1. AUTONOMOUS MICROSERVICES FOR A
FINANCIAL SYSTEM
Jeppe Cramon - @jeppec
Chief Architect - INPAY

2. “Inpay’s vision is to transform global trade
by making cross-border payments as fast,
easy, safe, and cost-efficient as local bank transfers”
Jacob Tackmann Thomsen
Founder & CEO
2

3. Our development teams main goal:
Build a flexible and scalable solution
that can grow with the company
3

4. WE WANT TO BE ABLE TO ADAPT
TO CHANGES QUICKLY

5. WE ALL WANT TO MOVE FAST
But many organization experience that they cannot move fast

6. THE MAIN IMPEDIMENT TO
MOVING FAST?

7. High coupling

8. It usually starts simple
“The” DB
UI
Logic
Data Access

9. And we end up
with…

10. Reality rears its ugly head

11. It’s hard to see
the sky

12. How to ensure ending up there?
Be in a hurry!
12

13. Being in a hurry results in writing bad code
Bad code slows us down every time we want
to change something
13

14. Bad code is rigid
Changing one thing requires you to modify a lot of
other code to make the codebase consistent again
14

15. Bad code is fragile
15
When you change a thing you end up
breaking something else!

16. Coupling
• Coupling is a measure of dependencies:
• Efferent coupling – who do you depend on
• Afferent coupling – who depends on you
• Different aspects on coupling:
• Temporal coupling (processing time of Y affects X – if Y is down can X continue?)
• Spatial coupling (where on the network are the Y instances and how does X know?)
• Behavioral coupling (how much about Y’s behavior does X understand)
X Y

17. End result
“Nobody is allowed to modify that module!”
or
“Let’s rewrite!”
17

18. If we want to go fast,
then we need to do a good job
This means:
• Take your time
• Study the problem
• Work iteratively to refine until you end up at a simple solution
18

19. Microservices promise a solution to our problem
Monolith
Microservice Microservice
Microservice MicroserviceMicroservice
Microservice
Microservice

20. There’s never time to do it right
But there’s always time to do it over
20

21. THERE IS VALUE IN MAKING
THINGS SMALLER
For one thing it is easier to reason about them in isolation
It’s easier to replace them – since coupling is explicit

22. Microservices as an architectural style
22

23. ARCHITECTURE IS ESSENTIAL
Because it influences how fast we can respond to changes
and what the cost of those changes are

24. What is the right level of coupling?
This highly depends on
• How likely are things to change
• And what parts that change together
24

25. MICROSERVICES TELL US TO
ADHERE TO THE SRP
Single Responsibility Principle (SRP) –
“A Microservice should have one reason – and only one reason – to change”

26. Does your domain
model adhere to
the SRP?

27. How to create a big entangled
domain model?
27

28. Result will be fragmented domain logic
Focus on Nouns (Entities)
and retrofit Verbs later
28

29. The Verbs capture the intent/task we wish to
accomplish – i.e. the use cases
29
A Use case typically involve more than one Entity

30. If we primarily model around nouns/entities
we can easily violate the SRP
Changes to requirements
is likely to require changes
to multiple entity classes
30

31. If a change to requirements forces us to
“refactor” the structure of your domain model
Then it wasn’t a model of the domain!
31

32. A change to requirements should be a
“change to how we do business”
Not a change to what business we’re in!
32

33. "We don't sell domains, WE SELL USE CASES!"
Jim Coplien
33

34. So instead of focusing on Nouns
We should focus on Verbs!
34

35. Second reason for big entangled
domain models
Trying to create a single model to serve
everyone
35

36. Every system has
Multiple Users with multiple roles
Multiple Stakeholders
36

37. It’s very hard to come up with one canonical
model that’s good for everybody
37

38. Different perspectives on data
With in a given Domain, e.g. Retail, there will exist multiple bounded contexts/sub-domains/business
capabilities such as:
• Product management
• Purchase
• Sales
• Pricing
• Inventory
• Shipping
• Support
• Accounting
• Management
Each of these lines of business have very specific and unique needs which are relevant for them alone in order
to conduct their business. They might use the same name for the entities they’re interested in or they might use
different names for the same logical entity.

