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WHEN MICROSERVICES MEET EVENT SOURCINGVinicius Gomes
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VINICIUS GOMES
Software developer at ThoughtWorks
vvgomes.com/blog
twitter.com/vvgomes
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AGENDA
The Traditional Approach
Introduction to Event Sourcing
CQRS
Demo
Summary
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THE TRADITIONAL APPROACH
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EXAMPLE
Online Restaurant
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ONLINE RESTAURANT
Data Model
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ONLINE RESTAURANT
Services
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Restaurant
ONLINE RESTAURANT
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📱POST
GET
I want to open an order!
200 ✔
Restaurant
ONLINE RESTAURANT
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📱POST
GET
I want to add an item to
my order!
200 ✔
THE TRADITIONAL APPROACH
• Simple to implement
• Technology agnostic
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GET POST PUT
DELETE
CHALLENGES
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CHALLENGES
Coupling
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CHALLENGES
Coupling
Resilience
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Restaurant
📱POST
GET
I want to add an item to my order!
503👎
CHALLENGES
Coupling
Resilience
Response time
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CHALLENGES
Coupling
Resilience
Response time
User intent
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POST /orders/8659a0d6/items/
Add item to order
CHALLENGES
Coupling
Resilience
Response time
User intent
Current / mutable state
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{ "id": “6a208c41…”, "status": "OPEN", "items": [ { "id": “be596c8e…”, "quantity": 1 }, { "id": “5d09509c…”, "quantity": 2 }, { "id": “cc52d1b6…”, "quantity": 1 } ] }
EVENT SOURCING
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EVENT SOURCING
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DOMAIN EVENTS
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{ "payload": { "orderId": “be596c8e…”, "itemId": “5d09509c…”, "quantity": 1 }, "payloadType": “com.restaurant.ItemAddedToOrderEvent”, "timestamp": “2017-03-25 08:48:51 -03:00”, "revision": “2”, "aggregateId": “6a208c41…”
}
Model the past
Immutable
Permanent
EVENT STORE
Append only
Sequential
Long-lived
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AGGREGATES
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state = reduce(handle, {}, events)
Reducer function
Reducing events into state
AGGREGATES
Example
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{ }
AGGREGATES
Example
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payload: { id: “42” }
{ "id": “42”, "status": "OPEN", "items": [ ] }
AGGREGATES
Example
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{ "id": “42”, "status": "OPEN", "items": [
{ "id": “43”, “quantity": 1
} ] }
payload: { orderId: “42”, itemId: “43”, quantity: 1 }
payload: { orderId: “42”, itemId: “44”, quantity: 2 }
AGGREGATES
Example
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{ "id": “42”, "status": "OPEN", "items": [
{ "id": “43”, “quantity": 1
}, { "id": “44”, “quantity": 2
} ] }
AGGREGATES
Example
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{ "id": “42”, "status": "OPEN", "items": [
{ "id": “44”, “quantity": 2
} ] }
payload: { orderId: “42”, itemId: “43” }
AGGREGATES
Example
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{ "id": “42”, "status": "PLACED", "items": [
{ "id": “44”, “quantity": 2
} ] }
payload: { orderId: “42” }
AGGREGATES
Example
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{ "id": “42”, "status": "PLACED", "items": [
{ "id": “44”, “quantity": 2
} ] }
Order Aggregate
COMMANDS
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Add item to order
{ "type": “com.restaurant.AddItemToOrderCommand”,
"payload": { "orderId": “be596c8e…”, "itemId": “5d09509c…”, "quantity": 1 }, "timestamp": “2017-03-25 08:48:51 -03:00”
}
COMMANDS
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(state, command) -> [event]
Handling a command
COMMANDS
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How do clients send commands?
📱
I want to add an item to my order!
?
COMMANDS
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Reviewing REST
Resource Collection
host/api/orders
COMMANDS
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Reviewing REST
Resource
host/api/orders/8659a0d6
COMMANDS
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Reviewing REST
Nested Resource
host/api/orders/8659a0d6/items/5d1a8457
COMMANDS
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Reviewing REST
HTTP MethodsGET POST PUT/PATCH DELETE
COMMANDS
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Reviewing REST
Hypermedia Formats
COMMANDS
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Commands can be resources
COMMANDS
Option #1
Generic “Commands” nested-resource
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POST /orders/8659a0d6/commands
{ “type": “AddItemToOrderCommand”, "menuItem": “/menu/items/8d30d99“, “quantity": 2 }
COMMANDS
Option #2
Command URI
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POST /orders/8659a0d6/items/commands/add
{ "menuItem": “/menu/items/8d30d99“, “quantity": 2 }
COMMANDS
Command discoverability
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EVENT SOURCING
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Queries?
QUERIES
Bad news 😔
The Event Store is not good for queries
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Most applications
•A few writes
•A lot of reads
CQRS
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CQRS
Command Query Responsibility Segregation
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Write ✍
Read 📖
Service
CQRS
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📱
Command
Query
Publish
Listen
Service A
CQRS
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Service B
CQRS
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Command model •Command definitions
•Event definitions
•The aggregate
•Aggregate repository
•Write only API
Query model •Event listeners
•Query entities
•Repositories
•Read only API
DEMO
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DEMO
Online Restaurant
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Service
DEMO
Online Restaurant
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EXAMPLE: ONLINE RESTAURANT
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Online Restaurant
Publishes Listen to
EXAMPLE: ONLINE RESTAURANT
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Online Restaurant
Publishes Listen to
Publishes Listen to
TECHNOLOGIES
Java
Spring Boot
Axon Framework
Spring Data REST
RabbitMQ
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https://github.com/vvgomes/event-driven-restaurant
SUMMARY
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MICROSERVICES + EVENT SOURCING
Benefits •History based queries
•Audit log by design
• Immutability
•User intent
•Decoupling
•Resilience
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Challenges •Complexity
•Snapshots
•Upcasting
•Race conditions
•Event contracts
•Eventual consistency
WHEN MICROSERVICES MEET EVENT SOURCINGVinicius Gomes
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