Headline

Implement conversion routines to unify different schemas and data structures.

Description

The data model of 101companies has no differences in its data structure or schema. Especially in the area of Business Intelligence it is a common task to unify data from various different systems. Adding structural differences, such as currencies will help us illustrating procedures on how to transform data.

Motivation

A globally operating company needs to analyze data from various locations (also meaning companies). Each location operates the 101companies HR-System. The HR-System is missing a feature to distinguish between currencies. But for analysis it is mandatory to unify all data to one target currency.

Illustration

This feature will be illustrated using Contribution:pyDWH.

Configuration

The configuration file config.cfg contains all needed information for the conversion. Each source database reflects a specific company or location (in a country). It has a currency for the salaries. There is also a base currency which is the target currency. In addition there is an option for an API to calculate the current exchange rates.

Excerpt of config.cfg

[source_1]
company_id = 1
currency = EUR
...

[source_2]
company_id = 2
currency = USD
...

[target]
...

[general]
base_currency = EUR

# usage: http://finance.yahoo.com/d/quotes.csv?f=sl1&s=USDEUR=X
exchange_rate_api = http://finance.yahoo.com/d/quotes.csv?f=sl1&s==X

While running the ETL (Extract, Transform, Load) it extracts all data from the given source databases, transforms* it and loads it in a unified database schema.

Transformation

First it checks whether we need to convert the currencies. In case the source and target currencies differ it will query the API to retrieve the current exchange rate between both currencies.

etl/exchange.py

import logging
import urllib2
from decimal import Decimal

# Supported currencies based on the list from European Central Bank:
# http://www.ecb.int/stats/exchange/eurofxref/html/index.en.html
CURRENCIES = [
    "AUD", "BGN", "BRL", "CAD", "CHF", "CNY", "CZK", "DKK", "EUR", "GBP",
    "HKD", "HRK", "HUF", "IDR", "ILS", "INR", "JPY", "KRW", "LTL", "LVL",
    "MXN", "MYR", "NOK", "NZD", "PHP", "PLN", "RON", "RUB", "SEK", "SGD",
    "THB", "TRY", "USD", "ZAR"
]


class ExchangeError(Exception):
    """
    Exchange exception for all conversion errors.
    """
    pass


class Exchange(object):
    """
    Performs currency conversion using an external API specified in the
    configuration file.
    """
    def (self, config):
        self.base_currency = config['base_currency']
        assert self.base_currency in CURRENCIES
        self.exchange_rate_api = config['exchange_rate_api']
        self.exchange_rate = None

    def set_exchange_rate(self, source_currency, target_currency=None):
        """
        Fetches current exchange rate from Yahoo! Finance API and set
        instance's exchange_rate with respect to the given target currency.
        """
        if target_currency is None:
            target_currency = self.base_currency

        if source_currency == target_currency:
            # Same currency, i.e. no conversion
            self.exchange_rate = 1
            return

        if source_currency not in CURRENCIES:
            raise ExchangeError("unknown currency: {}".format(source_currency))
        if target_currency not in CURRENCIES:
            raise ExchangeError("unknown currency: {}".format(target_currency))

        data = "{}{}".format(source_currency, target_currency)
        url = self.exchange_rate_api.replace("", data)
        response = ""
        logging.debug("url={}".format(url))
        request = urllib2.Request(url=url)
        try:
            response = urllib2.urlopen(request).read()
        except (urllib2.HTTPError, urllib2.URLError) as err:
            raise ExchangeError(err)
        response = response.strip()

        logging.debug("response={}".format(response))
        if data not in response:
            raise ExchangeError("invalid data from {}: {}".format(url, data))

        exchange_rate = response.split(",", 1)[1]
        self.exchange_rate = Decimal(exchange_rate)

During the transformation is where the conversion actually happens. For each employee 'apply_exchange_rate()' is being called.

Excerpt of etl/transform.py

    def _apply_exchange_rate(self, source_amount):
        """
        Applies exchange rate on the given amount, e.g. to convert it to
        an amount in base currency.
        """
        assert self.exchange is not None
        assert self.exchange.exchange_rate is not None
        if self.exchange.exchange_rate == 1:  # No conversion needed
            return source_amount
        target_amount = Decimal(source_amount) * self.exchange.exchange_rate
        logging.debug("exchange_rate={} [{}]->[{}]".format(
            self.exchange.exchange_rate, source_amount, target_amount))
        return target_amount.quantize(Decimal(".01"), rounding=ROUND_HALF_UP)

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