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<div class="section" id="gas-station-refueling">
<h1>Gas Station Refueling<a class="headerlink" href="#gas-station-refueling" title="Permalink to this headline">¶</a></h1>
<p>Covers:</p>
<ul class="simple">
<li>Resources: Resource</li>
<li>Resources: Container</li>
<li>Waiting for other processes</li>
</ul>
<p>This examples models a gas station and cars that arrive at the station for
refueling.</p>
<p>The gas station has a limited number of fuel pumps and a fuel tank that is
shared between the fuel pumps. The gas station is thus modeled as
<a class="reference internal" href="../api_reference/simpy.resources.html#simpy.resources.resource.Resource" title="simpy.resources.resource.Resource"><tt class="xref py py-class docutils literal"><span class="pre">Resource</span></tt></a>. The shared fuel tank is modeled
with a <a class="reference internal" href="../api_reference/simpy.resources.html#simpy.resources.container.Container" title="simpy.resources.container.Container"><tt class="xref py py-class docutils literal"><span class="pre">Container</span></tt></a>.</p>
<p>Vehicles arriving at the gas station first request a fuel pump from the
station. Once they acquire one, they try to take the desired amount of fuel
from the fuel pump. They leave when they are done.</p>
<p>The gas stations fuel level is reqularly monitored by <em>gas station control</em>.
When the level drops below a certain threshold, a <em>tank truck</em> is called to
refuel the gas station itself.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="sd">"""</span>
<span class="sd">Gas Station Refueling example</span>
<span class="sd">Covers:</span>
<span class="sd">- Resources: Resource</span>
<span class="sd">- Resources: Container</span>
<span class="sd">- Waiting for other processes</span>
<span class="sd">Scenario:</span>
<span class="sd"> A gas station has a limited number of gas pumps that share a common</span>
<span class="sd"> fuel reservoir. Cars randomly arrive at the gas station, request one</span>
<span class="sd"> of the fuel pumps and start refueling from that reservoir.</span>
<span class="sd"> A gas station control process observes the gas station's fuel level</span>
<span class="sd"> and calls a tank truck for refueling if the station's level drops</span>
<span class="sd"> below a threshold.</span>
<span class="sd">"""</span>
<span class="kn">import</span> <span class="nn">itertools</span>
<span class="kn">import</span> <span class="nn">random</span>
<span class="kn">import</span> <span class="nn">simpy</span>
<span class="n">RANDOM_SEED</span> <span class="o">=</span> <span class="mi">42</span>
<span class="n">GAS_STATION_SIZE</span> <span class="o">=</span> <span class="mi">200</span> <span class="c"># liters</span>
<span class="n">THRESHOLD</span> <span class="o">=</span> <span class="mi">10</span> <span class="c"># Threshold for calling the tank truck (in %)</span>
<span class="n">FUEL_TANK_SIZE</span> <span class="o">=</span> <span class="mi">50</span> <span class="c"># liters</span>
<span class="n">FUEL_TANK_LEVEL</span> <span class="o">=</span> <span class="p">[</span><span class="mi">5</span><span class="p">,</span> <span class="mi">25</span><span class="p">]</span> <span class="c"># Min/max levels of fuel tanks (in liters)</span>
<span class="n">REFUELING_SPEED</span> <span class="o">=</span> <span class="mi">2</span> <span class="c"># liters / second</span>
<span class="n">TANK_TRUCK_TIME</span> <span class="o">=</span> <span class="mi">300</span> <span class="c"># Seconds it takes the tank truck to arrive</span>
<span class="n">T_INTER</span> <span class="o">=</span> <span class="p">[</span><span class="mi">30</span><span class="p">,</span> <span class="mi">300</span><span class="p">]</span> <span class="c"># Create a car every [min, max] seconds</span>
<span class="n">SIM_TIME</span> <span class="o">=</span> <span class="mi">1000</span> <span class="c"># Simulation time in seconds</span>
<span class="k">def</span> <span class="nf">car</span><span class="p">(</span><span class="n">name</span><span class="p">,</span> <span class="n">env</span><span class="p">,</span> <span class="n">gas_station</span><span class="p">,</span> <span class="n">fuel_pump</span><span class="p">):</span>
<span class="sd">"""A car arrives at the gas station for refueling.</span>
<span class="sd"> It requests one of the gas station's fuel pumps and tries to get the</span>
<span class="sd"> desired amount of gas from it. If the stations reservoir is</span>
<span class="sd"> depleted, the car has to wait for the tank truck to arrive.</span>
<span class="sd"> """</span>
