The core concept of structured concurrency is that when control splits into concurrent tasks that they join up again. If a “main task” splits into several concurrent sub-tasks scheduled to be executed in fibers then those fibers must terminate before the main task can complete.
The main benefit of structured concurrency is abstraction. A caller of a method that is invoked to do a task should not care if the method decomposes the task and schedules a million fibers. When the method completes, any fibers scheduled by the method should have terminated.
In Project Loom, a prototype API has been developed called FiberScope that is a scope in which fibers are scheduled. Here is a basic example:
A thread or fiber creates and enters a scope by calling the FiberScope.open method. It exits the scope when the code in the block completes and any fibers scheduled in the scope have terminated. The example schedules two fibers. The thread/fiber executing the above code may have to wait (in the FiberScope’s close method) until the two fibers have terminated.
FiberScopes can be nested, consider the following:
In this example, a thread or fiber enters scope1, schedules a fiber, then enters scope2 where it schedules a fiber in that scope. The execution cannot exit scope2 until the fiber scheduled in that scope terminates. When it exits, it is back in scope1, where it schedules another fiber. It cannot exit scope1 until the two fibers scheduled in that scope have terminated.
More generally, the structured approach leads to trees of tasks. Consider the following method x that schedules two fibers to execute foo and bar. The code in foo and bar each schedule two fibers.
fiber1 is scheduled in scope1 to execute foo. It enters scope2 and schedules two fibers. It exits scope2 (and returns to scope1) when the two fibers terminate. fiber2 is scheduled in scope1 to execute bar. It enters scope3 and schedules two fibers. It exits scope3 (and returns to scope1) when the two fibers terminate. The thread or fiber executing will not exit scope1 until fiber1 and fiber2 have terminated.
FiberScope API also defines the static background() method to return the background scope which can be used to schedule fibers that are intended to outline the context where they are initially scheduled.
A FiberScope can be created with options that configure how cancellation is handled. At this time, the options are PROPAGATE_CANCEL, CANCEL_AT_CLOSE, and IGNORE_CANCEL.
The following example uses the PROPAGATE_CANCEL option:
If a fiber executing in this scope is cancelled then it will also cancel fiber1 and fiber2.
The IGNORE_CANCEL option is for recovery/cleanup tasks that cannot be cancelled. The Fiber.cancelled() method always returns false when executing in scope created with this option.
Decomposing deadline or timeouts is very difficult to get right.
FiberScope supports creating and entering a scope with a deadline. If the deadline is reached before the thread/fiber exits the scope then all fibers scheduled in the scope are cancelled.
Deadlines work with nested scopes. Consider the following:
scope1 is entered with a deadline that is now + 10s. It schedules a fiber in scope2 and cannot exit to scope1 until the fiber terminates. If the deadline is reached in the meantime then the fiber will be cancelled.
FiberScope also defines open(Duration timeout) to enter a scope with a timeout. If the timeout expires before thread/fiber exits the scope then all fibers scheduled in the scope are cancelled.
There is no support at this time for making context available to all fibers scheduled in the scope. InheritedThreadLocals can be used in the mean-time.