BB.Parameter.Store.CubDB

CubDB-backed parameter persistence for the Beam Bots robotics framework.

Tune a gain with BB.Parameter.set/3, and it's still there after a reboot.

Why CubDB

bb ships BB.Parameter.Store.Dets, which is fine when the machine shuts down politely. Robots don't always. CubDB stores its data in an append-only, immutable B-tree, so an unexpected shutdown can't corrupt the database or leave it needing a repair pass — which matters when the off switch is "it fell over". Keys and values are arbitrary terms, so parameter paths and BB.Unit values are stored as-is, and writes are flushed to disk as they happen.

Installation

mix igniter.install bb_parameter_store_cubdb

That adds the extension to your robot and declares a parameter_store_cubdb section, pointing at /root on a Nerves project and the build directory otherwise.

Or add it to mix.exs by hand:

def deps do
  [
    {:bb_parameter_store_cubdb, "~> 0.1.0"}
  ]
end

Usage

Add the extension to your robot and say where the database lives:

defmodule MyRobot.Robot do
  use BB, extensions: [BB.Parameter.Store.CubDB.Dsl]

  parameter_store_cubdb do
    data_dir "/root/my_robot_params"
  end

  parameters do
    group :balance do
      param :kp, type: :float, default: 180.0, min: 0.0, max: 500.0
      param :kd, type: :float, default: 4.0, min: 0.0, max: 50.0
    end
  end

  topology do
    link :base_link
  end
end

Then tune away, and the values will be there next boot:

iex> BB.Parameter.set(MyRobot.Robot, [:balance, :kp], 200.0)
:ok

Declaring the section is all that's needed — it sets the robot's parameter_store setting for you. If you'd rather configure the store directly and skip the extension, that works too:

settings do
  parameter_store {BB.Parameter.Store.CubDB, data_dir: "/root/my_robot_params"}
end

Options

Option	Default	Description
data_dir	required	Directory to store the database in, created if it doesn't exist
auto_compact	true	Compact automatically. true, false, or {min_write_operations, min_dirt_factor}
auto_file_sync	true	Flush the disk buffer on every write. Turning this off trades durability for write throughput

Where to put the data directory

On Nerves, use a path under /root — the application data partition. The firmware's root filesystem is read-only, and bb only logs a warning when a store fails to open, so a robot pointed at an unwritable path will boot with persistence quietly disabled.

Which is a problem if the same robot module has to run on your laptop and on the device, because /root isn't writable on the one and _build doesn't exist on the other. Take the directory from the application environment instead:

parameter_store_cubdb do
  data_dir Application.compile_env(:my_app, :params_dir)
end

Then give each environment and target its own value. In a Nerves project, config/config.exs ends with import_config "#{Mix.target()}.exs", so the per-target files are where the paths go:

# config/host.exs
config :my_app, params_dir: Path.expand("_build/params")

# config/rpi0_2.exs
config :my_app, params_dir: "/root/params"

For a plain (non-Nerves) project the same trick works per Mix.env(), with config/dev.exs and config/test.exs setting their own directories — handy for keeping a test run from inheriting the gains you tuned in dev.

Two things to know about this:

• Use Application.compile_env/2, not get_env/2 or runtime.exs. The DSL is compiled, so the value has to be available at compile time — a key set in config/runtime.exs is read far too late to end up in the section. The payoff is that compile_env also tracks the key, so changing it triggers a recompile rather than leaving a stale path baked into the BEAM file.
• An unset key is a compile error, not a nil path, because the section's schema requires a string. You'll hear about a missing target config while you're building the firmware rather than after it's on the robot.

Bounds aren't rechecked on load

bb applies persisted values without revalidating them against each parameter's min/max. A value that was in bounds when it was written will still be in bounds when it's read, so this only bites if something edits the database directly.

Licence

Apache-2.0. See LICENSE.txt.