Can you explain the concept of Event Sourcing and how it differs from traditional data handling methods?

Event Sourcing is an architectural pattern where all changes to an application's state are stored as a sequence of events, rather than as the current state of data. Instead of storing only the final state, the system records each event that led to that state.

Differences from traditional methods:

• Traditionally, databases store the current state of entities, and changes simply update records.
• In Event Sourcing, the state is reconstructed by sequentially applying all events.

Advantages:

• Complete history of changes, which facilitates auditing and debugging.
• Ability to reproduce the state at any point in time.
• Simplifies the implementation of CQRS (Command Query Responsibility Segregation).

Example in Go (simplified):

// Balance change event
type Event interface {
    Apply(account *Account)
}

type Deposit struct {
    Amount float64
}

func (d Deposit) Apply(account *Account) {
    account.Balance += d.Amount
}

// Account
type Account struct {
    Balance float64
    Events  []Event
}

func (a *Account) ApplyEvent(e Event) {
    e.Apply(a)
    a.Events = append(a.Events, e)
}

func main() {
    account := &Account{}
    account.ApplyEvent(Deposit{Amount: 100})
    fmt.Println(account.Balance) // 100
}

Thus, the account state is formed from a sequence of events.