Real-Time Simulation with SimPy: Syncing with the Real World

Published on February 10, 2026

Normal SimPy runs as fast as possible. Simulation years pass in seconds. But sometimes you need simulation time to match real time. <h2 id="when-to-use-real-time-simulation">When to Use Real-Time Simulation</h2> <ul> <li>Live demonstrations - Show stakeholders the simulation running</li> <li>Hardware-in-the-loop - Interface with real equipment</li> <li>Training systems - Operators interact in real time</li> <li>Testing - Validate timing-sensitive code</li> </ul> <h2 id="realtimeenvironment">RealtimeEnvironment</h2> <div class="code-block"><pre><code>import simpy.rt env = simpy.rt.RealtimeEnvironment(factor=1.0) def process(env): print(f"Start at real time") yield env.timeout(5) # Actually waits 5 seconds print(f"5 seconds have passed") env.process(process(env)) env.run(until=10) # Takes 10 real seconds </code></pre></div> <h2 id="speed-factor">Speed Factor</h2> Control how fast simulation runs relative to real time: <div class="code-block"><pre><code># Real-time (1 sim second = 1 real second) env = simpy.rt.RealtimeEnvironment(factor=1.0) # Half speed (1 sim second = 2 real seconds) env = simpy.rt.RealtimeEnvironment(factor=0.5) # Double speed (1 sim second = 0.5 real seconds) env = simpy.rt.RealtimeEnvironment(factor=2.0) # 10x speed (1 sim second = 0.1 real seconds) env = simpy.rt.RealtimeEnvironment(factor=10.0) </code></pre></div> <h2 id="strict-mode">Strict Mode</h2> By default, SimPy tries to keep up with real time but doesn't guarantee it: <div class="code-block"><pre><code># Non-strict (default): proceeds even if behind schedule env = simpy.rt.RealtimeEnvironment(factor=1.0, strict=False) # Strict: raises error if simulation can't keep up env = simpy.rt.RealtimeEnvironment(factor=1.0, strict=True) </code></pre></div> Use strict mode when timing accuracy is critical. <h2 id="practical-example-traffic-light">Practical Example: Traffic Light</h2> <div class="code-block"><pre><code>import simpy.rt import time def traffic_light(env): while True: print(f"[{time.strftime('%H:%M:%S')}] GREEN") yield env.timeout(30) print(f"[{time.strftime('%H:%M:%S')}] YELLOW") yield env.timeout(5) print(f"[{time.strftime('%H:%M:%S')}] RED") yield env.timeout(30) env = simpy.rt.RealtimeEnvironment(factor=1.0) env.process(traffic_light(env)) env.run(until=120) # Run for 2 minutes real time </code></pre></div> <h2 id="interactive-simulation">Interactive Simulation</h2> Accept user input during simulation: <div class="code-block"><pre><code>import simpy.rt import threading import queue command_queue = queue.Queue() def user_input_thread(): """Background thread for user input.""" while True: cmd = input("Enter command (add/quit): ") command_queue.put(cmd) if cmd == 'quit': break def command_processor(env, server): """Process user commands.""" while True: yield env.timeout(0.1) # Check every 0.1 sim seconds try: cmd = command_queue.get_nowait() if cmd == 'add': env.process(customer(env, f"Manual-{env.now:.1f}", server)) print(f"Added customer at {env.now:.1f}") elif cmd == 'quit': return except queue.Empty: pass # Start input thread input_thread = threading.Thread(target=user_input_thread, daemon=True) input_thread.start() env = simpy.rt.RealtimeEnvironment(factor=1.0) server = simpy.Resource(env, capacity=1) env.process(command_processor(env, server)) env.run(until=300) </code></pre></div> <h2 id="syncing-with-external-systems">Syncing with External Systems</h2> Interface with real hardware or APIs: <div class="code-block"><pre><code>import simpy.rt import requests def external_data_fetcher(env, url, interval): """Fetch data from external API at regular intervals.""" while True: yield env.timeout(interval) response = requests.get(url) data = response.json() print(f"[{env.now}] Fetched: {data}") # Process data, update simulation state, etc. env = simpy.rt.RealtimeEnvironment(factor=1.0) env.process(external_data_fetcher(env, 'http://api.example.com/data', 5)) env.run(until=60) </code></pre></div> <h2 id="visualisation-in-real-time">Visualisation in Real Time</h2> Update a display as simulation runs: <div class="code-block"><pre><code>import simpy.rt import matplotlib.pyplot as plt from matplotlib.animation import FuncAnimation queue_lengths = [] times = [] def monitor(env, server): global queue_lengths, times while True: times.append(env.now) queue_lengths.append(len(server.queue)) yield env.timeout(0.5) def animate(frame): plt.cla() plt.plot(times, queue_lengths) plt.xlabel('Time') plt.ylabel('Queue Length') plt.title('Real-Time Queue Monitor') # Setup env = simpy.rt.RealtimeEnvironment(factor=1.0) server = simpy.Resource(env, capacity=1) env.process(arrivals(env, server)) env.process(monitor(env, server)) # Animation fig = plt.figure() ani = FuncAnimation(fig, animate, interval=500) # Update every 500ms # Run simulation in background thread import threading sim_thread = threading.Thread(target=lambda: env.run(until=60)) sim_thread.start() plt.show() </code></pre></div> <h2 id="logging-with-real-timestamps">Logging with Real Timestamps</h2> <div class="code-block"><pre><code>import simpy.rt import logging import time logging.basicConfig( level=logging.INFO, format='%(asctime)s | sim_t=%(sim_time)s | %(message)s' ) class SimTimeFilter(logging.Filter): def __init__(self, env): self.env = env def filter(self, record): record.sim_time = f"{self.env.now:.2f}" return True env = simpy.rt.RealtimeEnvironment(factor=1.0) logger = logging.getLogger() logger.addFilter(SimTimeFilter(env)) def process(env): logger.info("Process started") yield env.timeout(5) logger.info("Process completed") </code></pre></div> <h2 id="performance-considerations">Performance Considerations</h2> Real-time simulation has constraints: <div class="code-block"><pre><code># Bad: Complex computation blocks real-time sync def slow_process(env): yield env.timeout(1) result = very_slow_computation() # Takes 3 real seconds # Now we're behind schedule! # Better: Offload heavy computation import concurrent.futures executor = concurrent.futures.ThreadPoolExecutor() def async_process(env): yield env.timeout(1) future = executor.submit(very_slow_computation) # Continue simulation while computation runs </code></pre></div> <h2 id="switching-between-modes">Switching Between Modes</h2> Same model, different execution: <div class="code-block"><pre><code>def create_simulation(realtime=False, factor=1.0): if realtime: env = simpy.rt.RealtimeEnvironment(factor=factor) else: env = simpy.Environment() # Setup simulation (same code either way) server = simpy.Resource(env, capacity=2) env.process(arrivals(env, server)) return env # Fast simulation for analysis env = create_simulation(realtime=False) env.run(until=10000) # Real-time for demonstration env = create_simulation(realtime=True, factor=1.0) env.run(until=60) </code></pre></div> <h2 id="gotchas">Gotchas</h2> <h3 id="simulation-falling-behind">Simulation Falling Behind</h3> <div class="code-block"><pre><code># With strict=True, this raises an error if can't keep up env = simpy.rt.RealtimeEnvironment(factor=1.0, strict=True) # With strict=False, simulation proceeds but timing drifts env = simpy.rt.RealtimeEnvironment(factor=1.0, strict=False) </code></pre></div> <h3 id="blocking-operations">Blocking Operations</h3> <div class="code-block"><pre><code># Don't do this - blocks the simulation def bad_process(env): import time time.sleep(5) # Blocks everything! yield env.timeout(0) # Use simulation timeout instead def good_process(env): yield env.timeout(5) # Proper simulation delay </code></pre></div> <h2 id="summary">Summary</h2> Real-time simulation: - Use <code>simpy.rt.RealtimeEnvironment</code> - Control speed with <code>factor</code> - Use <code>strict=True</code> when timing matters - Avoid blocking operations - Good for demos, testing, hardware integration Sometimes you need to slow down to see what's happening. <h2 id="next-steps">Next Steps</h2> <ul> <li><a href="/blog_posts/simpy-environment-explained.html">SimPy Environment Explained</a></li> <li><a href="/blog_posts/simpy-visualize-results.html">Visualising Results</a></li> <li><a href="/blog_posts/simpy-logging.html">Logging in SimPy</a></li> </ul>

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