39. Many perspectives on data
Online Retail System
Product
Unit Price
Promotional Price
Promotion End Date
Stock Keeping Unit (SKU)
Quantity On Hand (QOH)
Location Code
Price
Quantity Ordered
Name
The lifecycle of the data is VERY important!
Customer
Pricing
Inventory
Sales
Management Reporting

40. Smaller models & clear data ownership
Retail System
Pricing
Product
ProductID
Unit Price
Promotional
Price
…
Pricing
Inventory
Product
ProductID
SKU
QOH
Location Code
…
Inventory
Sales
Product
ProductID
Name
Description
Quantity
Ordered
…
Sales
Shared Entity identity

41. How can we discover bounded contexts?
41

42. BOUNDED CONTEXT
The setting in which a word or a statement
appears that determines its meaning
42

43. HOW TO GATHER REQUIREMENTS FROM MANY DIFFERENT
STAKEHOLDERS/DEPARTMENTS/SPECIALISTS?
Department C
Department E
Department A
Department D
Department B

44. Sequential data gathering?
Time

45. Or design by committee?

46. REQUIREMENTS GATHERING TENDS
TO FOLLOW THE STEPS OF CLASSICAL
OO DESIGN
We write use cases and afterwards look for nouns and verbs

47. SOFTWARE DEVELOPMENT IS
JOURNEY
Involving collective learning

48. SO LET’S FIND A FUN AND BETTER
WAY TO DO IT

49. Event Storming
• Event Storming is a fun way of bringing developers and business experts
together and drive your analysis from the outside and quickly explore
complex business domains in hours instead of days/weeks.

50. Event Storming
• Invented by Alberto Brandolini
• See http://ziobrando.blogspot.dk/2013/11/introducing-event-storming.html

51. Event Storming
• Invite the right people
• people with questions and people with answers. They provide the perfect mix of curiosity
and wisdom.
• Provide unlimited modeling space

52. Event Storming
• Explore the domain starting from Domain/Business Events
Event

53. Why Events
• Simple semantic and notation
• Represent the state transitions
• Gives us precision
• Easy to grasp by everybody in the room
53

54. Events
An Event is non-prescriptive of what should happen in other parts of
the system. It leaves this open to the recipients, so that they themselves
determine what to do based on occurrence of the event.
Events always carry a name in its past-tense form:
OrderWasAccepted, OrderHasShipped, CustomerWasReimbursed
Other qualities
• Immutable, i.e. content cannot be changed
• Always carries the ID of the Business Object it relates to
• An event can and will typically will be published to multiple
consumers.
• The publisher of the event does not know who the recipients are
• And it doesn’t know what the recipients intend to do with the event
“An Event describes something that HAS
happened”

55. Why Events
• Simple semantic and notation
• Represent the state transitions
• Gives us precision
• Easy to grasp by everybody in the room
• Place them along a timeline to visualize the process
• An event might be the predecessor or the follower of another one.
55

56. Event Storming
• What causes Domain Events?
• User Action model it as a Command
• External system
• Time
• Other Domain Events
• Documents
External
Time
Command Role
Event Event
Document

57. Events are often the side effect of Commands
A Command is prescriptive of what should happen. This is a stronger form of
coupling than Events or Documents.
A Command’s primary goal is to capture USER INTENT
A Command supports a single usecase and targets a single Business Object
Commands always carry a name in its imperative form: CreateOrder, ShipOrder,
CancelOrder, ReimburseCustomer, etc.
Other qualities
• Immutable, i.e. content cannot be changed
• Always carries the ID of the Business Object that it relates to
• A command can only be handled by ONE service
• The Operations that consume Commands don’t return values – but can throw
exceptions
“A command describes a Task that you want someone else to
carry out for you and the recipient can reject the Command”

