In the earlier sections, we learned that reactors delimit concurrent executions, and that event streams allow routing events within each reactor. This is already a powerful set of abstractions, and we can use reactors and event streams to write all kinds of distributed programs. However, such a model is restricted to reactor computations only. We cannot, for example, start blocking I/O operations, read from a temperature sensor implemented in hardware, wait until a GPU computation completes, or react to temporal events. In some cases, we need to interact with the native capabilities of the OS, or tap into a rich ecosystem of existing libraries. For this purpose, every reactor system has a set of services: protocols that relate event streams to the outside world.
In this section, we will take a closer look at various services that are available by default, and also show how to implement custom services and plug them into reactor systems.