<span class="n">fuel_tank_level</span> <span class="o">=</span> <span class="n">random</span><span class="o">.</span><span class="n">randint</span><span class="p">(</span><span class="o">*</span><span class="n">FUEL_TANK_LEVEL</span><span class="p">)</span>
<span class="k">print</span><span class="p">(</span><span class="s">'</span><span class="si">%s</span><span class="s"> arriving at gas station at </span><span class="si">%.1f</span><span class="s">'</span> <span class="o">%</span> <span class="p">(</span><span class="n">name</span><span class="p">,</span> <span class="n">env</span><span class="o">.</span><span class="n">now</span><span class="p">))</span>
<span class="k">with</span> <span class="n">gas_station</span><span class="o">.</span><span class="n">request</span><span class="p">()</span> <span class="k">as</span> <span class="n">req</span><span class="p">:</span>
<span class="n">start</span> <span class="o">=</span> <span class="n">env</span><span class="o">.</span><span class="n">now</span>
<span class="c"># Request one of the gas pumps</span>
<span class="k">yield</span> <span class="n">req</span>
<span class="c"># Get the required amount of fuel</span>
<span class="n">liters_required</span> <span class="o">=</span> <span class="n">FUEL_TANK_SIZE</span> <span class="o">-</span> <span class="n">fuel_tank_level</span>
<span class="k">yield</span> <span class="n">fuel_pump</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">liters_required</span><span class="p">)</span>
<span class="c"># The "actual" refueling process takes some time</span>
<span class="k">yield</span> <span class="n">env</span><span class="o">.</span><span class="n">timeout</span><span class="p">(</span><span class="n">liters_required</span> <span class="o">/</span> <span class="n">REFUELING_SPEED</span><span class="p">)</span>
<span class="k">print</span><span class="p">(</span><span class="s">'</span><span class="si">%s</span><span class="s"> finished refueling in </span><span class="si">%.1f</span><span class="s"> seconds.'</span> <span class="o">%</span> <span class="p">(</span><span class="n">name</span><span class="p">,</span>
<span class="n">env</span><span class="o">.</span><span class="n">now</span> <span class="o">-</span> <span class="n">start</span><span class="p">))</span>
<span class="k">def</span> <span class="nf">gas_station_control</span><span class="p">(</span><span class="n">env</span><span class="p">,</span> <span class="n">fuel_pump</span><span class="p">):</span>
<span class="sd">"""Periodically check the level of the *fuel_pump* and call the tank</span>
<span class="sd"> truck if the level falls below a threshold."""</span>
<span class="k">while</span> <span class="bp">True</span><span class="p">:</span>
<span class="k">if</span> <span class="n">fuel_pump</span><span class="o">.</span><span class="n">level</span> <span class="o">/</span> <span class="n">fuel_pump</span><span class="o">.</span><span class="n">capacity</span> <span class="o">*</span> <span class="mi">100</span> <span class="o"><</span> <span class="n">THRESHOLD</span><span class="p">:</span>
<span class="c"># We need to call the tank truck now!</span>
<span class="k">print</span><span class="p">(</span><span class="s">'Calling tank truck at </span><span class="si">%d</span><span class="s">'</span> <span class="o">%</span> <span class="n">env</span><span class="o">.</span><span class="n">now</span><span class="p">)</span>
<span class="c"># Wait for the tank truck to arrive and refuel the station</span>
<span class="k">yield</span> <span class="n">env</span><span class="o">.</span><span class="n">process</span><span class="p">(</span><span class="n">tank_truck</span><span class="p">(</span><span class="n">env</span><span class="p">,</span> <span class="n">fuel_pump</span><span class="p">))</span>
<span class="k">yield</span> <span class="n">env</span><span class="o">.</span><span class="n">timeout</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span> <span class="c"># Check every 10 seconds</span>
<span class="k">def</span> <span class="nf">tank_truck</span><span class="p">(</span><span class="n">env</span><span class="p">,</span> <span class="n">fuel_pump</span><span class="p">):</span>
<span class="sd">"""Arrives at the gas station after a certain delay and refuels it."""</span>
<span class="k">yield</span> <span class="n">env</span><span class="o">.</span><span class="n">timeout</span><span class="p">(</span><span class="n">TANK_TRUCK_TIME</span><span class="p">)</span>
<span class="k">print</span><span class="p">(</span><span class="s">'Tank truck arriving at time </span><span class="si">%d</span><span class="s">'</span> <span class="o">%</span> <span class="n">env</span><span class="o">.</span><span class="n">now</span><span class="p">)</span>