58. Commands & Events
Commands mutate Business Object state which results in
one or more Events being published
Command Event(s)
AcceptOrder OrderAccepted
ShipOrder OrderShipped
AddComment CommentAdded
QuarantineReview ReviewQuarantined
UnquarantineReview ReviewUnquarantined

59. Also, UI’s give you valuable input
Create Task based UI mockups
59

60. 60

61. Domain modeling
61

62. When discussing with Business Experts
Focus on data fields and NOT entities
62

63. Focus on which data fields clump together
(or are talked about together)
and what fields are separate
Place the fields in piles,
that later will become your
bounded contexts
63

64. Don’t name the piles before you know what
they are
This avoids cognitive bias
64

65. Give the piles gibberish or color names
65

66. UI
BL
DAO
UI
BL
Entity
UI
BL
Entity
UI
BL
Entity
UI
BL
Entity
What we see, is that we move from

67. Towards Bounded Context that are fairly small (but not too small)
and rarely need to call each other
Sales Pricing InventoryShipping
✕ ✕ ✕
UI
BL
DAO
UI
BL
DAO
UI
BL
DAO
UI
BL
DAO

68. So where does (micro) services fit into this?
68

69. A Service is
• The technical authority for a given bounded context
• It is the owner of all the data and business rules that support this bounded
context – everywhere (including the UI)
• It forms a single source of truth for that bounded context

70. SOA PRINCIPLE
SERVICES ARE AUTONOMOUS
Autonomy means that our service is independent and self-
contained and as far as possible doesn’t directly depend on
other services to be functional.

71. Service autonomy
Component
B
Component
C
Component
A
System X
Service
A
Component
B
Component
C
System X
Slow/unreliable network
Different SLA
Slow system

72. SERVICES ARE AUTONOMOUS
For a service to be autonomous is must NOT share state

73. SERVICES ARE AUTONOMOUS
Autonomy is essential for
Scalability (scale out clustering)
Reliability (fail over clustering)

74. SERVICES ARE AUTONOMOUS
Autonomy is essential for
Reusability
Adaptability

75. How do we communicate between our
services?
75

76. Synchronous calls are the crystal meth of programming
At first you make good progress but then the sheer horror
becomes evident when you realise the scalability
limitations and how the brittleness holds back both
performance and development flexibility. By then it is too
late to save.
http://www.infoq.com/news/2014/10/thompson-reactive-manifesto-2
We need the reactive properties and then apply
protocols for the message interactions. Without
considering the protocols of interaction this world
of micro-services will become a coordination
nightmare.
Martin Thompson

77. But all the cool kids tell us to use REST
between (micro)services?
77

78. Service Dependencies
Source: https://www.infoq.com/presentations/netflix-ipc

79. WHAT’S THE CHALLENGE
WITH USING RPC/REST/…
BETWEEN SERVICES?

80. Synchronous calls lower our tolerance for faults
• When you get an IO error
• When servers crash or restarts
• When databases are down
• When deadlocks occurs in our databases
• Do you retry?
With synchronous style Service interaction we can loose business data, there’s no automatic retry or we risk creating
data more than once because idempotence* often is an after though
80
Client Server
Duplicated
Response
Duplicated Request
Processing
Response
Request Processing
The same message can be
processed more than once
*Idempotence describes the quality of an
operation in which result and state does not
change if the operation is performed more
than 1 time

