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https://github.com/pkrawat1/ex_banking

simple banking OTP application
https://github.com/pkrawat1/ex_banking

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simple banking OTP application

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# elixir-test

Test task for Elixir developers. Candidate should write a simple banking OTP application in Elixir language.



## General acceptance criteria

- All code is in git repo (candidate can use his/her own github account).

- OTP application is a standard mix project.

- Application name is :ex_banking (main Elixir module is ExBanking).

- Application interface is just set of public functions of ExBanking module (no API endpoint, no REST / SOAP API, no TCP / UDP sockets, no any external network interface).

- Application should not use any database / disc storage. All needed data should be stored only in application memory.

- Candidate can use any Elixir or Erlang library he/she wants to (but app can be written in pure Elixir / Erlang / OTP).

- Solution will be tested using our auto-tests for this task. So, please follow specifications accurately.

Public functions of ExBanking module described in this document is the only one thing tested by our auto-tests. If anything else needs to be called for normal application functioning then probably tests will fail.

Code accuracy also matters. Readable, safe, refactorable code is a plus.



## Money amounts

- Money amount of any currency should not be negative.

- Application should provide 2 decimal precision of money amount for any currency.

- Amount of money incoming to the system should be equal to amount of money inside the system + amount of withdraws (money should not appear or disappear accidentally).

- User and currency type is any string. Case sensitive. New currencies / users can be added dynamically in runtime. In the application, there should be a special public function (described below) for creating users. Currencies should be created automatically (if needed).



## API reference

Requirements for public functions provided by ExBanking module. Any function should return success result or error result.



```

@spec create_user(user :: String.t) :: :ok | {:error, :wrong_arguments | :user_already_exists}



Function creates new user in the system

New user has zero balance of any currency

```



```

@spec deposit(user :: String.t, amount :: number, currency :: String.t) :: {:ok, new_balance :: number} | {:error, :wrong_arguments | :user_does_not_exist | :too_many_requests_to_user}



Increases user’s balance in given currency by amount value

Returns new_balance of the user in given format

```



```

@spec withdraw(user :: String.t, amount :: number, currency :: String.t) :: {:ok, new_balance :: number} | {:error, :wrong_arguments | :user_does_not_exist | :not_enough_money | :too_many_requests_to_user}



Decreases user’s balance in given currency by amount value

Returns new_balance of the user in given format

```



```

@spec get_balance(user :: String.t, currency :: String.t) :: {:ok, balance :: number} | {:error, :wrong_arguments | :user_does_not_exist | :too_many_requests_to_user}



Returns balance of the user in given format

```



```

@spec send(from_user :: String.t, to_user :: String.t, amount :: number, currency :: String.t) :: {:ok, from_user_balance :: number, to_user_balance :: number} | {:error, :wrong_arguments | :not_enough_money | :sender_does_not_exist | :receiver_does_not_exist | :too_many_requests_to_sender | :too_many_requests_to_receiver}



Decreases from_user’s balance in given currency by amount value

Increases to_user’s balance in given currency by amount value

Returns balance of from_user and to_user in given format

```



## Performance

- In every single moment of time the system should handle 10 or less operations for every individual user (user is a string passed as the first argument to API functions). If there is any new operation for this user and he/she still has 10 operations in pending state - new operation for this user should immediately return too_many_requests_to_user error until number of requests for this user decreases < 10

- The system should be able to handle requests for different users in the same moment of time

Requests for user A should not affect to performance of requests to user B (maybe except send function when both A and B users are involved in the request)