pyjevsim 2.0.1

A DEVS(Discrete Event System Specification) Modeling & Simulation environment with journaling functionality

pyjevsim

Introduction

pyjevsim is a DEVS (discrete event system specification) modeling and simulation environment with built-in journaling. It supports snapshot and restore of individual models or the full simulation engine, virtual-time and real-time execution, and HLA federate integration via a stepped execution mode. Compatible with Python 3.10+.

Full documentation: https://pyjevsim.readthedocs.io/en/latest/

What's new in 2.0

• Two-phase tick. SysExecutor evaluates every imminent model's output() first, then routes outputs and applies transitions — fixing confluent-event ordering under Parallel-DEVS semantics.
• HLA stepped execution. step(granted_time) and get_next_event_time() let an IEEE 1516-2010 RTI federate drive pyjevsim without owning the main loop.
• V_TIME jump-to-next-event. The virtual-time scheduler hops directly to the next scheduled event instead of advancing by a fixed time_resolution, eliminating idle ticks on sparse models.
• Opt-in uncaught-message tracking for debugging dangling outputs.
• DEVStone benchmark suite with cross-engine comparison adapters.

Installing

From PyPI (recommended):

pip install pyjevsim

From source:

git clone https://github.com/eventsim/pyjevsim
cd pyjevsim
pip install -e .

Dependencies

• Python >= 3.10
• dill >= 0.3.6 (installed automatically) — used for model serialization and restoration.

pytest is required only to run the test suite and is declared under the dev extra:

pip install pyjevsim[dev]

Quick Start

A minimal generator → sink simulation:

from pyjevsim.behavior_model import BehaviorModel
from pyjevsim.definition import ExecutionType, Infinite
from pyjevsim.system_executor import SysExecutor
from pyjevsim.system_message import SysMessage


class Gen(BehaviorModel):
    def __init__(self, name):
        super().__init__(name)
        self.init_state("Generate")
        self.insert_state("Generate", 1)
        self.insert_output_port("out")

    def ext_trans(self, port, msg): pass
    def int_trans(self): pass
    def output(self, md):
        msg = SysMessage(self.get_name(), "out")
        msg.insert("tick")
        md.insert_message(msg)
    def time_advance(self):
        return 1


class Sink(BehaviorModel):
    def __init__(self, name):
        super().__init__(name)
        self.init_state("Idle")
        self.insert_state("Idle", Infinite)
        self.insert_input_port("in")

    def ext_trans(self, port, msg):
        print(f"received: {msg.retrieve()}")
    def int_trans(self): pass
    def output(self, md): pass
    def time_advance(self):
        return Infinite


se = SysExecutor(1, ex_mode=ExecutionType.V_TIME)
gen = Gen("g")
sink = Sink("s")
se.register_entity(gen)
se.register_entity(sink)
se.coupling_relation(gen, "out", sink, "in")
se.simulate(5)

See the quick-start guide for structural models, snapshots, and HLA stepped execution.

Examples

The examples/ directory contains:

• banksim/ — bank queue simulation demonstrating BehaviorModel, StructuralModel, and snapshot/restore.
• atsim/ — anti-torpedo simulator with self-propelled and stationary decoy models.
• mwmsim/ — municipal waste management agent-based model.

Output messages are shared by reference

When a model's output port has multiple downstream subscribers, every subscriber receives the same SysMessage object. pyjevsim does not deep-copy outputs during propagation — and neither does any other major Python DEVS engine (xdevs.py and PythonPDEVS share references the same way; benchmark/aliasing_test.py empirically demonstrates this for all four engines in the comparison set). Treat received messages as immutable; if your model needs to mutate a payload, copy it on the receiver side:

def ext_trans(self, port, msg):
    payload = list(msg.retrieve())   # local copy, safe to mutate
    payload.append(my_local_data)
    ...

See benchmark/results/ALIASING.md for the full investigation and per-engine source pointers.

Benchmarks

The benchmark/ directory contains a DEVStone suite plus adapters that run the same workload against other Python DEVS engines so the pyjevsim baseline can be tracked over time.

benchmark/
├── devstone/                     # original pyjevsim-only DEVStone (flat)
│   ├── atomic.py
│   └── topology.py
├── engines/                      # cross-engine canonical DEVStone
│   ├── common.py                 # shared RunResult dataclass
│   ├── pyjevsim/                 # adapter for this repo
│   ├── xdevs/                    # adapter for xdevs.py (pip install xdevs)
│   ├── pypdevs/                  # adapter for PythonPDEVS minimal kernel
│   └── reference/                # hand-rolled flat-FEL engine (floor)
├── run_devstone.py               # pyjevsim-only runner
├── run_compare.py                # cross-engine comparison runner
└── results/
    ├── BASELINE.md               # captured baseline numbers
    ├── baseline.csv
    └── devstone_sweep.csv

pyjevsim-only sweep

python -m benchmark.run_devstone --sweep \
    --output benchmark/results/devstone_sweep.csv

Cross-engine comparison

pip install xdevs                                       # optional
python -m benchmark.run_compare --list-engines
python -m benchmark.run_compare \
    --output benchmark/results/baseline.csv

Sparse-time baseline

run_sparse runs a tiny periodic-generator-plus-sink topology while sweeping the inter-event simulated period. Holds the work constant at 100 events; only the simulated-time gap between events varies. Isolates per-tick overhead in V_TIME mode (see benchmark/results/SPARSE.md):

python -m benchmark.run_sparse --output benchmark/results/sparse.csv

Output aliasing test

benchmark/aliasing_test.py empirically demonstrates that all four engines share output value references across multiple subscribers — see benchmark/results/ALIASING.md. The prevailing convention is "treat received values as immutable; copy on the receiver if you need to mutate".

