The existence of a parallel statement raises several questions regarding how trails scheduled during execution. Nonetheless, the concurrency properties of Céu are quite simple and easy to grasp.

Céu doesn't require semaphores, locks, or any other synchronization primitive!

Some rules are respected during the execution of programs, the first of which is the synchronous execution model.

A running program in Céu is synchronous with respect to input events. This means that while trails are reacting to the current input event, no further events are handled in any other trail.

The period in which trails are reacting to a given input event is named as a reaction chain. A trail only halts (i.e. stops to react) when it awaits again or terminates.

In the example, suppose the event A occurs just before B.

Initially, both trails in parallel are awaiting, and the occurrence of A awakes the first trail that performs the increment on v.

If the event B occurs during the increment operation, it is enqueued by the Céu runtime until the running reaction chain terminates to respect the synchronous execution model.

This way, there is no possible race condition on accessing v and, given the sequence A->B, the only possible result for the program is 2 ((0+1)*2).

Try also to invert the emits in the Input panel and observe the output.

Reactions to input events do not overlap.

But what if a trail executes endlessly and never halts, how further input events could be handled?

And what if multiple trails react to the same input event and access the same variables?

Check the following lessons for answers to these questions.

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