Scheduling callbacks and output writers

Callbacks and output writers in Simulation actuate on objects that are subtypes of AbstractSchedule. Schedules are small callable objects that return true when an action should fire and false otherwise. This page collects the built-in schedules, when to use them, and how to combine them for more complex behavior.

Aligned time steps

Simulation automatically shortens the next time step so that callback and output events scheduled by model time happen exactly when requested. Set align_time_step = false in the Simulation constructor to disable this.

For the examples here, we use the following simple simulation and progress:

using Oceananigans
grid = RectilinearGrid(size=(1, 1, 1), extent=(1, 1, 1))
model = NonhydrostaticModel(grid)
simulation = Simulation(model, Δt=0.1, stop_time=2.5, verbose=false)
dummy(sim) = @info string("Iter: ", iteration(sim), " -- I was called at t = ", time(sim),
                          " and wall time = ", prettytime(sim.run_wall_time))

dummy (generic function with 1 method)

During the examples below, we use Oceananigans.Simulations.reset!(simulation) so that we reuse the simulation in the next example without requiring to recreate it.

Basic schedules

IterationInterval

IterationInterval actuates every interval iterations:

schedule = IterationInterval(11)
add_callback!(simulation, dummy, schedule, name=:dummy)
run!(simulation)

[ Info: Iter: 0 -- I was called at t = 0.0 and wall time = 0 seconds
[ Info: Iter: 11 -- I was called at t = 1.0999999999999999 and wall time = 43.406 ms
[ Info: Iter: 22 -- I was called at t = 2.2000000000000006 and wall time = 47.729 ms

Use the offset keyword argument to shift the trigger so that, for example,

Oceananigans.Simulations.reset!(simulation)
simulation.stop_time = 2.5

schedule = IterationInterval(7; offset=-2)
add_callback!(simulation, dummy, schedule, name=:dummy)
run!(simulation)

[ Info: Iter: 0 -- I was called at t = 0.0 and wall time = 0 seconds
[ Info: Iter: 5 -- I was called at t = 0.5 and wall time = 2.405 ms
[ Info: Iter: 12 -- I was called at t = 1.2 and wall time = 5.648 ms
[ Info: Iter: 19 -- I was called at t = 1.9000000000000006 and wall time = 8.818 ms

Notice that other than iteration 0, the callback is actuated on iterations 5, 12, 19, ...

Above, we have overwritten the original callback called "dummy" with the new one with the offset schedule. An alternative way is to construct the Callback manually and add it in the simulation:

simulation.callbacks[:dummy] = Callback(dummy, schedule)

Callback of dummy on IterationInterval(7) with offset -2

TimeInterval

TimeInterval actuates every interval of model time, in units corresponding to model.clock.time. For example,

Oceananigans.Simulations.reset!(simulation)
simulation.stop_time = 2.5

schedule = TimeInterval(1.11)
add_callback!(simulation, dummy, schedule, name=:dummy)
run!(simulation)

[ Info: Iter: 0 -- I was called at t = 0.0 and wall time = 0 seconds
[ Info: Iter: 12 -- I was called at t = 1.11 and wall time = 20.551 ms
[ Info: Iter: 24 -- I was called at t = 2.22 and wall time = 28.223 ms

When model.clock.time isa AbstractTime such as DateTime, then interval can be Dates.Period:

using Dates

start_time = DateTime(2025, 1, 1)
clock = Clock(time = start_time)
datetime_model = NonhydrostaticModel(grid; clock)

stop_time = start_time + Dates.Minute(3)
datetime_simulation = Simulation(datetime_model; Δt=Dates.Second(25), stop_time, verbose=false)

schedule = TimeInterval(Dates.Minute(1))
add_callback!(datetime_simulation, dummy, schedule)
run!(datetime_simulation)

[ Info: Iter: 0 -- I was called at t = 2025-01-01T00:00:00 and wall time = 0 seconds
[ Info: Iter: 3 -- I was called at t = 2025-01-01T00:01:00 and wall time = 717.233 ms
[ Info: Iter: 6 -- I was called at t = 2025-01-01T00:02:00 and wall time = 718.683 ms
[ Info: Iter: 9 -- I was called at t = 2025-01-01T00:03:00 and wall time = 719.867 ms

WallTimeInterval

WallTimeInterval uses wall-clock seconds instead of model time. This is mostly useful for writing checkpoints to disk after consuming a fixed amount of computational resources. For example, using the previous simulation without the

Oceananigans.Simulations.reset!(simulation)
simulation.stop_time = 2.5

schedule = WallTimeInterval(0.005)
add_callback!(simulation, dummy, schedule, name=:dummy)
run!(simulation)

[ Info: Iter: 0 -- I was called at t = 0.0 and wall time = 0 seconds
[ Info: Iter: 1 -- I was called at t = 0.1 and wall time = 0 seconds
[ Info: Iter: 7 -- I was called at t = 0.7 and wall time = 23.184 ms
[ Info: Iter: 20 -- I was called at t = 2.0000000000000004 and wall time = 28.108 ms

SpecifiedTimes

SpecifiedTimes actuates when model.clock.time reaches the given values. The constructor accepts numeric times or Dates.DateTime values and sorts them automatically. This schedule is helpful for pre-planned save points or events tied to specific model times.

