Mucon: Not Just Events: Developing Asynchronous Microservices

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@crichardson
Not Just Events: Developing
Asynchronous Microservices
Chris Richardson
Founder of Eventuate.io
Founder of t...

@crichardson
Presentation goal
Using asynchronous
messaging to implement
transactions and queries in a
microservice archit...

@crichardson
Presentation goal
Microservices != REST
Microservices != Events
Microservices != Event Sourcing
Apache Kafka ...

@crichardson
About Chris

@crichardson
About Chris
Consultant and trainer
focussed on helping
organizations adopt the
microservice architecture
(htt...

@crichardson
About Chris
Founder of a startup that is creating
an open-source/SaaS platform
that simpliﬁes the development...

@crichardson
About Chris
https://www.manning.com/books/microservices-patterns
40% discount with code
ctwmucon18

About Chris: microservices.io
Microservices pattern
language
Articles
Code examples
Microservices Assessment
Platform - co...

@crichardson
Agenda
Transactions, queries and microservices
Managing transactions with sagas
Implementing queries with CQRS

The microservice architecture
structures
an application as a
set of loosely coupled
services

@crichardson
API
Service = independently deployable
component
Operations
Event
Publisher
Commands
Queries
Synchronous
REST...

@crichardson
Microservices enable
continuous delivery/deployment
Process:
Continuous delivery/deployment
Organization:
Sma...

@crichardson
Let’s imagine that you are
building an online store API
createCustomer(creditLimit)
createOrder(customerId, o...

@crichardson
Order
Service
createCustomer()
createOrder()
ﬁndOrdersForCustomer()
ﬁndRecentCustomers()
API Gateway
createCu...

@crichardson
No ACID transactions that span
services
BEGIN TRANSACTION
…
SELECT ORDER_TOTAL
FROM ORDERS WHERE CUSTOMER_ID ...

@crichardson
Querying across services is
not straightforward
SELECT *
FROM CUSTOMER c, ORDER o
WHERE
c.id = o.ID
AND c.id ...

@crichardson
Agenda
Transactions, queries and microservices
Managing transactions with sagas
Implementing queries with CQRS

@crichardson
From a 1987 paper

@crichardson
Saga
Use Sagas instead of 2PC
Distributed transaction
Service A Service B
Service A
Local
transaction
Service...

@crichardson
Order Service
Create Order Saga
Local transaction
Order
state=PENDING
createOrder()
Customer Service
Local tr...

@crichardson
Saga design challenges
API design
Synchronous REST API initiates asynchronous saga
When to send back a respon...

How do the saga participants
communicate?
Synchronous
communication, e.g. REST
= temporal coupling
Client and server need ...

@crichardson
Collaboration using asynchronous,
broker-based messaging
Order Service
Customer
Service
….
Message broker

@crichardson
About the message broker
At least once delivery
Ensures a saga completes when its participants are
temporaril...

@crichardson
Saga step = a transaction
local to a service
Service
Database Message Broker
update publish message/event
How...

@crichardson
How to sequence the saga
transactions?
After the completion of transaction Ti “something” must
decide what st...

@crichardson
Choreography: distributed decision making
vs.
Orchestration: centralized decision making

@crichardson
Agenda
Overview
Choreography
Orchestration
Transactional
messaging
Transactions, queries and microservices
Ma...

@crichardson
Choreography-based coordination
using events
Order cancelled
Order
Service
Customer
Service
DB DB
Order creat...

@crichardson
Message broker
Choreography-based Create Order
Saga
Order created
Credit Reserved
Credit Limit Exceeded
Creat...

@crichardson
API
Order Service
Operations
createOrder()
Event
Publisher
Order
events
Event
Subscriber
Customer
Events
Serv...

@crichardson
API
Customer Service
Operations
createCustomer()
Event
Publisher
Customer
Events
Event
Subscriber
Order
Event...

Beneﬁts and drawbacks of
choreography
Beneﬁts
Simple, especially when using
event sourcing
Participants are loosely couple...

@crichardson
Overview
Choreography
Orchestration
Transactional
messaging
Agenda
Transactions, queries and microservices
Ma...

@crichardson
Order Service
Orchestration-based coordination using
command messages
Local transaction
Order
state=PENDING
c...

@crichardson
Request/asynchronous reply-
based communication
Release credit
Order
Service
Customer
Service
DB
DB
Reserve c...

@crichardson
A saga (orchestrator)
is a persistent object
that
tracks the state of the saga
and
invokes the participants

Saga orchestrator behavior
On create:
Invokes a saga participant
Persists state in database
Wait for a reply
On reply:
Loa...

@crichardson
Order Service
CreateOrderSaga orchestrator
Customer Service
Create Order
Customer
creditLimit
creditReservati...

