Event replay in Microservices - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary purpose of event replay in microservices?

In a microservices architecture, event replay is often used. What is its main goal?

A To rebuild the current state of a service by reprocessing past events

B To send real-time notifications to users about new events

C To delete old events from the event store to save space

D To encrypt events before sending them to other services

💡 Hint

Think about why a service might need to process old events again.

Event replay allows a service to reconstruct its state by reprocessing past events, especially after failures or schema changes.

❓ Architecture

intermediate

Which component is essential for enabling event replay in a microservices system?

To support event replay, which component must be part of the system design?

A A message queue that deletes events after delivery

B A load balancer to distribute incoming requests

C A cache to store the latest state snapshot only

D An event store that persists all events in order

💡 Hint

Consider where events must be kept to allow replay.

An event store that saves all events in order is necessary to replay events and rebuild state.

❓ scaling

advanced

How to efficiently scale event replay when the event log grows very large?

When the event log becomes huge, replaying all events can be slow. Which approach best improves replay efficiency?

A Use snapshots to save intermediate states and replay only recent events

B Delete old events regularly to keep the log small

C Replay events on multiple services simultaneously without coordination

D Replay events only during off-peak hours to reduce load

💡 Hint

Think about saving partial progress to avoid replaying everything from the start.

Snapshots store intermediate states so replay can start from a recent snapshot, reducing the number of events to process.

❓ tradeoff

advanced

What is a key tradeoff when enabling event replay in microservices?

Choosing to keep all events for replay has benefits and drawbacks. What is a major tradeoff?

A Reduced network traffic versus higher latency in event processing

B Increased storage and complexity versus ability to recover and audit state

C Simpler codebase versus inability to handle failures

D Faster response times versus less accurate state reconstruction

💡 Hint

Consider what storing all events costs and what benefits it brings.

Keeping all events requires more storage and system complexity but enables full recovery and auditing of state changes.

❓ estimation

expert

Estimate the storage needed for event replay given event size and volume

A microservice generates 1000 events per second. Each event is 1 KB in size. How much storage is needed to keep events for 30 days to support event replay?

A About 26 TB

B About 260 GB

C About 2.6 TB

D About 26 GB

💡 Hint

Calculate total events per day, multiply by event size, then by 30 days.

1000 events/sec * 86400 sec/day = 86,400,000 events/day. Each event 1 KB = 86.4 GB/day. For 30 days: 86.4 GB * 30 = 2.592 TB ≈ 2.6 TB.