81. Also remember: REST isn’t magic!

82. WITH CROSS SERVICE INTEGRATION
WE’RE BOUND BY THE LAWS OF
DISTRIBUTED COMPUTING

83. The 8 Fallacies of Distributed Computing
These fallacies are assumptions architects, designers and developers of
distributed systems are likely to make. The fallacies will be proven
wrong in the long run - resulting in all sorts of troubles and pains for
the solution and architects who made the assumptions.
1. The network is reliable.
2. Latency is zero.
3. Bandwidth is infinite.
4. The network is secure.
5. Topology doesn't change.
6. There is one administrator.
7. Transport cost is zero.
8. The network is homogeneous.
See http://www.rgoarchitects.com/Files/fallacies.pdf for a walkthrough of the fallacies and why they’re fallacies

84. A DISTRIBUTED SYSTEM IS ONE
WHERE A MACHINE I’VE NEVER
HEARD OF CAN CAUSE MY PROGRAM
TO FAIL.
— Leslie Lamport

85. Essential complexity of 2 way integration
Component
C
Component
B
Component
A
UI
Service Service
B:Service()
call C:Service()
call A:Service()
commit()
Service
Local transaction
between System A, B
and C

86. B:Service()
call C:Service()
call A:Service()
if (A:Call-Failed:Too-Busy?)
Wait-A-While()
call A:Service()
if (A:Call-Failed:Too-Busy?)
Wait-A-Little-While-Longer()
call A:Service()
if (A:Call-Failed:IO-Error?)
Save-We-Need-Check-If-Call-A-Succeded-After-All
AND We-Need-To-Retry call C:Service and call B:Service
AND Tell-Customer-That-This-Operation-Perhaps-Went-Well
if (A:Call-Went-Well?)
commit()
Accidental complexity from distributed service integration
Component
C
Component
B
System A
UI
Service Service Service
Local transaction
between System B and
C

87. What’s wrong with distributed transactions?
• Transactions lock resources while active
• Services are autonomous
• Can’t be expected to finish within a certain time interval
• Locking keeps other transactions from completing their job
• Locking doesn’t scale
• And what about timeouts?
• X Phase Commit is fragile by design

88. WE NEED TO CHANGE FOCUS FROM
SHORT TECHNICAL TRANSACTIONS
To long running business transactions
supporting business processes

89. Using Business Events to drive Business Processes
Sales Service
Shipping
Billing
Sales
Customers
MessageChannel
Online Ordering System
Web Shop
(Composite UI)
Billing Service
Shipping Service
Order
Accepted
Event
AcceptOrder
Command
The sales
fulfillment
processing
can now
begin…

90. Choreographed Event Driven Processes
Sales Service
Order
Accepted
Invoicing Service
Order Fulfilment
(Saga/
Process-Manager)
Shipping Service
Online Ordering System
MessageChannel(e.g.aTopic)
Order
Accepted
Order
Accepted
Customer
Billed
Customer
Billed
Order
Approved
Order
Approved
Works as a Finite
State Machine
(WorkFlow)
handling the life
cycle of Shipping
and thereby forms
a very central new
Aggregate in the
System

91. SERVICES AT INPAY

92. Services/Business-capabilities in INPAY
Currency
Service
Banking
Service
Identity
Management
Service
Sales
Service
PSP
Service
3rd party
Providers
Service
Virtual
Banking
Service
IT
Operations
Compliance
Service

93. Service and deployment
• A Service represents a logical responsibility
boundary
• Logical responsibility and physical deployment of a Service DOES
NOT have to be 1-to-1
• It’s too constraining
• We need more degrees of freedom
• Philippe Krutchen 4+1 views of architecture: Logical and Physical designs
should be independent of each other
A service needs to be deployed everywhere its data is needed

94. We need more fine grained building blocks
Autonomous Components

95. A Service represents a logical boundary
95
Service
Autonomous
Component
Autonomous
Component
UI component
UI component