Current baseline (best-of-three, no synthetic CPU work) — see benchmark/results/BASELINE.md:

variant	d × w	pyjevsim tr/s	xdevs tr/s	pypdevs tr/s	reference tr/s
LI	4 × 4	175 k	689 k	765 k	1.68 M
HI	4 × 4	233 k	546 k	888 k	2.00 M
HO	4 × 4	241 k	757 k	918 k	1.97 M

Use --int-cycles N / --ext-cycles N to inject synthetic CPU work per transition and shift the measurement toward user-code cost.

Debugging Uncaught Output Messages

By default SysExecutor drops output messages that hit a port with no downstream coupling — the simulator stays on its fast path and the events disappear silently. When wiring up a model graph it is often useful to know which events are leaking; pass track_uncaught=True and they get routed to the built-in DefaultMessageCatcher (accessible as se.dmc) so you can observe them:

se = SysExecutor(1, ex_mode=ExecutionType.V_TIME, track_uncaught=True)

The flag costs ~10-15% throughput on dense graphs with many dangling outputs (every uncoupled emit pays for one ext_trans + reschedule on the catcher), so leave it off in production runs.

Execution Modes

SysExecutor supports three execution modes via ExecutionType:

Mode	Description
V_TIME	Virtual time — simulation runs as fast as possible
R_TIME	Real time — simulation paces itself to wall-clock time
HLA_TIME	HLA/RTI-controlled time — time advancement is driven externally

from pyjevsim.system_executor import SysExecutor
from pyjevsim.definition import ExecutionType

se = SysExecutor(1, ex_mode=ExecutionType.V_TIME)

Multi-threading Support

SysExecutor provides thread-safe APIs for multi-threaded simulation environments where external threads inject events while the simulation runs.

Pause / Resume

Pause the simulation to allow external threads to accumulate events, then resume.

se.pause_sim()    # Pauses the simulation loop
# External threads can safely call insert_external_event() while paused
se.resume_sim()   # Resumes the simulation loop

External Event Injection

Insert events from external threads into the simulation. Thread-safe via internal synchronization.

se.insert_external_event("port_name", message, scheduled_time=0)

Output Event Callback

Register a callback to be notified when output events are generated, avoiding polling.

se.set_output_event_callback(lambda: print("output ready"))
events = se.handle_external_output_event()

HLA/RTI Integration (HLA_TIME Mode)

For HLA/RTI-controlled simulations, use HLA_TIME mode with step() and get_next_event_time().

se = SysExecutor(1, ex_mode=ExecutionType.HLA_TIME)
se.register_entity(model)
se.init_sim()

# RTI-driven loop
while not se.is_terminated():
    next_time = se.get_next_event_time()
    # ... request time advance from RTI, wait for grant ...
    granted_time = ...  # time granted by RTI
    output_events = se.step(granted_time)
    # ... publish output_events to RTI ...

step(granted_time)

Runs one RTI-granted simulation step using the same Parallel-DEVS four-phase tick that the standalone V_TIME path uses, so HLA federates get correct δ_int / δ_ext / δ_con semantics:

• Every event whose req_time <= granted_time fires inside the call.
• Multiple cascade rounds at the same simulated instant complete in one step() (sigma=0 chains do not require multiple grants).
• During each round, global_time reflects the actual event time so models observe correct simulated time inside their transitions.
• Per IEEE 1516-2010, global_time lands at granted_time when the call returns, even if the last processed event was earlier.
• Returns the output_event_queue contents drained during this step (a deque of (time, message) tuples) so the federate can republish them as RTI interactions.

get_next_event_time()

Returns the earliest scheduled event time across the FEL and the external-event queue. Use it to compute the Time Advance Request value for the RTI.

Federate ambassador

pyjevsim ships the simulator-side hooks (above) but not an RTI ambassador. Wire step / get_next_event_time / insert_external_event / set_output_event_callback into the federate ambassador of your chosen IEEE 1516-2010 RTI client.

Graceful Termination

se.terminate_simulation()  # Sets SIMULATION_TERMINATED state
se.is_terminated()         # Returns True if terminated

Signal handlers (SIGTERM, SIGINT) automatically invoke terminate_simulation() on all registered SysExecutor instances.

License

Author: Changbeom Choi (@cbchoi)
Copyright (c) 2014-2020 Handong Global University
Copyright (c) 2021-2024 Hanbat National University
License: MIT. The full license text is available at:

• https://github.com/eventsim/pyjevsim/blob/main/LICENSE