<span class="n">ammount</span> <span class="o">=</span> <span class="n">fuel_pump</span><span class="o">.</span><span class="n">capacity</span> <span class="o">-</span> <span class="n">fuel_pump</span><span class="o">.</span><span class="n">level</span>
<span class="k">print</span><span class="p">(</span><span class="s">'Tank truck refuelling </span><span class="si">%.1f</span><span class="s"> liters.'</span> <span class="o">%</span> <span class="n">ammount</span><span class="p">)</span>
<span class="k">yield</span> <span class="n">fuel_pump</span><span class="o">.</span><span class="n">put</span><span class="p">(</span><span class="n">ammount</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">car_generator</span><span class="p">(</span><span class="n">env</span><span class="p">,</span> <span class="n">gas_station</span><span class="p">,</span> <span class="n">fuel_pump</span><span class="p">):</span>
<span class="sd">"""Generate new cars that arrive at the gas station."""</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="n">itertools</span><span class="o">.</span><span class="n">count</span><span class="p">():</span>
<span class="k">yield</span> <span class="n">env</span><span class="o">.</span><span class="n">timeout</span><span class="p">(</span><span class="n">random</span><span class="o">.</span><span class="n">randint</span><span class="p">(</span><span class="o">*</span><span class="n">T_INTER</span><span class="p">))</span>
<span class="n">env</span><span class="o">.</span><span class="n">process</span><span class="p">(</span><span class="n">car</span><span class="p">(</span><span class="s">'Car </span><span class="si">%d</span><span class="s">'</span> <span class="o">%</span> <span class="n">i</span><span class="p">,</span> <span class="n">env</span><span class="p">,</span> <span class="n">gas_station</span><span class="p">,</span> <span class="n">fuel_pump</span><span class="p">))</span>
<span class="c"># Setup and start the simulation</span>
<span class="k">print</span><span class="p">(</span><span class="s">'Gas Station refuelling'</span><span class="p">)</span>
<span class="n">random</span><span class="o">.</span><span class="n">seed</span><span class="p">(</span><span class="n">RANDOM_SEED</span><span class="p">)</span>
<span class="c"># Create environment and start processes</span>
<span class="n">env</span> <span class="o">=</span> <span class="n">simpy</span><span class="o">.</span><span class="n">Environment</span><span class="p">()</span>
<span class="n">gas_station</span> <span class="o">=</span> <span class="n">simpy</span><span class="o">.</span><span class="n">Resource</span><span class="p">(</span><span class="n">env</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span>
<span class="n">fuel_pump</span> <span class="o">=</span> <span class="n">simpy</span><span class="o">.</span><span class="n">Container</span><span class="p">(</span><span class="n">env</span><span class="p">,</span> <span class="n">GAS_STATION_SIZE</span><span class="p">,</span> <span class="n">init</span><span class="o">=</span><span class="n">GAS_STATION_SIZE</span><span class="p">)</span>
<span class="n">env</span><span class="o">.</span><span class="n">process</span><span class="p">(</span><span class="n">gas_station_control</span><span class="p">(</span><span class="n">env</span><span class="p">,</span> <span class="n">fuel_pump</span><span class="p">))</span>
<span class="n">env</span><span class="o">.</span><span class="n">process</span><span class="p">(</span><span class="n">car_generator</span><span class="p">(</span><span class="n">env</span><span class="p">,</span> <span class="n">gas_station</span><span class="p">,</span> <span class="n">fuel_pump</span><span class="p">))</span>
<span class="c"># Execute!</span>
<span class="n">env</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">until</span><span class="o">=</span><span class="n">SIM_TIME</span><span class="p">)</span>
</pre></div>
</div>
<p>The simulation’s output:</p>
<div class="highlight-python"><div class="highlight"><pre>Gas Station refuelling
Car 0 arriving at gas station at 87.0
Car 0 finished refueling in 18.5 seconds.
Car 1 arriving at gas station at 129.0
Car 1 finished refueling in 19.0 seconds.
Car 2 arriving at gas station at 284.0
Car 2 finished refueling in 21.0 seconds.
Car 3 arriving at gas station at 385.0
Car 3 finished refueling in 13.5 seconds.
Car 4 arriving at gas station at 459.0
Calling tank truck at 460
Car 4 finished refueling in 22.0 seconds.
Car 5 arriving at gas station at 705.0
Car 6 arriving at gas station at 750.0
Tank truck arriving at time 760
Tank truck refuelling 188.0 liters.
Car 6 finished refueling in 29.0 seconds.
Car 5 finished refueling in 76.5 seconds.
Car 7 arriving at gas station at 891.0
Car 7 finished refueling in 13.0 seconds.
</pre></div>
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