96. An Application is the plate where Components are
co-deployed
96
Sales service components
PSP service components
…

97. Service deployment
• Many services can be deployed to the same physical server
• Many services can be deployed in the same application
• Application boundary is a Process boundary which is a physical boundary
• A Service is a logical boundary
• Service deployment is not restricted to tiers either
• Part of service A and B can be deployed to the Web tier
• Another part of Service A and B can be deployed to the backend/app-service
tier of the same application
• The same service can be deployed to multiple tiers / multiple applications
• ie. applications and services are not the same and does not share the
same boundaries
• Multiple services can be “deployed” to the same UI page (service mashup)
• Multiple services can cooperate to fulfill a larger use-case (e.g. a workflow or a
business process)

98. Service
Autonomous
Component
1..*
Is implemented by
A Service is the technical authority of a specific
Bounded-Context/Business Capability
e.g. Sales, Shipping, Billing
Services support business processes.
Business processes naturally span multiple services, but
there will always be a single service that is the actual
authority on the business process.
Service vs Autonomous Components

99. Also known as Microservices
Service
Autonomous
Component
1..*
Is implemented by
Service vs Autonomous Components
Autonomous-Components/Microservices are a division of Services along Transactional
boundaries (a transaction stays within the boundary of a Microservice)
Microservices are the individually logical deployable units of a Service with their own
Endpoints. Could e.g. be the split between Read and Write models (CQRS) - each would be
their own Microservice

100. Autonomous Component
• Can be deployed alone or co-located, together with one or more adapters
from the same service
• Works transparently in a clustered environment
Core logic
(use case controllers/aggregates)
Primary/Driving Adapter
GUI / API / …
Primary/Driving Port
Secondary/Driven Adapter
Database/Notifications/…
Secondary/Driven Port

101. Services are the corner stone
• We talk in terms of Services/business capabilities and the processes/use-
cases they support
• Autonomous-Components/Microservices are an implementation detail
• They are much less stable (which is a good thing – it means they’re easier to replace)
• With regards to other Services
• subscribe to events from
• send commands to (less common)
• call operations (in rare occasions)

102. Services/Bounded Contexts and Aggregates
Sales
Service
PSP
Service
Virtual Banking
Service
Customer
customerId
…
Contract
contractId
customerId
…
VBFeeSchedule
contractId
…
PSPFeeSchedule
contractId
…
BillingTemplate
contractId
…

103. CQRS
A single model cannot be
appropriate for reporting,
searching and transactional
behavior
Greg Young, 2008

104. Commands, Events and Query Models
Read model
Read model
Events
UI
Domain modelQuery/Read model
”AcceptOrder”
command
”OrderAccepted”
event
”Find all
Accepted Orders”
Query
Commands are Imperative: DoStuff
Events are Past tense: StuffDone

105. Event Sourcing
Aggregates track their own Domain Events
and derive state from them
Time
07:39
Time
07:40
Time
07:41
Time
07:45
Time
07:46
Time
07:50

106. Infrastructure patterns
106

107. Create Contract
Sales Service : Contracts_Ac
Contract Manager Application
PSP Service : Agreement_Ac
ContractCreated
ContractCreated
Publish
PayInTemplateCreated
ContractEvents
<<Topic>>

108. Client handled subscriptions
• Highly resilient pattern for an Event Driven Architecture that’s backed by Event-
Sourced AC’s
• In this model the publisher of the Events is responsible for the durability of all its
Events, typically to an EventStore/EventLog.
• Each client (subscriber) maintains durable information of the last event it has
received from each publisher.
• When ever the client starts up it makes a subscription to the publisher where it
states from which point in time it wants events published.
• This effectively means that publisher can remain simple and the client (subscriber)
can remain simple and we don’t need additional sophisticated broker infrastructure
such as Kafka+ZooKeeper.

