A detailed single cell recipe

This example builds the same single cell model as the previous tutorial, except using a arbor.recipe and arbor.simulation instead of a arbor.single_cell_model.


Concepts covered in this example:

  1. Building a arbor.recipe.

  2. Building an arbor.context and a arbor.domain_decomposition

  3. Using the recipe, context and domain decomposition to create a arbor.simulation

  4. Running the simulation and visualizing the results,

Recipes are an important concept in Arbor. They represent the most versatile tool for building a complex network of cells. We will go though this example of a model of a single cell, before using the recipe to represent more complex networks in subsequent examples.

We outline the following steps of this example:

  1. Define the cell. This is the same cell we have seen before.

  2. Define the recipe of the model.

  3. Define the execution context of the model: a description of the underlying system on which the simulation will run.

  4. Define the domain decomposition of the network: how the cells are distributed on the different ranks of the system.

  5. Define the simulation.

  6. Run the simulation.

  7. Collect and visualize the results.

The cell

We can copy the cell description code or reuse single_cell_detailed.swc from the previous example where it is explained in detail.

We will need to add one more thing to the cell. We will create the voltage probe at the “custom_terminal” locset. In the previous example, this probe was registered directly using the arbor.single_cell_model object. Now it has to be explicitly created and registered in the recipe.

# (6) Create a class that inherits from arbor.recipe
class single_recipe(arbor.recipe):

The recipe

The arbor.single_cell_model of the previous example created a arbor.recipe under the hood, and abstracted away the details so we were unaware of its existence. In this example, we will examine the recipe in detail: how to create one, and why it is needed.

    # (6.1) Define the class constructor
    def __init__(self, cell, probes):
        # The base C++ class constructor must be called first, to ensure that
        # all memory in the C++ class is initialized correctly.
        self.the_cell = cell
        self.the_probes = probes

        self.the_props = arbor.cable_global_properties()
        self.the_props.set_property(Vm=-65, tempK=300, rL=35.4, cm=0.01)
            ion="na", int_con=10, ext_con=140, rev_pot=50, method="nernst/na"
        self.the_props.set_ion(ion="k", int_con=54.4, ext_con=2.5, rev_pot=-77)
        self.the_props.set_ion(ion="ca", int_con=5e-5, ext_con=2, rev_pot=132.5)
        self.the_props.catalogue.extend(arbor.allen_catalogue(), "")

    # (6.2) Override the num_cells method
    def num_cells(self):
        return 1

    # (6.3) Override the num_targets method
    def cell_kind(self, gid):
        return arbor.cell_kind.cable

    # (6.4) Override the cell_description method
    def cell_description(self, gid):
        return self.the_cell

    # (6.5) Override the probes method
    def probes(self, gid):
        return self.the_probes

    # (6.6) Override the connections_on method
    def connections_on(self, gid):
        return []

    # (6.7) Override the gap_junction_on method
    def gap_junction_on(self, gid):
        return []

    # (6.8) Override the event_generators method
    def event_generators(self, gid):
        return []

    # (6.9) Overrode the global_properties method
    def global_properties(self, gid):
        return self.the_props

Let’s go through the recipe point by point.

Step (6) creates a single_recipe class that inherits from arbor.recipe. The base recipe implements all the methods defined above with default values except arbor.recipe.num_cells(), arbor.recipe.cell_kind() and arbor.recipe.cell_description() which always have to be implemented by the user. The arbor.recipe.global_properties() also needs to be implemented for arbor.cell_kind.cable cells. The inherited recipe can implement any number of additional methods and have any number of instance or class variables.

Step (6.1) defines the class constructor. In this case, we pass a cell and a set of probes as arguments. These will be used to initialize the instance variables self.the_cell and self.the_probes, which will be used in the overloaded cell_description and get_probes methods. Before variable initialization, we call the base C++ class constructor arbor.recipe.__init__(self). This ensures correct initialization of memory in the C++ class.

We also create the self.the_cat variable and set it to arbor’s default mechanism catalogue. This will expose the hh and pas mechanisms but not the Ih mechanism, which is present in the allen catalogue. To be able to use Ih, we extend self.the_cat to include the allen catalogue.

Finally we create the self.the_props variable. This will hold the global properties of the model, which apply to all the cells in the network. Initially it is empty. We set all the properties of the system similar to what we did in the previous example. One last important step is to register self.the_cat with self.the_props.


The mechanism catalogue needs to live in the recipe as an instance variable. Its lifetime needs to extend to the entire duration of the simulation.

Step (6.2) overrides the arbor.recipe.num_cells() method. It takes no arguments. We simply return 1, as we are only simulating one cell in this example.

Step (6.3) overrides the arbor.recipe.cell_kind() method. It takes one argument: gid. Given the gid, this method returns the kind of the cell. Our defined cell is a arbor.cell_kind.cable, so we simply return that.

Step (6.4) overrides the arbor.recipe.cell_description() method. It takes one argument: gid. Given the gid, this method returns the cell description which is the cell object passed to the constructor of the recipe. We return self.the_cell.

