Understanding the Command System

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This document explains the design of Beam Bots' command system - why commands are short-lived processes, how they integrate with the state machine, and the patterns they enable.

The Core Model

Commands in BB are short-lived GenServers that execute discrete operations. They're not background services - they start, do something, and stop.

execute command
      │
      ▼
┌─────────────────┐
│ Command Process │ ─── receives messages, updates state
│   (GenServer)   │ ─── decides when complete
└─────────────────┘
      │
      ▼
  return result

This model is borrowed from action servers in ROS, but adapted for Erlang/OTP semantics.

Why Processes?

Commands could be simple function calls. Why make them processes?

Async by Default

Commands can take time. A movement command might wait for motors to reach position. Making commands processes means:

  • Callers don't block
  • Multiple commands can (potentially) run concurrently
  • Timeouts are handled naturally

Message-Driven

Commands often need to react to external events:

  • Sensor feedback (reached position?)
  • Hardware errors (servo overheating?)
  • User cancellation

As processes, commands can subscribe to PubSub and receive messages.

Supervision

Command processes are supervised. If a command crashes:

  • The robot returns to a safe state
  • Awaiting callers receive an error
  • Resources are cleaned up

State Machine Integration

Commands interact with the robot state machine. Being processes lets them:

  • Check state before executing
  • Hold the robot in a state while running
  • Transition state on completion

The State Machine

Every robot has an operational state:

:disarmed ─arm─→ :idle ─disarm─→ :disarmed

Commands declare which states they can run in:

command :move_to do
  handler MoveCommand
  allowed_states [:idle]
end

command :arm do
  handler BB.Command.Arm
  allowed_states [:disarmed]
end

The Runtime enforces these constraints:

BB.Robot.Runtime.state(MyRobot)  #=> :disarmed
MyRobot.move_to(position: 1.0)   #=> {:error, %NotAllowed{}}

Custom States

For complex robots, you can define custom states beyond :idle using the states DSL section:

states do
  state :recording do
    doc "Recording trajectory data"
  end

  state :playback do
    doc "Playing back recorded trajectory"
  end

  state :calibrating do
    doc "Running calibration routine"
  end
end

commands do
  command :start_recording do
    handler {BB.Command.SetState, to: :recording}
    allowed_states [:idle]
  end

  command :start_playback do
    allowed_states [:recording]
  end
end

The built-in states (:idle, :disarmed) are always available. Your custom states extend what's possible. See the Custom States and Categories tutorial for comprehensive coverage.

Command Lifecycle

1. Execution Request

{:ok, cmd_pid} = MyRobot.move_to(target: %{shoulder: 0.5})

The DSL generates this function. It calls BB.Robot.Runtime.execute/3.

2. State Validation

Runtime checks:

  • Robot is in an allowed state
  • Command category has capacity (for concurrent commands)

3. Process Start

A GenServer starts under the Runtime's DynamicSupervisor:

DynamicSupervisor.start_child(supervisor, {CommandServer, opts})

4. Handler Invocation

The CommandServer calls your handler's handle_command/3:

@impl BB.Command
def handle_command(goal, context, state) do
  # goal: arguments from the execute call
  # context: robot module, struct, state handle
  # state: command's internal state (includes :result)

  # Return GenServer-style tuple
  {:noreply, updated_state}
end

5. Execution

The command runs as a GenServer:

  • Receives messages via handle_info/2
  • Can make calls with handle_call/3
  • Subscribes to PubSub for sensor feedback

6. Completion

When done, return {:stop, reason, state}:

def handle_info(:done, state) do
  {:stop, :normal, %{state | result: {:ok, :completed}}}
end

7. Result Extraction

On termination, result/1 is called:

@impl BB.Command
def result(%{result: result}), do: result

The result goes to awaiting callers.

