CEP 20 - Time Step Execution Stack

Title:Time Step Execution Stack
Author:Matthew Gidden
Type:Standards Track


An update to the time step execution stack is proposed. A timestep will now progress through phases, including a building phase, a tick phase, a resource exchange phase, a tock phase, and a decommissioning phase. Phases are grouped in two categories: kernel phases and agent phases. Kernel phases have required or guaranteed actions that occur during them, and include the build, exchange, and decommission phases. Agent phases include the Tick and Tock phases, and allow agents to inspect simulation state and update their own state.


The current time step in Cyclus consists of the following stack:

  • Tick
  • Resource Exchange
  • Tock

Until very recently, only Regions received ticks/tocks from the Timer class. They were then expected to pass the Tick/Tock “message” to their children, and so on, until all Models received a Tick or Tock. It was pointed out recently that this behavior, when mixed with entity entry and exit into/from the simulation could produce unexpected behavior, e.g., an entity could enter/exit in the middle of a time step. Furthermore, modules were developed specifically taking advantage of this behavior (i.e., the ordering of ticks/tocks) in order to guarantee that entities that were constructed during the Tick phase also received a Tick in that phase. Furthermore, this imposed a requirement on Regions and Institutions that they pass Ticks and Tocks along to their children in a trickle-down-like manner.

Accordingly, there is a need to standardize what can/should be expected to occur in each phase of a given time step. Guarantees should be given to module developers that if an entity enters a simulation, that it will experience the entire time step on the time step it enters, and if an entity leaves a simulation, that it will experience an entire time step on the time it leaves.


By Law’s definition [LK99], a Cyclus simulation is, broadly, a dynamic, discrete-event simulation that uses a fixed-increment time advance mechanism. In general, fixed-increment time advance scenarios assume a time step (dt), and assume that all events that would happen during a time occur simultaneously at the end of the time step. This situation can be thought of as an event-based time advance mechanism, i.e., one that steps from event to event, that executes all events simultaneously that were supposed to have occurred in the time step.

Two key types of events happen in a Cyclus simulation (at present):

  • the exchange of resources
  • agent entry into and exit from the simulation

Simulation entities can have arbitrarily complex state which is dependent on the results of the exchange and the present status of agents in the simulation. Accordingly, methods that allow entities to update state must occur in response to these events and to schedule agent entry and exit.

Because there is a key event that defines agent interaction in a given time step, it is necessary to involve all agents in that interaction. Accordingly it is necessary that there be an ordering between these two key types of events, deviating slightly from Law’s description of fixed-increment time advance. Specifically, we want to preserve the following invariant: any agent that exists in a given time step should be included in the resource exchange, or, equivalently, experience the entire time step execution stack.

This leads to the following ordering, or phases, of time step execution:

  • agents enter simulation (Building Phase)
  • agents respond to current simulation state (Tick Phase)
  • resource exchange execution (Exchange Phase)
  • agents respond to current simulation state (Tock Phase)
  • agents leave simulation (Decommissioning Phase)

The Building, Exchange, and Decommissioning phases each include critical, core-based events, and will be called Kernel phases. The Tick and Tock phases do not include core-based events, and instead let agents react to previous core-based events and inspect core simulation state. Furthermore, they are periods in which agents can update their own state and are accordingly considered Agent phases. In general, Agent phases must bracket critical Kernel phases, of which only the Exchange Phase exists for now. If another critical core phase is added in the future it must provide a similar invariant, i.e., that it is bracketed by Agent phases. For example, if a new phase is added before Exchange, then the time execution stack would follow as: Building, Tick, New Kernel Phase, New Agent Phase, Exchange, Tock, Decommission.

Technically, whether agent entry occurs simultaneously with agent exit or not does not matter from a simulation-mechanics point of view, because the two phases have a direct ordering. It will, however, from the point of view of module development. It is simpler cognitively to think of an agent entering the simulation and acting in that time step, rather than entering a simulation at a given time and taking its first action in the subsequent time step.

In the spirit of Law’s definition of a fixed-increment time advance mechanism, there is a final important invariant: there is no guaranteed agent ordering of within-phase execution. This invariant allows for:

  • a more cognitively simple process
  • paralellization

Any future addition of phases in the timestep execution stack neccessarily guarantee the three invariants described above.

Specification & Implementation

Two primary concerns exist for changing the current state of Cyclus to incorporate this CEP:

  • how to implement agent entry/exit as described
  • what name to give to the response phases

Currently, the response phases are called Tick and Tock. These names have been criticized for not being specific/informative about the expected actions agents will/should take during the phases. I propose we instead use PreExchange and PostExchange. Wordsmithing and/or other suggestions are welcome.

The agent entry/exit question is a bit more involved because of the parent-child (or manager-managed) relationship agents have in Cyclus. Specifically, the entry and exit of agents should be managed by the agent’s manager. The following provides one possible specification.

/// @brief execute time step stack
def Step(context):
    time = context.time()

    for each builder, prototype in build_list[time]:

    for each agent in agent_list:

    for each manager in resource_exchange_managers:

    for each agent in agent_list:

    for each agent in decomm_list[time]:

The primary change here is the notion of a build_list and decomm_list. Managers of agents, nominally their parent, can add agents to each list as required during the Pre- and PostExchange phases. At some future time, the building and decommissioning lists can be made queryable in order to determine future overall or sub-simulation state (e.g., the power level at a future point in time). Accordingly, prototypes (which know their initial state) are used in the build_list and to-be decommissioned agents in the decomm_list.

As described above, the notion of build and decommission lists can change in a time step. When combined with the invariant that the order of agent execution within a phase is unordered, future simulation predictions would be unreliable if both lists could be changed in within a phase. Therefore, these lists must be immutable during phases. This issue can be remedied by using staging data structures and merging the staging data structures into the lists after the completion of a phase.

Backwards Compatibility

The overall Cyclus implementation/framework will remain largely unchanged, with the exception of the core’s handling of agent entry/exit registration. Cycamore modules that deal with agent entry/exit will have to be redesigned to incorporate the new execution stack.

Document History

This document is released under the CC-BY 3.0 license.

References and Footnotes


[LK99]Averill M. Law and David M. Kelton. Simulation Modeling and Analysis. McGraw-Hill Higher Education, 3rd edition, 1999. ISBN 0070592926.