arkataev/currency_converter

web service for converting currencies

REST web service for currency conversion

• Build pre-commit hooks: black, isort, mypy, Dockerfile Line and YAMLLint
• Monitoring metrics:
  • TODO

Requirements

• Exchange rates updated from web-resources once a day
• Initially supported currencies : Czech koruna, Euro, Polish złoty and US dollar, but can be extended
• Speed of conversion is important

Components

Exchange Rates Map (ERM)

Allows fast retrieval of exchange rate for a given pair of currency codes.

• Highly available
• Read frequently
• Updated at least once a day

Interface

• get_cer(code_x: str, code_y: str) -> float
• update_cer(code_x: str, code_y: str) -> None

Storing strategy

ERM should be fast to read and highly available. Storing ERM in memory will be fast to read, but if server fails we'll need to update ERM once again and that might be expensive. Since that, we might consider some persistable in-memory storage, e.g Redis.

Exchange rates update

ERM is basically a map of permutations of all values in SCC to corresponent exchange rates.

Each exchange rate is retrieved from resource and updated in ERM. As soon as each CER (Currency Exchange Rate) can be updated independently this operation might be performed asynchronously, utilizing a task queue.

Supported Currency Codes Set (SCCS)

Allows fast retrieval of currency codes available for exchange.

• Highly available
• Read frequently
• Updated on demand, but potentially very rarely.

Currency Converter (CC)

Converts given amount of supported currency X to correspondent amount of supported currency Y, using exchange rate of (X,Y). Provides web-api for converting operations.

• Highly available
• Read frequently

Interface

• exchange(exchange_rate: float, amount: float) -> float

sequenceDiagram
  User ->> CC: GET `/currencies/{code_x}/{code_y}/convert?amount={amount}`
  
  activate CC
    CC ->> CC: data validation
    Note left of CC: amount is valid? codes comply to ISO 4217?
  deactivate CC
  alt validation fails
    CC ->> User: 400 Bad Request
  end
  CC ->> SCCS: mcontains(code_x, code_y)
  activate SCCS
  alt all supported 
    SCCS ->> CC: [1,1]
  else any supported
     SCCS ->> CC: [0,1]
     CC ->> User: 404 Not Found
  else all not supported
    SCCS ->> CC: [0,0]
    CC ->> User: 404 Not Found
  end
  deactivate SCCS
  
  CC ->> ERM: get_cer(code_x, code_y)
  ERM ->> CC: cer
  
  activate CC
    CC ->> CC: exchange(cer, amount)
    Note left of CC: result = cer * amount
    CC ->> User: result
  deactivate CC

Loading

WEB-API

To allow greater scalability no state should be stored in API component.

• GET /currencies/{code}/{code}/convert?n={n}