###### tags `interview`
Wallet System Design
===
Imagine that you are running a project that helps customers manage
their money inside your company.

### Requirements
- 1 Customer can have multiple wallets
- Cusotmers can transfer their money to others
- Customers are able to check their own wallet balance
- Customers are able to query for their transaction history

### Question:
- How would you design from Database layer to API layer

# Database schema

wallet
  id

transaction
  amount +-
  wallet_id

SELECT SUM(transaction.amount)
FROM transaction
WHERE wallet_id = ''

#### customer

# API Layer
Request example:

Pseudo Code:

# Additional Requirements
- Let's say your have servers that can deal with multiple requests at the same time.
- When there are serveral customer doing high frequency transactions, how do you ensure your system is correct
- When system has uncaught error, that causes user's balance not to be updated correctly:
- what would you do to recover your system?
- how do you get the alert as early as you can?

- If you are leading a team, how would you do the technical scoping?