A Digital Wallet lets users:

Store and manage balances,
Make payments and transfers,
Top up or withdraw funds,
All securely, consistently, and auditable — often as part of a larger payment system.

Client → API Gateway → Wallet Service → Ledger Service
                           ↓                 ↓
                         Queue        → Notification / Risk Engine
                           ↓
                 External Payment Gateway / Bank

Wallet Table

Field	Description
wallet_id	Unique ID per user
user_id	Owner
balance	Current computed balance
status	Active, frozen, closed
currency	Currency type

Ledger Table (Immutable)

Field	Description
entry_id	Unique entry
txn_id	Transaction ID
debit_account_id	Source wallet
credit_account_id	Destination wallet
amount	Transaction amount
created_at	Timestamp
status	Pending / Completed / Reversed

Top-up

User → Wallet Service → External Payment Gateway → Ledger Service

Steps:

Call gateway (e.g., card/bank API) to debit external source.
On success, credit wallet account in ledger.
Update wallet balance (eventually or synchronously).
Notify user.

Challenges:

Handling delayed gateway callbacks (idempotency required).