Event sourcing in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is event sourcing in system design?

Event sourcing is a design pattern where all changes to application state are stored as a sequence of events. Instead of saving only the current state, the system saves every change as an event, allowing the state to be rebuilt by replaying these events.

beginner

How does event sourcing differ from traditional state storage?

Traditional storage saves only the current state, overwriting previous data. Event sourcing saves every change as an event, preserving the full history and allowing state reconstruction at any point in time.

intermediate

What are the main benefits of using event sourcing?

Benefits include: full audit trail of changes, easier debugging by replaying events, ability to rebuild state after failure, and support for complex business logic with event replay.

intermediate

What is an event store in event sourcing?

An event store is a specialized database that stores all events in order. It acts like a journal or log where each event is appended and never deleted, ensuring the history is complete and immutable.

intermediate

How do you rebuild the current state in an event sourced system?

You rebuild the current state by replaying all stored events in order, applying each event to an initial empty state until you reach the latest state.

What does event sourcing primarily store?

A Only the latest state snapshot

B Database schema changes

C User session data

D A sequence of all state changes as events

Event sourcing stores every change as an event, not just the latest state.

Which of the following is a key advantage of event sourcing?

A No need for backups

B Faster writes than traditional databases

C Ability to replay events to rebuild state

D Simpler database schema

Replaying events allows rebuilding the state and auditing changes.

What is an event store?

A A database that stores events in order

B A cache for user data

C A tool for monitoring system health

D A backup storage system

An event store is a database designed to store events sequentially and immutably.

How is the current state obtained in event sourcing?

A By replaying all events from the start

B By reading the last event only

C By querying a snapshot only

D By using a cache

The current state is rebuilt by applying all events in order.

Which scenario best suits event sourcing?

A Simple static websites

B Systems needing full audit trails

C Temporary session storage

D Single-use scripts

Event sourcing is ideal for systems requiring detailed history and auditability.

Explain event sourcing and how it helps in rebuilding application state.

Think about how a bank keeps track of every transaction to know the current balance.

Describe the role and characteristics of an event store in event sourcing.

Imagine a diary where you only add new entries and never erase old ones.