@crichardson
API
Order Service
Operations
createOrder()
API
Client
Invokes
Customer
Service
Service
Database
reserveCredit...

@crichardson
API
Customer Service
Operations
createCustomer()
reserveCredit()
releaseCredit()
Service
Database
Simple
API

@crichardson
Eventuate Tram Sagas
Open-source Saga orchestration framework
Currently for Java
https://github.com/eventuate...

Beneﬁts and drawbacks of
orchestration
Beneﬁts
Centralized coordination
logic is easier to understand
Reduced coupling, e....

@crichardson
Overview
Choreography
Orchestration
Transactional
messaging
Agenda
Transactions, queries and microservices
Ma...

@crichardson
Messaging must be
transactional
Service
Database Message Broker
update publish
How to
make atomic
without 2PC?

Publish to message broker
ﬁrst?
Guarantees atomicity
BUT
Service can’t read its own
writes
Difﬁcult to write business
logi...

@crichardson
Option: Event sourcing
=
Event centric approach to
business logic and persistence
http://eventuate.io/

@crichardson
Event sourcing: persists an object
as a sequence of events
Event table
Entity type
Event
id
Entity
id
Event
d...

@crichardson
Replay events to recreate in memory state
Order
state
apply(event)
Event table
Entity type
Event
id
Entity
id...

@crichardson
FYI:
Apache Kafka != event store

@crichardson
Event sourcing guarantees: state
change event is published
Event table
Entity type
Event
id
Entity
id
Event
d...

@crichardson
Preserves history of domain objects
Supports temporal queries
Simpliﬁes retroactive correction
Built-in audit...

@crichardson
Unfamiliar programming model
Evolving the schema of long-lived events
Event store only supports PK-based acce...

@crichardson
Good ﬁt for choreography-based sagas
BUT orchestration is more challenging
Use event handler to translate eve...

@crichardson
Option:
Traditional persistence
(JPA, MyBatis,…)
+
Transactional outbox pattern
https://github.com/eventuate-...

@crichardson
Spring Data for JPA example
Publish event
Save order
http://eventuate.io/exampleapps.html

@crichardson
Customer Service: consuming
domain events…
https://github.com/eventuate-tram/eventuate-tram-examples-customer...

@crichardson
Atomically updating state and
publishing events
ACID
transaction
See BASE: An Acid Alternative, http://bit.ly...

@crichardson
Transaction log tailing
Order
Service
Datastore
ORDER table
Transaction log
Update
Transaction log
miner
Mess...

@crichardson
Polling the message table
Simple
Works for all databases
BUT what about polling frequency
MESSAGE
Table
Messa...

@crichardson
Agenda
Transactions, queries and microservices
Managing transactions with sagas
Implementing queries with CQRS

@crichardson
Queries often retrieve data
owned by multiple services

@crichardson
API Composition pattern
Customer Service
Customer
…
Order Service
Order
…
API Gateway
ﬁndOrdersForCustomer(cu...

@crichardson
Find recent, valuable
customers
SELECT *
FROM CUSTOMER c, ORDER o
WHERE
c.id = o.ID
AND o.ORDER_TOTAL > 10000...

API Composition would be
inefﬁcient
1 + N strategy:
Fetch recent customers
Iterate through customers
fetching their shippe...

@crichardson
Using events to update a queryable
replica = CQRS
Order
Service
Customer
Service
Order events
Customer events...

@crichardson
API
Order Service
Operations
createOrder()
Event
Publisher
Order
events
Service
Database

@crichardson
API
Customer Service
Operations
createCustomer()
Event
Publisher
Customer
events
Service
Database

@crichardson
Customer Order View Service
Operations
ﬁndOrder()
ﬁndOrders….()
Event
Subscriber
Order
Events
Service
Databas...

@crichardson
Query side data model
Command Query Responsibility
Segregation (CQRS)
Command side data model
Commands
Aggreg...

@crichardson
Queries database (type)
Command side
POST
PUT
DELETE
Aggregate
Event Store/Message Broker
Events
Query side
G...

@crichardson
A CQRS view can be part of a
service
Restaurant
Service
Restaurant events channel
Transactional
Source of tru...

@crichardson
Use a CQRS replica to validate
commands
Order Service
createOrder()
Customer
Service
Orders
Customers
creditL...

@crichardson
CQRS views are disposable
Rebuild when needed from source of truth
Using event sourcing
(Conceptually) replay...

@crichardson
Handling replication lag
Lag between updating command side and CQRS view
Risk of showing stale data to user
E...

@crichardson
Summary
Use synchronous protocols, e.g. REST or gRPC:
External APIs
(Read only) API Composition
Use asynchron...

@crichardson
@crichardson chris@chrisrichardson.net
http://learn.microservices.io
Questions?
40% discount with code
ctwmuc...

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