a.exs

defmodule KV.Bucket do
  @doc """
  Starts a new bucket.
  """
  def start_link do
    Agent.start_link(fn -> %{} end)
  end

  @doc """
  Gets a value from the `bucket` by `key`.
  """
  def get(bucket, key) do
    Agent.get(bucket, fn b -> Map.get(b, key) end)
  end

  @doc """
  Puts the `value` for the given `key` in the `bucket`.
  """
  def put(bucket, key, value) do
    Agent.update(bucket, fn b -> Map.put(b, key, value) end)
  end
end

b.exs

defmodule KV.Bucket do
  @doc """
  Starts a new bucket.
  """
  def start_link do
    Agent.start_link(fn -> %{} end)
  end

  @doc """
  Gets a value from the `bucket` by `key`.
  """
  def get(bucket, key) do
    Agent.get(bucket, &Map.get(&1, key))
  end

  @doc """
  Puts the `value` for the given `key` in the `bucket`.
  """
  def put(bucket, key, value) do
    Agent.update(bucket, &Map.put(&1, key, value))
  end
end