Event Sourcing

Storing only the current state loses history

A bank account row says balance = 100. That's it. How did it get there? Which deposits, which withdrawals, in what order? A bad update silently overwrites the truth forever.

class Account {
  balance = 100;

  deposit(amt)  { this.balance += amt; }
  withdraw(amt) { this.balance -= amt; }
  // how did we reach 100? no idea — overwritten.
}

The log is the source of truth

Every change is appended as an event — never mutated, never deleted. Current state is derived by folding (replaying) the log.

const events = [
  { type: 'Deposited', amount: 100 },
  { type: 'Withdrew',  amount: 30  },
  { type: 'Deposited', amount: 50  },
];

// state = fold of history, never mutated in place
const apply = (bal, e) =>
  e.type === 'Deposited' ? bal + e.amount : bal - e.amount;

const balance = events.reduce(apply, 0); // 120

Append events — replay or time-travel

Add events to the log below, then Replay to fold them into a balance step by step, or drag the slider to see the balance as of any past event.

The log never lies

• Full audit trail — every change is a fact you can inspect, forever.
• Time-travel & debugging — rebuild state as of any point in history to see exactly how a bug happened.
• Rebuild any projection — replay the same log into new read models without touching the source of truth.
• Natural fit with events — domain events, not table rows, become the primary data.
• Caution: event schema versioning, replay cost at scale (use snapshots), and eventual consistency of derived views.
• Used in: ledgers, git commits, Kafka topics, banking systems.