One of the more popular reply-driven programs is a fractal graphics program distributed over the network. The master program divides the screen into several areas, for example, 64 regions. At startup, the master program is given a list of nodes that can participate in this activity. The master program starts up worker (subserver) programs, one on each of the nodes, and then waits for the worker programs to send to the master.
The master then repeatedly picks "unfilled" regions (of the 64 on screen) and delegates the fractal computation work to the worker program on another node by replying to it. When the worker program has completed the calculations, it sends the results back to the master, which displays the result on the screen.
Because the worker program sent to the master, it's now up to the master to again reply with more work. The master continues doing this until all 64 areas on the screen have been filled.