Event sourcing pattern in Microservices - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the primary benefit of using event sourcing in microservices?

Consider a microservices system that uses event sourcing. What is the main advantage of this pattern?

A It stores the current state only, reducing storage needs.

B It guarantees zero network latency between services.

C It eliminates the need for any database in the system.

D It allows reconstructing past states by replaying events.

💡 Hint

Think about how event sourcing handles data changes over time.

Event sourcing stores all changes as events, allowing the system to rebuild any past state by replaying these events.

❓ Architecture

intermediate

Which component is essential in an event sourcing architecture?

In an event sourcing system, which component is responsible for storing all the events?

A Event Store

B Command Handler

C Read Model

D API Gateway

💡 Hint

Think about where the system keeps the history of all changes.

The Event Store is the component that stores all events in the system, acting as the source of truth.

❓ scaling

advanced

How does event sourcing help with scaling read operations in microservices?

In a microservices system using event sourcing, how can read operations be scaled efficiently?

A By replaying events on demand for every read request.

B By blocking writes during heavy read traffic.

C By using separate read models built from events for queries.

D By storing all data in a single monolithic database.

💡 Hint

Consider how read models can be optimized separately from writes.

Event sourcing allows building specialized read models from events, which can be scaled independently to handle queries efficiently.

❓ tradeoff

advanced

What is a common tradeoff when using event sourcing in microservices?

Which of the following is a typical downside of adopting event sourcing?

A Increased complexity in rebuilding current state from events.

B Immediate consistency without eventual consistency concerns.

C Simpler debugging due to fewer stored data points.

D No need for backups because events are immutable.

💡 Hint

Think about the effort needed to get the current state from stored events.

Event sourcing requires replaying events to rebuild state, which adds complexity and can impact performance.

❓ estimation

expert

Estimate the storage growth in an event sourcing system over time.

A microservice using event sourcing records 1000 events per minute. Each event is 1 KB in size. Estimate the storage needed after 1 year (assume 365 days).

A About 525 TB

B About 525 GB

C About 525 MB

D About 525 KB

💡 Hint

Calculate total events per year and multiply by event size.

1000 events/min × 60 min/hr × 24 hr/day × 365 days = 525,600,000 events. Each event is 1 KB, so total storage ≈ 525,600,000 KB ≈ 525 GB.

Practice

1. What is the main idea behind the Event sourcing pattern in microservices?

easy

A. Store all changes as a sequence of events instead of only the current state

B. Store only the latest snapshot of data for faster access

C. Use events only for communication between services, not for storage

D. Store events only when errors occur in the system

Solution

1. Step 1: Understand event sourcing concept

   Event sourcing means saving every change as an event, not just the final state.

2. Step 2: Compare options with definition

   Only Store all changes as a sequence of events instead of only the current state correctly describes storing all changes as events, others focus on snapshots or partial use.

3. Final Answer:

   Store all changes as a sequence of events instead of only the current state -> Option A

4. Quick Check:

   Event sourcing = storing all changes as events [OK]

Hint: Event sourcing saves every change, not just the final state [OK]

Common Mistakes:

• Confusing event sourcing with snapshot storage
• Thinking events are only for communication
• Believing events are stored only on errors

2. Which of the following is the correct sequence in event sourcing when a user updates their profile?

easy

A. Update state directly, then create an event for the change

B. Send event to other services before updating state

C. Create an event representing the change, then update the state from the event

D. Store the new state without creating any event

Solution

1. Step 1: Recall event sourcing flow

   In event sourcing, changes are first recorded as events, then state is updated from those events.

2. Step 2: Match options to flow

   Create an event representing the change, then update the state from the event correctly shows event creation before state update; others do not follow this order.

3. Final Answer:

   Create an event representing the change, then update the state from the event -> Option C

4. Quick Check:

   Event first, then state update = Create an event representing the change, then update the state from the event [OK]

Hint: Events come before state updates in event sourcing [OK]

Common Mistakes:

• Updating state before creating events
• Skipping event creation
• Sending events before state is updated

3. Given this simplified event sourcing code snippet, what will be the final user balance?

events = [
  {"type": "Deposit", "amount": 100},
  {"type": "Withdraw", "amount": 30},
  {"type": "Deposit", "amount": 50}
]

balance = 0
for event in events:
    if event["type"] == "Deposit":
        balance += event["amount"]
    elif event["type"] == "Withdraw":
        balance -= event["amount"]
print(balance)

medium

A. 80

B. 150

C. 100

D. 120

Solution

1. Step 1: Calculate deposits and withdrawals

   Deposits: 100 + 50 = 150; Withdrawals: 30

2. Step 2: Compute final balance

   Balance = 0 + 150 - 30 = 120

3. Final Answer:

   120 -> Option D

4. Quick Check:

   Deposits minus withdrawals = 120 [OK]

Hint: Add deposits, subtract withdrawals to find balance [OK]

Common Mistakes:

• Adding withdrawals instead of subtracting
• Ignoring last deposit event
• Starting balance not zero

4. In an event sourcing system, which issue is caused by missing an event during state reconstruction?

medium

A. System crashes immediately

B. State becomes inconsistent or incorrect

C. Events are duplicated in the event store

D. Commands fail to generate new events

Solution

1. Step 1: Understand event replay in event sourcing

   State is rebuilt by replaying all events in order.

2. Step 2: Effect of missing an event

   If an event is missing, the rebuilt state will not reflect all changes, causing inconsistency.

3. Final Answer:

   State becomes inconsistent or incorrect -> Option B

4. Quick Check:

   Missing event = incorrect state [OK]

Hint: Missing events cause wrong state, not crashes [OK]

Common Mistakes:

• Assuming system crashes on missing event
• Confusing missing event with duplicate events
• Thinking commands stop working due to missing event

5. You are designing a microservice using event sourcing. To improve performance, you want to avoid replaying all events every time. Which approach is best to achieve this?

hard

A. Use snapshots to save periodic state and replay only recent events

B. Store only the latest event and discard older ones

C. Send all events to other services for processing

D. Update state directly and ignore events after initial load

Solution

1. Step 1: Identify performance issue in event sourcing

   Replaying all events from the start can be slow as event count grows.

2. Step 2: Choose solution to reduce replay time

   Snapshots save the current state at intervals, so only recent events need replaying.

3. Final Answer:

   Use snapshots to save periodic state and replay only recent events -> Option A

4. Quick Check:

   Snapshots improve replay speed [OK]

Hint: Snapshots speed up state rebuild by reducing event replay [OK]

Common Mistakes:

• Discarding old events breaks event sourcing
• Sending all events elsewhere doesn't reduce replay
• Ignoring events after initial load loses audit trail