Bookk

Bookk is a simple library that provides building blocks for operating journal entries and manipulating double-entry bookkeeping accounting ledgers.

See full documentation at hexdocs.

This library aims to decrease the friction between domain specialists (mainly accountants) and developers by providing a DSL that enables developers to write code for journal entries with a syntax that's familiar to specialists. That way, it should be easy for specialists to review code for journal entries and, likewise, it whould be easy for developers to implement journal entries based on instructions provided by specialists.

def to_interledger_entry(%Deposit{} = tx) do
  journalize! using: ACME.ChartOfAccounts do
    on ledger(:acme) do
      debit account(:cash), tx.amount
      credit account({:unspent_cash, {:user, tx.user_id}}), tx.amount
    end

    on ledger({:user, tx.user_id}) do
      debit account(:cash), tx.amount
      credit account(:deposits), tx.amount
    end
  end
end

(A journal entry from a deposit operation affecting two ledgers written with Bookk's DSL)

Persisting state, such as accounts balances and log of transactions per accounts is considered off scope for this library at the moment — and honestly, might never becomes part of its scope — but you can still do it own your own. An example of how to persist state using Ecto is provided in section Examples at Persist State using Ecto.

Visit page API Reference for a brief introduction to double-entry bookkeeping concepts implemented by this library.

Installation

The package can be installed by adding bookk to your list of dependencies in mix.exs:

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

Examples

Chart of Accounts

defmodule ACME.ChartOfAccounts do
  @behaviour Bookk.ChartOfAccounts

  alias Bookk.AccountClass, as: C
  alias Bookk.AccountHead, as: H

  # some of the most common account classes
  @classes %{
     A: %C{id:  "A", parent_id: nil, natural_balance:  :debit, name: "Assets"},
    CA: %C{id: "CA", parent_id: "A", natural_balance:  :debit, name: "Current Assets"},
    AR: %C{id: "AR", parent_id: "A", natural_balance:  :debit, name: "Accounts Receivables"},
     E: %C{id:  "E", parent_id: nil, natural_balance:  :debit, name: "Expenses"},
    OE: %C{id: "OE", parent_id: nil, natural_balance: :credit, name: "Owner's Equity"},
     L: %C{id:  "L", parent_id: nil, natural_balance: :credit, name: "Liabilities"},
    AP: %C{id: "AP", parent_id: "L", natural_balance: :credit, name: "Accounts Payables"},
     I: %C{id:  "I", parent_id: nil, natural_balance: :credit, name: "Income"},
     G: %C{id:  "G", parent_id: "I", natural_balance: :credit, name: "Gains"},
     R: %C{id:  "R", parent_id: "I", natural_balance: :credit, name: "Revenue"}
  }

  @impl Bookk.ChartOfAccounts
  def ledger(:acme), do: "acme"
  def ledger({:user, <<id::binary>>}), do: "user(#{id})"

  @impl Bookk.ChartOfAccounts
  def account(:cash), do: %H{name: "cash/CA", class: @classes.CA}
  def account(:deposits), do: %H{name: "deposits/OE", class: @classes.OE}
  def account({:unspent_cash, {:user, <<id::binary>>}}), do: %H{name: "unspent-cash:user(#{id})/L", class: @classes.L}
  def account({:deposit_expenses, <<provider::binary>>}), do: %H{name: "deposit-expenses:#{provider}/E", class: @classes.E}

  @impl Bookk.ChartOfAccounts
  def account_id(ledger_name, %H{} = account_head), do: ledger_name <> ":" <> account_head.name
end

(DSL) Interledger Entry

Here's demonstrated how to journalize (create a journal entry template, if you may) for an specific accounting transaction. Examples are using ACME.ChartOfAccounts from the previous section.

When user deposits balance (using a single ledger):

import Bookk.Notation, only: [journalize!: 2]

interledger_entry =
  journalize! using: ACME.ChartOfAccounts do
    on ledger(:acme) do
      debit account(:cash), deposited_amount
      credit account({:unspent_cash, {:user, user_id}}), deposited_amount
    end
  end

updated_state =
  Bookk.NaiveState.empty()
  |> Bookk.NaiveState.post(interledger_entry)

When user deposited balance (using multiple ledgers):

import Bookk.Notation, only: [journalize!: 2]

interledger_entry =
  journalize! using: ACME.ChartOfAccounts do
    on ledger(:acme) do
      debit account(:cash), deposited_amount
      credit account({:unspent_cash, {:user, user_id}}), deposited_amount
    end

    on ledger({:user, user_id}) do
      debit account(:cash), deposited_amount
      credit account(:deposits), deposited_amount
    end
  end

updated_state =
  Bookk.NaiveState.empty()
  |> Bookk.NaiveState.post(interledger_entry)

Persist state using Ecto

This section demostrantes how state can be persisted to a database using Ecto instead of posting (apply side-effects) to the in-memory structs provided by the library (such as Bookk.Ledger and Bookk.NaiveState).

First, we'll need two models.

