This PR introduces a scheduling frontier which is a frontier that tracks the dataflows that can be scheduled in the cluster. This is accomplished using a timestamp that can be one of two variants:
An Installed(id) variant, which represents the fact that id might be scheduled.
A Future(lower) variant, which represents the fact that dataflows with id >= lower might be scheduled.
Each worker initializes a local MutableAntichain with num_worker copies of Future(0) elements (the minimum) which represents the fact that any worker might schedule any id. A channel is allocated through the allocator so that changes to that local view of the frontier can be communicated between all the workers.
When a dataflow is installed in a worker Installed(id) and Future(id+1) elements are inserted into the frontier and a Future(id) element is retracted.
When a dataflow naturally terminates an Installed(id) element is retracted from the frontier.
When a dataflow is dropped through drop_dataflow is is first moved into a frozen dataflow list but all its resources are maintained. Then Installed(id) is retracted from the frontier to let the other workers know that it will not be scheduled anymore.
Is is ensured that only one of the two paths above are taken to ensure that we only retract a Installed(id) element once.
Eventually all workers will retract their copies of a Dataflow(id), either because it was frozen or because it naturally terminated. When this happens id stops being beyond the global scheduling frontier and at that moment each worker is free to drop the dataflow and its associated resources since no worker will scheduled it again.
This PR introduces a scheduling frontier which is a frontier that tracks the dataflows that can be scheduled in the cluster. This is accomplished using a timestamp that can be one of two variants:
Installed(id)variant, which represents the fact thatidmight be scheduled.Future(lower)variant, which represents the fact that dataflows withid >= lowermight be scheduled.Each worker initializes a local
MutableAntichainwithnum_workercopies ofFuture(0)elements (the minimum) which represents the fact that any worker might schedule any id. A channel is allocated through the allocator so that changes to that local view of the frontier can be communicated between all the workers.When a dataflow is installed in a worker
Installed(id)andFuture(id+1)elements are inserted into the frontier and aFuture(id)element is retracted.When a dataflow naturally terminates an
Installed(id)element is retracted from the frontier.When a dataflow is dropped through
drop_dataflowis is first moved into a frozen dataflow list but all its resources are maintained. ThenInstalled(id)is retracted from the frontier to let the other workers know that it will not be scheduled anymore.Is is ensured that only one of the two paths above are taken to ensure that we only retract a
Installed(id)element once.Eventually all workers will retract their copies of a
Dataflow(id), either because it was frozen or because it naturally terminated. When this happensidstops being beyond the global scheduling frontier and at that moment each worker is free to drop the dataflow and its associated resources since no worker will scheduled it again.