel reads, and enforced isolation. 
Domains: Sharing state in the communicating event-loop actor model