1. Account, which holds the account's balance:

   defmodule Account do
     use Ecto.Schema
   
     import Ecto.Changeset
   
     @primary_key false
     schema "accounts" do
       field :id, :string, primary_key: true
       field :ledger_id, :string
       field :balance, :integer
       field :created_at, :utc_datetime_usec
       field :updated_at, :utc_datetime_usec
     end
   
     @doc false
     @spec changeset(t, map) :: Ecto.Changeset.t()
   
     def changeset(account \\ %__MODULE__{}, %{} = fields) do
       account
       |> cast(fields, [:id, :ledger_id, :balance, :created_at, :udpated_at])
       |> validate_required([:id, :ledger_id, :balance, :created_at, :udpated_at])
     end
   end
   

2. AccountTransaction, which serves as a log of changes to an accounts:

   defmodule AccountTransaction do
     use Ecto.Schema
   
     import Ecto.Changeset
   
     @primary_key false
     schema "accounts_transactions" do
       field :account_id, :string, primary_key: true
       field :transaction_id, Ecto.UUID, primary_key: true
       field :delta_amount, :integer
       field :balance_after, :integer
       field :created_at, :utc_datetime_usec
     end
   
     @doc false
     @spec changeset(t, map) :: Ecto.Changeset.t()
   
     def changeset(account_transfer \\ %__MODULE__{}, %{} = fields) do
       account_transfer
       |> cast(fields, [:account_id, :transaction_id, :delta_amount, :balance, :created_at])
       |> validate_required([:account_id, :transaction_id, :delta_amount, :balance, :created_at])
     end
   end
   

We'll also have a Transactionable protocol that specifies what functions are expected from structs that describe side effects to our accounting system:

defprotocol Transactionable do
  @moduledoc false

  @typedoc false
  @type t :: %{
              required(__struct__) => atom,
              required(:id) => String.t(),
              optional(atom) => any
            }

  @doc false
  @spec to_interledger_entry(t) :: Bookk.InterledgerEntry.t()

  def to_interledger_entry(tx)
end

Now we have a DepositTransaction describing that a user deposited balance into their account — note that it implements Transactionable protocol:

defmodule DepositTransaction do
  @moduledoc false

  @typedoc false
  @type t :: %DepositTransaction{
              id: String.t(),
              user_id: String.t(),
              amount: pos_integer
            }

  defstruct [:id, :user_id, :amount]
end

defimpl Transactionable, for: DepositTransaction do
  use Bookk.Notation

  @impl Transactionable
  def to_interledger_entry(tx) do
    journalize! using: ACME.ChartOfAccounts do
      on ledger(:acme) do
        debit account(:cash), tx.amount
        credit account({:unspent_cash, {:user, tx.user_id}})
      end

      on ledger({:user, tx.user_id}) do
        debit account(:cash), tx.amount
        credit account(:deposits), tx.amount
      end
    end
  end
end

And finally, we have our Accounting module that knows how to take a Transactionable struct and persist its side-effects to the database using Ecto:

defmodule Accounting do
  @moduledoc false

  @doc false
  @spec transact(Transactionable.t()) :: {:ok, Ecto.Multi.t()} | {:error, term}

  def transact(tx) do
    interledger_entry = Transactionable.to_interledger_entry(tx)
    now = DateTime.utc_now()

    multis =
      for {ledger_name, journal_entry} <- Bookk.InterledgerEntry.to_journal_entries(interledger_entry),
          op <- Bookk.JournalEntry.to_operations(journal_entry),
          do: op_to_multi(op, leder_name, tx.id, now)

    multis
    |> Enum.reduce(Ecto.Multi.new(), &Ecto.Multi.append(&2, &1))
    |> ACME.Repo.transaction()
  end

  defp op_to_multi(%Bookk.Operation{} = op, ledger_name, tx_id, now) do
    #   we need uniq names for each multi operation, there will be 2 of them for
    # ↓ each `Bookk.Operation`
    multi_a_name = Ecto.UUID.generate()
    multi_b_name = Ecto.UUID.generate()

    #   the amount by which the account's balance should change (either a
    #   positive or negative integer — in cents or the smallest fraction of the
    # ↓ currency you're using)
    delta_amount = Bookk.Operations.to_delta_amount(op)
    account_id = ACME.ChartOfAccounts.account_id(ledger_name, op.account_head)

    account_changeset =
      Account.changeset(%{
        id: account_id,
        ledger_id: ledger_name,
        balance: delta_amount,
        created_at: now,
        updated_at: now
      })

    Ecto.Multi.new()
    #  ↓ upserts the account
    |> Ecto.Multi.insert(multi_a_name, account_changeset, [
      conflict_target: :id,
      on_conflict: [
        inc: [balance: delta_amount],
        set: [updated_at: now]
      ],
      returning: [:balance]
    ])
    #    creates the log entry recording the amount by which the account's
    #  ↓ balance changed in this accounting transaction
    |> Ecto.Multi.insert(multi_b_name, fn %{^multi_a_name => updated_accocunt} ->
      AccountTransaction.changeset(%{
        account_id: account_id,
        transaction_id: tx_id,
        delta_amount: delta_amount,
        balance_after: updated_account.balance,
        created_at: now
      })
    end)
  end
end