Event sourcing pattern in Microservices - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Event Sourcing pattern?

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

intermediate

How does Event Sourcing help with data consistency in microservices?

By storing all changes as events, Event Sourcing ensures a clear, immutable history of state changes. This helps microservices maintain consistency by replaying events to reach the current state, avoiding conflicts from concurrent updates.

beginner

What is an event store in Event Sourcing?

An event store is a specialized database that saves all events in order. It acts like a journal, recording every change. The event store allows replaying events to rebuild the current state or audit past changes.

advanced

What is a common challenge when using Event Sourcing?

One challenge is handling event versioning when the event structure changes over time. Systems must support backward compatibility or migration strategies to replay old events correctly.

intermediate

How does Event Sourcing relate to CQRS (Command Query Responsibility Segregation)?

Event Sourcing often pairs with CQRS, where commands change state by generating events, and queries read from a separate, optimized read model built from those events. This separation improves scalability and performance.

What does Event Sourcing store in its database?

A A sequence of events representing all state changes

B Only the current state snapshot

C User session data

D Database schema changes

Event Sourcing stores every change as an event, not just the current state.

Which component is responsible for saving events in Event Sourcing?

A Load balancer

B Event store

C Cache

D API gateway

The event store is the database that saves all events in order.

Why is Event Sourcing useful for auditing?

A Because it keeps a full history of all changes

B Because it deletes old data

C Because it encrypts data

D Because it caches queries

Event Sourcing records every change, making it easy to audit the full history.

What is a key benefit of combining Event Sourcing with CQRS?

A Simplifying database schema

B Encrypting all events

C Reducing network latency

D Separating write and read models for better scalability

CQRS separates commands and queries, improving scalability when used with Event Sourcing.

What is a common difficulty when evolving an Event Sourcing system?

A Encrypting the event store

B Scaling the user interface

C Handling changes in event formats over time

D Managing user sessions

Event versioning is challenging because old events must still be replayed correctly after changes.

Explain how Event Sourcing works and why it is useful in microservices.

Think about saving every change instead of just the final state.

Describe the relationship between Event Sourcing and CQRS and how they improve system design.

Consider how commands and queries are handled differently.

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