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# Real-time simulations

Sometimes, you might not want to perform a simulation as fast as possible but synchronous to the wall-clock time. This kind of simulation is also called real-time simulation.

Real-time simulations may be necessary

- if you have hardware-in-the-loop,
- if there is human interaction with your simulation, or
- if you want to analyze the real-time behavior of an algorithm.

To convert a simulation into a real-time simulation, you only need to replace SimPy’s default `Environment` with a `simpy.rt.RealtimeEnvironment`. Apart from the initial_time argument, there are two additional parameters: factor and strict: `RealtimeEnvironment(initial_time=0, factor=1.0, strict=True)`.

The factor defines how much real time passes with each step of simulation time. By default, this is one second. If you set `factor=0.1`, a unit of simulation time will only take a tenth of a second; if you set `factor=60`, it will take a minute.

Here is a simple example for converting a normal simulation to a real-time simulation with a duration of one tenth of a second per simulation time unit:

    >>> import time
    >>> import simpy
    >>>
    >>> def example(env):
    ...     start = time.perf_counter()
    ...     yield env.timeout(1)
    ...     end = time.perf_counter()
    ...     print('Duration of one simulation time unit: %.2fs' % (end - start))
    >>>
    >>> env = simpy.Environment()
    >>> proc = env.process(example(env))
    >>> env.run(until=proc)
    Duration of one simulation time unit: 0.00s
    >>>
    >>> import simpy.rt
    >>> env = simpy.rt.RealtimeEnvironment(factor=0.1)
    >>> proc = env.process(example(env))
    >>> env.run(until=proc)
    Duration of one simulation time unit: 0.10s

If the strict parameter is set to `True` (the default), the `step()` and `run()` methods will raise a `RuntimeError` if the computation within a simulation time step take more time than the real-time factor allows. In the following example, a process will perform a task that takes 0.02 seconds within a real-time environment with a time factor of 0.01 seconds:

    >>> import time
    >>> import simpy.rt
    >>>
    >>> def slow_proc(env):
    ...     time.sleep(0.02)  # Heavy computation :-)
    ...     yield env.timeout(1)
    >>>
    >>> env = simpy.rt.RealtimeEnvironment(factor=0.01)
    >>> proc = env.process(slow_proc(env))
    >>> try:
    ...     env.run(until=proc)
    ...     print('Everything alright')
    ... except RuntimeError:
    ...     print('Simulation is too slow')
    Simulation is too slow

To suppress the error, simply set `strict=False`:

    >>> env = simpy.rt.RealtimeEnvironment(factor=0.01, strict=False)
    >>> proc = env.process(slow_proc(env))
    >>> try:
    ...     env.run(until=proc)
    ...     print('Everything alright')
    ... except RuntimeError:
    ...     print('Simulation is too slow')
    Everything alright

That’s it. Real-time simulations are that simple with SimPy!

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