Step (6.5) overrides the arbor.recipe.get_probes() method. It takes one argument: gid. Given the gid, this method returns all the probes on the cell. The probes can be of many different kinds measuring different quantities on different locations of the cell. We pass these probes explicitly to the recipe and they are stored in self.the_probes, so we return that variable.

Step (6.6) overrides the arbor.recipe.connections_on() method. It takes one argument: gid. Given the gid, this method returns all the connections ending on that cell. These are typically synapse connections from other cell sources to specific targets on the cell with id gid. Since we are simulating a single cell, and self-connections are not possible, we return an empty list.

Step (6.7) overrides the arbor.recipe.gap_junctions_on() method. It takes one argument: gid. Given the gid, this method returns all the gap junctions on that cell. Gap junctions require 2 separate cells. Since we are simulating a single cell, we return an empty list.

Step (6.8) overrides the arbor.recipe.event_generators() method. It takes one argument: gid. Given the gid, this method returns event generators on that cell. These generators trigger events (or spikes) on specific targets on the cell. They can be used to simulate spikes from other cells, to kick-start a simulation for example. Our cell uses a current clamp as a stimulus, and has no targets, so we return an empty list.

Step (6.9) overrides the arbor.recipe.global_properties() method. It takes one argument: kind. This method returns the default global properties of the model which apply to all cells in the network of that kind. We return self.the_props which we defined in step (1).


You may wonder why the method arbor.recipe.cell_kind() is required, since it can be inferred by examining the cell description. The recipe was designed to allow building simulations efficiently in a distributed system with minimum communication. Some parts of the model initialization require only the cell kind, not the full cell description which can be quite expensive to build. Providing these descriptions separately saves time and resources for the user.

More information on the recipe can be found here.

Now we can instantiate a single_recipe object using the cell and probe we created in the previous section:

# Instantiate recipe
# Pass the probe in a list because that it what single_recipe expects.
recipe = single_recipe(cell, [probe])

The execution context

An execution context describes the hardware resources on which the simulation will run. It contains the thread pool used to parallelise work on the local CPU, and optionally describes GPU resources and the MPI communicator for distributed simulations. In the previous examples, the arbor.single_cell_model object created the execution context arbor.context behind the scenes.

The details of the execution context can be customized by the user. We may specify the number of threads in the thread pool; determine the id of the GPU to be used; or create our own MPI communicator. However, the ideal settings can usually be inferred from the system, and Arbor can do that with a simple command.

# (7) Create a simulation
sim = arbor.simulation(recipe)

The domain decomposition

The domain decomposition describes the distribution of the cells over the available computational resources. The arbor.single_cell_model also handled that without our knowledge in the previous examples. Now, we have to define it ourselves.

The arbor.domain_decomposition class can be manually created by the user, by deciding which cells go on which ranks. Or we can use a load balancer that can partition the cells across ranks according to some rules. Arbor provides arbor.partition_load_balance, which, using the recipe and execution context, creates the arbor.domain_decomposition object for us.

Our example is a simple one, with just one cell. We don’t need any sophisticated partitioning algorithms, so we can use the load balancer, which does a good job distributing simple networks.

# Instruct the simulation to record the spikes and sample the probe

The simulation

Finally we have the 3 components needed to create a arbor.simulation object.

probe_id = arbor.cell_member(0, 0)
handle = sim.sample(probe_id, arbor.regular_schedule(0.02))

Before we run the simulation, however, we need to register what results we expect once execution is over. This was handled by the arbor.single_cell_model object in the previous example.

We would like to get a list of the spikes on the cell during the runtime of the simulation, and we would like to plot the voltage registered by the probe on the “custom_terminal” locset.

# (8) Run the simulation
sim.run(tfinal=100, dt=0.025)

# (9) Print or display the results
spikes = sim.spikes()

The lines handling probe sampling warrant a second look. First, we declared probe_id to be a arbor.cell_member, with arbor.cell_member.gid = 0 and arbor.cell_member.index = 0. This variable serves as a global identifier of a probe on a cell, namely the first declared probe on the cell with gid = 0, which is id of the only probe we created on the only cell in the model.

Next, we instructed the simulation to sample probe_id at a frequency of 50 kHz. That function returns a handle which we will use to extract the results of the sampling after running the simulation.

The execution

We can now run the simulation we just instantiated for a duration of 100 ms with a time step of 0.025 ms.

The results

The last step is result collection. We instructed the simulation to record the spikes on the cell, and to sample the probe.

We can print the times of the spikes:

data = []
meta = []
for d, m in sim.samples(handle):

The probe results, again, warrant some more explanation:

df_list = []
for i in range(len(data)):

sim.samples() takes a handle of the probe we wish to examine. It returns a list of (data, meta) terms: data being the time and value series of the probed quantity; and meta being the location of the probe. The size of the returned list depends on the number of discrete locations pointed to by the handle. We placed the probe on the “custom_terminal” locset which is represented by 2 locations on the morphology. We therefore expect the length of sim.samples(handle) to be 2.

We plot the results using pandas and seaborn as we did in the previous example, and expect the same results:

                "U/mV": data[i][:, 1],
                "Location": str(meta[i]),
                "Variable": "voltage",

The following plot is generated. Identical to the plot of the previous example.


The full code

You can find the full code of the example at python/examples/single_cell_detailed_recipe.py.