Awaiting Results

Callers have options:

Blocking Wait

{:ok, cmd} = MyRobot.move_to(target: %{shoulder: 0.5})
{:ok, result} = BB.Command.await(cmd)  # blocks until done

Timeout

case BB.Command.await(cmd, 5000) do
  {:ok, result} -> handle_result(result)
  {:error, :timeout} -> handle_timeout()
end

Non-Blocking Check

case BB.Command.yield(cmd, 0) do
  {:ok, result} -> done(result)
  nil -> still_running()
end

Fire and Forget

{:ok, _cmd} = MyRobot.move_to(target: %{shoulder: 0.5})
# Don't await - let it run

Command Categories

By default, only one command runs at a time. Categories enable concurrency:

commands do
  category :motion do
    doc "Physical movement commands"
    concurrency_limit 1
  end

  category :sensing do
    doc "Sensor and data collection commands"
    concurrency_limit 2  # Allow concurrent sensing
  end

  command :move_to do
    category :motion
    allowed_states [:idle]
  end

  command :read_sensor do
    category :sensing
    allowed_states [:idle]
  end
end

Each category has a concurrency_limit (default: 1). Commands in different categories can run concurrently. Commands in the same category are limited by that category's concurrency limit.

Cancellation

Commands can declare they cancel others:

command :move_to do
  cancel [:motion]  # Cancels running motion commands
end

command :emergency_stop do
  cancel :*  # Cancels everything
end

When cancelled, the command process terminates and result/1 is called with the current state. Awaiting callers receive whatever result/1 returns. Commands should handle cancellation with a fallback clause:

@impl BB.Command
def result(%{result: result}) do
  {:ok, result}
end

def result(_state), do: {:error, :cancelled}

State Transitions

Commands can change robot state:

@impl BB.Command
def result(%{result: {:ok, value}}) do
  {:ok, value, next_state: :recording}
end

This is how BB.Command.Arm works - it returns {:ok, :armed, next_state: :idle}.

Safety Integration

Commands receive safety state changes:

@impl BB.Command
def handle_safety_state_change(:disarming, state) do
  # Robot is being disarmed - stop gracefully
  {:stop, :disarmed, %{state | result: {:error, :disarmed}}}
end

The default implementation stops on any safety change. Override for commands that should continue (use with care).

Design Patterns

Request-Feedback-Result

The canonical pattern for motion commands:

  1. Request: Send target to actuators
  2. Feedback: Subscribe to sensors, monitor progress
  3. Result: Complete when target reached or timeout
def handle_command(goal, context, state) do
  # Request
  publish_targets(goal.target, context)
  subscribe_to_sensors(goal.target, context)

  {:noreply, %{state | target: goal.target}}
end

def handle_info({:bb, [:sensor, _], msg}, state) do
  if reached_target?(msg, state.target) do
    # Result
    {:stop, :normal, %{state | result: {:ok, :reached}}}
  else
    # Feedback (continue waiting)
    {:noreply, state}
  end
end

Coordination

For commands coordinating multiple subsystems:

def handle_command(goal, context, state) do
  # Start multiple operations
  start_arm_motion(goal, context)
  start_gripper_action(goal, context)

  {:noreply, %{state | arm_done: false, gripper_done: false}}
end

def handle_info({:arm_complete, _}, state) do
  check_completion(%{state | arm_done: true})
end

def handle_info({:gripper_complete, _}, state) do
  check_completion(%{state | gripper_done: true})
end

defp check_completion(%{arm_done: true, gripper_done: true} = state) do
  {:stop, :normal, %{state | result: {:ok, :complete}}}
end

defp check_completion(state), do: {:noreply, state}

Timeout with Progress

For commands that might hang:

def handle_command(goal, _context, state) do
  schedule_timeout(5000)
  {:noreply, %{state | last_progress: now()}}
end

def handle_info(:timeout, state) do
  if stale?(state.last_progress) do
    {:stop, :normal, %{state | result: {:error, :timeout}}}
  else
    schedule_timeout(5000)
    {:noreply, state}
  end
end

def handle_info({:progress, _}, state) do
  {:noreply, %{state | last_progress: now()}}
end