Consider the core functionalities a personal financial manager should have, such as expense tracking, budgeting, and financial reporting. How would you design the system to handle these features efficiently?

Think about the data structures that could be used to store and manage financial transactions. What are the trade-offs between using a hash table, a tree, or a graph for this purpose?

How would you design the system to handle concurrent transactions and ensure data consistency? Consider the use of locks, transactions, or eventual consistency models.