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BrmBar v3 - Architectural Overview

BrmBar v3 is written in Python, with the database stored in PostgreSQL and the primary user interface modelled in QtQuick. All user interfaces share a common brmbar package that provides few Python classes for manipulation with the base objects.

Objects and Database Schema


The most essential brmbar object is an Account, which can track balances of various kinds (described by acctype column) in the classical accounting paradigm:

  • Cash: A physical stash of cash. One cash account is created by default, corresponding to the cash box where people put money when buying stuff (or depositing money in their user accounts). Often, that's the only cash account you need.
  • Debt: Represents brmbar's debt to some person. These accounts are actually the "user accounts" where people deposit money. When a deposit of 100 is made, 100 is subtracted from the balance, the balance is -100 and brmbar is in debt of 100 to the user. When the user buys something for 200, 200 is added to the balance, the balance is 100 and the user is in debt of 100 to the brmbar. This is correct notation from accounting point of view, but somewhat confusing for the users, so in the user interface (and crbalance column of some views), this balance is negated!
  • Inventory: Represents inventory items (e.g. Club Mate bottles). The account balance represents the quantity of items.
  • Income: Represents pure income of brmbar, i.e. the profit; there is usually just a single account of this type where all the profit (sell price of an item minus the buy price of an item) is accumulated.
  • Expense: This type is currently not used.
  • Starting balance and ending balance: This may be used in the future when transaction book needs to be compressed.

As you can see, the amount of cash, user accounts, inventory items etc. are all represented as Account objects that are of various types, are named and have a certain balance (calculated from a transaction book). That balance is a number represented in certain currency. It also has a set of barcodes associated.

Currency, Exchange rate

Usually, all accounts that deal with cash (the cash, debt, income, ... accounts) share a single currency that corresponds to the physical currency locally in use (the default is ). However, inventory items have balances corresponding to item quantities - to deal with this correctly, each inventory item has its own currency; i.e. Club Mate bottle is a currency associated with the Club Mate account.

Currencies have defined (uni-directional) exchange rates. The exchange rate of "Kč to Club Mate bottles" is the buy price of Club Mate, how much you pay for one bottle of Club Mate from the cash box when you are stocking in Club Mate. The exchange rate of "Club Mate bottle to Kč" is the sell price of Club Mate, how much you pay for one bottle of Club Mate to the cash box when you are buying it from brmbar (sell price should be higher than buy price if you want to make a profit).

Exchange rate is valid since some defined time; historical exchange rates are therefore kept and this allows to account for changing prices of inventory items. (Unfortunately, at the time of writing this, the profit calculation actually didn't make use of that yet.)

Transactions, Transaction splits

A transaction book is used to determine current account balances and stores all operations related to accounts - depositing or withdrawing money, stocking in items, and most importantly buying stuff (either for cash or from a debt account). A transaction happenned at some time and was performed by certain responsible person.

The actual accounts involved in a transaction are specified by a list of transaction splits that either put balance into the transaction (credit side) or grab balance from it (debit side). For example, a typical transaction representing a sale of Club Mate bottle to user "pasky" would be split like this:

  • credit of 1 Club Mate on Club Mate account with memo "pasky".
  • debit of 35 Kč on "pasky" account with memo "Club Mate" (indeed we add 35Kč to the debt account for pasky buying the Club Mate; if this seems weird, refer to the "debt" account type description).
  • debit of 5 Kč on income account Profits with memo "Margin on Club Mate" (this represents the sale price - buy price delta, i.e. the profit we made in brmbar by selling this Club Mate).

The brmbar Python Package

The brmbar package (in brmbar/ subdirectory) provides common brmbar functionality for the various user interfaces:

  • Database: Layer for performing SQL queries with some error handling.
  • Currency: Class for querying and manipulating currency objects and converting between them based on stored exchange rates.
  • Account: Class for querying and manipulating the account objects and their current balance.
  • Shop: Class providing the "business logic" of all the actual user operations: selling stuff, depositing and withdrawing moeny, adding stock, retrieving list of accounts of given type, etc.
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