109. Client handled subscriptions
Publisher
Subscriber
A
Local storage
EventStore
Subscriber
B
Local storage
Topic
Subscription
Topic
Subscription
TopicSubscriptionHandler
TopicSubscriptionHandler
EventEvent
Event Event
EventBus
Event
Event

110. psp_fees_ac
(deployed on 10.25.26.102)
psp_fees_ac
(deployed on 10.25.26.101)
Bus Bus
Bus Bus
sales_contract_ac
(deployed on 10.25.26.104)
sales_contract_ac
(deployed on 10.25.26.103)
Federated Bus

111. Distributed
Bus
Topic
Queue
Topic
Publisher
side
Topic
Subscriber
side
Queue
Sender
side
Queue
Receiver
side
Local
Topic
Publisher
Local
Topic
Subscriber
Local
Queue
Sender
Local
Queue
Receiver
Distributed per Service
EventBus
Local
Topic
Subscriber
Local
Topic
Subscriber
Local
Queue
Sender
Distributed
Notifications
Distributed
Broadcast
Distributed
SingleInstance
Task

112. What about applications?
112

113. Applications in INPAY
Identity
Management
Application
Contract
Manager
Application
Treasury
Application
Compliance
Application
Operations
Application
CRM
PSP
Gateway
PSP
Merchant
Application
ERP

114. Who owns the UI?
• For a service to be fully autonomous
is must be self contained – which
means it must:
• Own its UI
• Own its Business Logic
• Own its Data model
User interface
Business Logic
Data model &
storage
Service
A similar approach is available under the name Self Contained Systems http://scs-architecture.org/

115. Applications and Services
iOS Homebanking Call center support portal
Bank Backoffice application
Customer information service
Legal and contract information service
Accounts service
Credit card service
Mortgage loans service

116. Screen view != View model
Think in composites
116

117. 117

118. 118

119. Application UI’s
• An applications is a composition of different services
• The composition can be:
• Resource Oriented Client Architecture (ROCA) style service
integration (http://roca-style.org/)
• Mashup of Service UI components in a dedicated Application –
aka. Composite UI
Both solutions involve
integration via Services
web interfaces to minimize
coupling to other services.
Image from http://scs-architecture.org/

120. Composite UI - example
Page Context:
{id: ISBN-10 0-321-83457-7 }
ImageService
BookService
ReviewService
PriceService
InventoryService
OthersAlsoBoughtService
PriceService
ReviewService
BookService
ImageService
BookService

121. Widget Widget
Page
Widget
Service A Service B Service C
Widget
Widget
Widget
Service A
Service B
Service C
Widget
Service C
• Overall structure of the page
is “owned” by the application.
• Each widget is owned and
delivered by the
underlying Service.
Page layout

122. AUTONOMOUS-COMPONENTS/
MICROSERVICES ARE LOGICAL
DEPLOYABLE UNITS
That doesn’t mean they HAVE to be deployed individually.
Design for Distribution
But take advantage of locality

123. Autonomous Components can be co-deployed together
with Application backends
contract_manager (Spring Boot fat–jar)
sales_contract_ac
sales_customer_ac
sales_contract_manager_adapters
api_psp
psp_fees_ac
psp_contract_manager_adapters
frontend
api_sales
app
libs
contract
customer
fees

124. Be pragmatic
Some services are more stable
In which case we allow other services to call them using local
calls
124

125. Application in code
@Configuration
@ComponentScan(basePackages = { "com.inpay.contractmanager",
"com.inpay.adapters",
"com.inpay.itops.spring" })
public class Application extends InpaySpringBootApplication {
public Application() {
super();
}
@Override
protected ApplicationIdentifier getApplicationIdentifier() {
return ApplicationIdentifier.from("ContractManager");
}
@Override
protected Collection<AutonomousComponent> getAutonomousComponentsHostedInThisApplication() {
CurrencyExchangeRateAc currencyExchangeRateAc = new CurrencyExchangeRateAc();
return list(
new PSPFeeScheduleAc(currencyExchangeRateAc.getCurrencyConverter()),
new VBFeeScheduleAc(currencyExchangeRateAc.getCurrencyConverter()),
new ContractAc(),
new CustomersAc(),
currencyExchangeRateAc
);
}
public static void main(String[] args) {
SpringApplication.run(Application.class, args);
}
}

