The nodes of a program graph, which correspond to processes or dataflow actors, are implemented in Ptolemy by objects derived from the class Star. The firing function of a dataflow actor is implemented by the run method of Star. The edges of the program graph, which correspond to communication channels, are implemented by the class Geodesic. A Geodesic is a first-in first-out (FIFO) queue that is accessed by the put and get methods. The connections between stars and geodesics are implemented by the class PortHole. Each PortHole has an internal buffer. The methods sendData and receiveData transfer data between this buffer and a Geodesic using the put and get methods.

Several existing domains in Ptolemy, such as SDF and BDF, implement dataflow process networks by scheduling the firings of dataflow actors. The firing of a dataflow actor is implemented as a function call to the run method of a Star object. A scheduler executes the system as a sequence of function calls. Thus, the repeated actor firings that make up a dataflow process are interleaved with the actor firings of other dataflow processes. Before invoking the run method of a Star, the scheduler must ensure that enough data is available to satisfy the actor's firing rules. This makes it necessary for a Star object to inform the scheduler of the number of tokens it requires from its inputs. With this information, a scheduler can guarantee that an actor will not attempt to read from an empty channel.

By contrast, the PN domain creates a separate thread of execution for each node in the program graph. Threads are sometimes called lightweight processes. Modern operating systems, such as Unix, support the simultaneous execution of multiple processes. There need not be any actual parallelism. The operating system can interleave the execution of the processes. Within a single process, there can be multiple lightweight processes or threads, so there are two levels of multi-threading. Threads share a single address space, that of the parent process, allowing them to communicate through simple variables. There is no need for more complex, heavyweight inter-process communication mechanisms such as pipes.

Synchronization mechanisms are available to ensure that threads have exclusive access to shared data and cannot interfere with one another to corrupt shared data structures. Monitors and condition variables are available to synchronize the execution of threads. A monitor is an object that can be locked and unlocked. Only one thread may hold the lock on a monitor. If a thread attempts to lock a monitor that is already locked by another thread, it is suspended until the monitor is unlocked. At that point it wakes up and tries again to lock the monitor. Condition variables allow threads to send signals to each other. Condition variables must be used in conjunction with a monitor; a thread must lock the associated monitor before using a condition variable.

The scheduler in the PN domain creates a thread for each node in the program graph. Each thread implements a dataflow process by repeatedly invoking the run method of a Star object. The scheduler itself does very little work, leaving the operating system to interleave the execution of threads. The put and get methods of the class Geodesic have been re-implemented using monitors and condition variables so that a thread attempting to read from an empty channel is automatically suspended, and threads automatically wake up when data becomes available.

The classes PtThread, PtGate, and PtCondition define the interfaces for threads, monitors, and condition variables in Ptolemy. Different implementations can be used as long as they conform to the interfaces defined in these base classes. At different points in the development of the PN domain, we experimented with implementations based on Sun's Lightweight Process library, AWESIME (A Widely Extensible Simulation Environment) by Dirk Grunwald [Gru91}, and Solaris threads [Pow91,Eyk92,Kha92,Kle92a,Kle92b,Ste92,Sun94]. The current implementation is based on a POSIX thread library by Frank Mueller [Mue92,Mue93,Gie93,Mue95]. This library, which runs on several platforms, is based on Draft 6 of the POSIX standard. Parts of our implementation will need to be updated to be compliant with the final POSIX thread standard.

By choosing the POSIX standard, we improve the portability of our code. Sun and Hewlett Packard already include an implementation of POSIX threads in their operating systems, Solaris 2.5 and HPUX 10. Having threads built into the kernel of the operating system, as opposed to a user library implementation, offers the opportunity for automatic parallelization on multiprocessor workstations. Thus, the same program runs properly on uniprocessor workstations and multiprocessor workstations without needing to be recompiled. This is important because it would be impractical to maintain different binary executables of Ptolemy for each workstation configuration.