Event sourcing in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary benefit of using event sourcing in a system?

Consider a system that records all changes as events instead of just storing the current state. What is the main advantage of this approach?

A It allows rebuilding the current state by replaying all past events.

B It reduces the total number of database writes by storing only the latest state.

C It eliminates the need for backups since events are immutable.

D It automatically encrypts all data for security.

💡 Hint

Think about how you can recover or understand the system state at any point in time.

Event sourcing stores every change as an event, enabling the system to rebuild the current state by replaying these events. This provides a full history and audit trail.

❓ Architecture

intermediate

Which component is essential in an event sourcing architecture to reconstruct the current state?

In an event sourcing system, which component is responsible for rebuilding the current state from stored events?

A Event Store

B API Gateway

C Event Processor (or Event Replay Engine)

D Command Handler

💡 Hint

Think about the part that reads events and applies them to get the current state.

The Event Processor or Event Replay Engine reads stored events and applies them in order to reconstruct the current state of the system.

❓ scaling

advanced

How can event sourcing systems handle very large event streams efficiently?

When the number of events grows very large, replaying all events to rebuild state becomes slow. What is a common technique to improve performance?

A Use snapshots to store periodic full states and replay only recent events.

B Delete old events after a fixed time to keep the event store small.

C Store only the latest event and discard previous ones.

D Switch to a relational database to speed up queries.

💡 Hint

Think about saving intermediate states to avoid replaying everything from the start.

Snapshots capture the full state at a point in time, so the system only needs to replay events after the snapshot, improving rebuild speed.

❓ tradeoff

advanced

What is a key tradeoff when using event sourcing compared to traditional state storage?

Event sourcing offers benefits but also has downsides. Which of the following is a common tradeoff?

A Loss of historical data due to overwriting events.

B Increased complexity in rebuilding state and handling event versioning.

C Inability to audit changes since only current state is stored.

D Reduced fault tolerance because events are mutable.

💡 Hint

Consider what extra work developers must do when using event sourcing.

Event sourcing requires managing event versioning and rebuilding state from events, which adds complexity compared to storing only current state.

❓ estimation

expert

Estimate the storage growth for an event sourcing system with 1000 events per second over 1 year, assuming each event is 1 KB.

Calculate the approximate storage needed to keep all events for one year if the system generates 1000 events every second and each event is 1 KB in size.

A About 315 GB

B About 315 TB

C About 3.15 TB

D About 31.5 TB

💡 Hint

Calculate total seconds in a year, multiply by events per second and event size.

There are 31,536,000 seconds in a year. 1000 events/second * 1 KB/event * 31,536,000 seconds = 31,536,000,000 KB = ~31.5 TB.