126. AC in code public class PSPAgreementAc extends HzBackedAutonomousComponent {
public static AutonomousComponentId SERVICE_AC_ID = PSP_SERVICE_ID.ac("psp_agreement_ac");
…
public PSPAgreementAc(CurrencyConverter currencyConverter) {
this.currencyConverter = currencyConverter;
}
@Override
public void onInitialize(IConfigureACEnvironment acSetup) {
acSetup.withAutonomousComponentId(SERVICE_AC_ID).usingServiceDataSource()
.withBusConfiguration(cfg -> {
cfg.getAxonContext()
.subscribeAnnotatedCommandHandler(new TemplateCmdHandler(
cfg.getAxonContext().eventSourcedRepository(PSPTemplate.class),
currencyConverter));
….
manageLifecycleFor(templateViewRepository = new TemplateViewRepository(cfg, currencyConverter));
})
.runOnBusStartup((bus, axonContext) -> {
bus.registerAxonReplayableTopicPublisher(InternalTemplateEvents.TOPIC_NAME,
replayFromAggregate(PSPTemplate.class)
.dispatchAggregateEventsOfType(InternalTemplateEvents.class));
bus.subscribeTopic(SERVICE_AC_ID.topicSubscriber("ContractEvents"),
ExternalContractEvents.TOPIC_NAME,
new SalesTopicSubscription(bus));
});
}
public TemplateViewRepository getTemplateViewRepository() { return templateViewRepository; }
}

127. AC, autonomy and “shared” service data
Service
DB
DB
Autonomous
Component
Autonomous
Component
Autonomous
Component
Autonomous
Component DB

128. 50 shades of inter service AC Autonomy*
Endpoint Process Database Storage
Shared Shared Shared Shared
Own Shared Shared Shared
Own Own Shared Shared
Own Shared Own Shared
Own Own Own Shared
Own Own Own Own
Lower Autonomy
Higher Autonomy
* No RPC in use!

129. 5 Microservices Do’s
• Identify Business Capabilities (or Bounded Contexts) and split your services according
to them. A service is owned by one team that builds and runs the service. This gives you
proper business and IT alignment and allows pin point accuracy with regards to spending
money to solve problems.
• Spend time to understand the business processes and the domain. At first you must go
slow to go fast. Building microservices properly takes time and is not trivial. Identify how
likely things are to change and what things change together.
• Focus on the business side effects - also know as the Events and make them a first class
principle. Avoid RPC/REST/Request-Response between Services - events are the API.
• Consider building composite application and Backend’s For Frontend's (BFF’s) to
decouple services further. An application is owned by a dedicated team, but may borrow
developers from service teams.
• Learn from history. Don’t repeat the mistakes that gave (misapplied) SOA a bad name.
Also, microservice might not be as small as you think - we need low coupling as well as high
cohesion :)

130. 5 Microservices Don’ts
• Do not introduce a network boundary as an excuse to write better code - many have
troubles with poorly modularized monoliths and believe that introducing network between
modules magically solves the problem. If you don’t change your thinking and design, you
will end up with a distributed monolith, which has all the disadvantages of a monolith, the
disadvantages of a distributed system and none of the advantages of microservices
• Don't split the atom! Distributed Transactions are never easy. Learn about the CAP
theorem and avoid Request/Response API’s between Services.
• Don't fall into the trap of “Not my problem”. When working on isolated code bases teams
can loose sight of the big picture.
• Identify the bottlenecks and possible solutions before deciding to split a problem into
one or more microservices. There’s nothing the guarantees that your microservice scales
better than your monolith.
• Don't do big bang rewrites. Move towards microservices gradually while focusing on
functional areas that can replace or support the old monolith. Don’t rewrite core business
while being new to microservices.

131. Thanks :)