This paper describes the Active Object pattern, which decouples method execution from method invocation in order to simplify synchronized access to an object that resides in its own thread of control. The Active Object pattern allows one or more independent threads of execution to interleave their access to data modeled as a single object. A broad class of producer/consumer and reader/writer applications are wellsuited to this model of concurrency. This pattern is commonly used in distributed systems requiring multi-threaded servers. In addition, client applications, such as windowing systems and network browsers, employ active objects to simplify concurrent, asynchronous network operations.

In theory, multi-threading an application can improve performance by excecuting multiple instruction streams simultaneously. In addition, they can simplify program structure by allowing each thread to execute synchronously rather than reactively or asynchronously. In practice, however, multi-threaded applications often perform no better, or even worse, than single-threaded applications due to the overhead of acquiring and releasing locks. In addition, threads are hard to program correctly due to subtle concurrency control protocols (e.g., avoiding race conditions and deadlocks). This paper describes the Thread-Specific Storage pattern, which helps to alleviate several problems with thread performance and complexity. The Thread-Specific Storage pattern improves performance and simplifies multi-threaded applications by allowing multiple threads to use one logical access point to retrieve thread local data without incurring locking overhead for each access.

This paper will be submitted to the 3rd annual Pattern Languages of Programming conference held at AllertonPark, Illinois, September 4-6, 1996. 1 Introduction