Event sourcing
- Node.js course
- Expert
- Lesson 34/47
- 5 min
Learn the event sourcing pattern.
What is event sourcing?
Event sourcing stores the events that produce state instead of storing state directly. State is rebuilt by replaying the events. This gives audit history and the ability to rewind time.
A replayEvents(events) function models this: state starts at 0, each event's effect is applied. Example: replayEvents([{amount: 10}, {amount: 5}, {amount: -3}]) returns 0 + 10 + 5 - 3 = 12. Events are immutable; they are only appended. This fits data with history such as bank balances and order status. Combined with CQRS, writes and reads run through separate models.
The event chain
An event chain is a sequence of immutable events appended in order. Each event carries an effect; the sum of effects yields the state. replayEvents walks this chain with a for...of loop.
Example: let state = 0; for (const e of events) { state += e.amount; }. For events = [{amount: 10}, {amount: 5}, {amount: -3}] state becomes 10, 15, 12 in turn. This is the logic of a real event store: events go to a log, the consumer replays them whenever needed. Because events never change, audit is safe; instead of deleting an old event, you append a compensating event.
Practice Write the event replay function
Write function replayEvents(events). Define let state = 0. Add for (const e of events) { state += e.amount; }. End with return state. Print console.log(replayEvents([{amount: 10}, {amount: 5}, {amount: -3}])).
Starter code
// Write the replayEvents function
// Start from state 0, apply each event's effect
// Print with [{amount: 10}, {amount: 5}, {amount: -3}]
// write hereSample solution
function replayEvents(events) {
let state = 0;
for (const e of events) {
state += e.amount;
}
return state;
}
console.log(replayEvents([{amount: 10}, {amount: 5}, {amount: -3}]));Expected output
12
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 How is state obtained in event sourcing?
replayEvents([{amount: 10}, {amount: 5}, {amount: -3}])- It is read directly from the table
- It is taken from the cache
- Events are replayed Correct answer
- It is written to a constant
Explanation: State is obtained by replaying events.
2 Match each piece to its role
| Term | Meaning |
|---|---|
event |
Immutable record |
replay |
Applies events in order |
amount |
Event's effect |
compensating |
Offsetting event |
3 Complete the loop body
for (const e of events) {
state += ____.amount;
}for (const e of events) {
state += e.amount;
}Explanation: Each event's amount is added.