Oceananigans.Simulations.reset!(simulation)
simulation.stop_time = 2.5

schedule = SpecifiedTimes(0.2, 1.5, 2.1)
add_callback!(simulation, dummy, schedule, name=:dummy)
run!(simulation)

[ Info: Iter: 0 -- I was called at t = 0.0 and wall time = 0 seconds
[ Info: Iter: 2 -- I was called at t = 0.2 and wall time = 23.341 ms
[ Info: Iter: 15 -- I was called at t = 1.5 and wall time = 28.915 ms
[ Info: Iter: 21 -- I was called at t = 2.1 and wall time = 31.128 ms

Arbitrary functions of model

Any function of model that returns a Bool can be used as a schedule:

Oceananigans.Simulations.reset!(simulation)
simulation.stop_time = 2.5

after_two(model) = model.clock.time > 2
add_callback!(simulation, dummy, after_two, name=:dummy)
run!(simulation)

[ Info: Iter: 0 -- I was called at t = 0.0 and wall time = 0 seconds
[ Info: Iter: 20 -- I was called at t = 2.0000000000000004 and wall time = 23.555 ms
[ Info: Iter: 21 -- I was called at t = 2.1000000000000005 and wall time = 24.291 ms
[ Info: Iter: 22 -- I was called at t = 2.2000000000000006 and wall time = 25.029 ms
[ Info: Iter: 23 -- I was called at t = 2.3000000000000007 and wall time = 25.736 ms
[ Info: Iter: 24 -- I was called at t = 2.400000000000001 and wall time = 26.477 ms
[ Info: Iter: 25 -- I was called at t = 2.5 and wall time = 27.192 ms

Combining schedules

Some applications benefit from running extra steps immediately after an event or from combining multiple criteria.

ConsecutiveIterations

ConsecutiveIterations actuates when the parent schedule does and for the next N iterations. For example, averaging callbacks often need data at the scheduled time and immediately afterwards.

Oceananigans.Simulations.reset!(simulation)
simulation.stop_time = 2.5

times = SpecifiedTimes(0.55, 1.5, 2.12)
schedule = ConsecutiveIterations(times)
add_callback!(simulation, dummy, schedule, name=:dummy)
run!(simulation)

[ Info: Iter: 0 -- I was called at t = 0.0 and wall time = 0 seconds
[ Info: Iter: 1 -- I was called at t = 0.1 and wall time = 0 seconds
[ Info: Iter: 6 -- I was called at t = 0.55 and wall time = 25.516 ms
[ Info: Iter: 7 -- I was called at t = 0.65 and wall time = 31.586 ms
[ Info: Iter: 16 -- I was called at t = 1.5 and wall time = 37.811 ms
[ Info: Iter: 17 -- I was called at t = 1.6 and wall time = 38.541 ms
[ Info: Iter: 23 -- I was called at t = 2.12 and wall time = 42.682 ms
[ Info: Iter: 24 -- I was called at t = 2.22 and wall time = 43.400 ms

AndSchedule and OrSchedule

Use AndSchedule when an action should fire only if every child schedule actuates in the same iteration. Use OrSchedule when any one of the child schedules should trigger the action. Both accept any mix of AbstractSchedules, so you can require, for example, output every hour and every 1000 iterations:

Oceananigans.Simulations.reset!(simulation)
simulation.stop_time = 2.5

after_one_point_seven(model) = model.clock.time > 1.7
schedule = AndSchedule(IterationInterval(2), after_one_point_seven)
add_callback!(simulation, dummy, schedule, name=:dummy)
run!(simulation)

[ Info: Iter: 0 -- I was called at t = 0.0 and wall time = 0 seconds
[ Info: Iter: 18 -- I was called at t = 1.8000000000000005 and wall time = 55.817 ms
[ Info: Iter: 20 -- I was called at t = 2.0000000000000004 and wall time = 57.313 ms
[ Info: Iter: 22 -- I was called at t = 2.2000000000000006 and wall time = 58.761 ms
[ Info: Iter: 24 -- I was called at t = 2.400000000000001 and wall time = 60.205 ms

Stateful schedules

Stateful schedules such as TimeInterval, SpecifiedTimes, and ConsecutiveIterations store their own counters, so we need to create a fresh instance (or call copy) for each callback or output writer that needs an identical pattern.

Output-specific schedules

Some schedules only apply to output writers because they keep extra state or require file access.

AveragedTimeInterval

AveragedTimeInterval asks an output writer to accumulate data over a sliding time window before writing. The window ends at each actuation time, runs for window seconds, and samples every stride iterations inside the window.

AveragedSpecifiedTimes

AveragedSpecifiedTimes behaves like SpecifiedTimes but with a trailing averaging window. Pass either a SpecifiedTimes instance or raw times.

FileSizeLimit

FileSizeLimit actuates when the target file grows beyond size_limit bytes. Output writers update the internal path automatically so you usually only pass the size limit. Combine it with OrSchedule to rotate files when either the clock reaches a value or the file becomes too large.