Event sourcing pattern in Microservices - Architecture Diagram

System Overview - Event sourcing pattern

The Event Sourcing pattern stores all changes to application state as a sequence of events. Instead of saving only the current state, the system records every state-changing event. This allows rebuilding the current state by replaying events, ensuring a full history and audit trail.

Key requirements include reliable event storage, event replay for state reconstruction, and eventual consistency between services.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  v
Command Service ---> Event Store ---> Event Processor ---> Read Model Database
                          |                                   ^
                          v                                   |
                    Event Bus -------------------------------

Components

User

actor

Initiates commands to change system state

Load Balancer

load_balancer

Distributes incoming requests evenly to API Gateway instances

API Gateway

api_gateway

Receives client requests and routes them to appropriate services

Command Service

service

Handles commands, validates them, and creates events

Event Store

database

Stores all events in an append-only log for durability and replay

Event Bus

message_queue

Distributes events asynchronously to interested services

Event Processor

service

Consumes events from the bus and updates the read model

Read Model Database

database

Stores query-optimized views of current state for fast reads

Request Flow - 9 Hops

User → Load Balancer

Load Balancer → API Gateway

API Gateway → Command Service

Command Service → Event Store

Event Store → Event Bus

Event Bus → Event Processor

Event Processor → Read Model Database

Command Service → API Gateway

API Gateway → User

Failure Scenario

Component Fails:Event Store

Impact:New events cannot be saved, so state changes are lost and system cannot rebuild state from events.

Mitigation:Use replication and backups for Event Store. If failure occurs, switch to replica or restore from backup to resume event logging.

Architecture Quiz - 3 Questions

Which component is responsible for storing all changes as events?

A Event Store

B Read Model Database

C API Gateway

D Event Processor

Design Principle

This architecture demonstrates the Event Sourcing pattern by storing all state changes as immutable events in an append-only log. It separates command handling from query models, enabling reliable state reconstruction and auditability. Asynchronous event processing ensures scalability and eventual consistency.

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