The "Why", "What" and "How" of Microservices

The “Why”, “What” and “How”
of Microservices
Jeppe Cramon - @jeppec
Chief Architect - INPAY

WHY WOULD WE WANT TO USE
MICROSERVICES?
And what are the alternatives?

Cost of Software Development
• The 60/60 rule*
• The cost of software maintenance consumes 40%-80%
(60 % in average)
• Of this, 60% is consumed by enhancements
• And only 17% is consumed by error corrections
* "Frequently Forgotten Fundamental Facts about Software Engineering" by Robert L. Glass, IEEE Software May/June 2001

What are the sources of enhancements?
• New business requirements
• Changed processes (e.g. direct booking)
• Changed calculations (new pricing models)
• New features (space yield)
• New corporate strategies
• Ability to sell individual “components” of our application
• Management decisions to purchase 3rd party applications
• Upgrades/replacement to existing 3rd party systems
• Replacing existing functionality in our application with new components/systems (e.g. best
of breed or golf course deals)

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

ULTIMATELY WE WANT TO BE
ABLE TO ADAPT TO CHANGES IN
A CONSTANT AMOUNT OF TIME

E.G. IF YOUR PRICING MODEL WAS
WRONG – YOU SHOULD BE ABLE
TO CHANGE IT IN HOURS/DAYS
AND REDEPLOY

SOME ORGANIZATIONS
CALCULATE CHANGE TIMES IN
MONTHS OR YEARS

WE ALL WANT TO MOVE FAST
But can’t always move fast -
for various reasons…

WHAT IS THE MAIN TECHNICAL,
BUSINESS AND ARCHITECTURAL
IMPEDIMENT TO OBTAINING HIGH
VELOCITY?

High Coupling
• Coupling is a measure of dependencies:
• Efferent coupling – who do you depend on
• Afferent coupling – who depends on you
• 4 different aspects on coupling:
• Temporal coupling (processing time of Y affects X – if Y is down can X continue?)
• Platform coupling (e.g. using platform specific protocols)
• 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

Coupling
Coupling is a challenge when you want to makes changes

The playing field
Zero coupling The Enterprise Canonical model

Coupling causes ripple effect –
much like circles in the water
• You want to change a little thing and all of a sudden you need to change 25
other seemingly unrelated things
• Zero coupling is impossible
• Question is what is the right level of coupling?
• This highly depends on how likely the component/system/service is it to change and
what parts that change together

It starts simple
“The” DB
UI
Logic
Data Access

Next step is spaghetti in layers

And often we end up with a
Big Ball Of Mud

AS AN UMBRELLA NAME WE CALL
THESE SYSTEMS MONOLITHS
Which roughly translates to mean that they deployed as one
unit

Characteristics of a monolith?
• Often an indivisible unit
• Hard to adapt to new technology
• Low Reusability
• Functionality of a part cannot be reused alone
• Slow Delivery train
• Introducing a new feature often requires coordination with other features to
deliver all of them at the same time
• Higher and higher maintenance cost over time
• Change to control flow is often hard
• Starting the application often takes a long time
• Testing the application often takes a long time
• Forces high requirements on mental capacity to keep the entire
monolith in your head
• Tends to grow bigger and bigger and bigger
• Reliability
• When one thing fails => potentially everything fails (e.g. due to
OutOfMemoryException)

MONOLITHS ARE UNFAIRLY
CATEGORIZED AS BEING BAD
Many of the problems mentioned before can be solved using
design principles that we will investigate in this talk

Advantages of Monolith
• Can take advantage of locality:
• In memory calls and avoid distributed transactions
• Can perform Joins with other components SQL tables
• Can take advantage of development IDE’s
• Refactoring
• Code completion
• Code searching
• Less spinning wheels
• Less deployment and monitoring requirements

WHAT OTHER ALTERNATIVES
EXISTS?
SOA?

FIRST QUESTIONS IS
What is SOA?

To many this is SOA
Data
Storage
Data
Storage
Data
Storage
Data
Service
Data
Service
Data
Service
Activity
Service
Activity
Service
Process
Service
Process
Service
Client Client
Client
Data
Service
If one service is
down, you risk
everything is down

Classic Service reuse multiplies our direct
and especially indirect dependencies
which creates high coupling
My new
Service that
wants to reuse
other services
Service that is
going to be
reused
Another
Service that is
going to be
reused
Reusable
Service
Reusable
Service
Reusable
Service
Reusable
Service
Reusable
Service
DB
service

The cost/benefit of reuse
• Layered SOA has typically been recommended because it increases reuse
• Really, this is use and not reuse.
• Use results in more coupling
• The thesis is that the more we reuse, the faster we will be done
• But the thesis rest on the false assumption that writing code is the most expensive
part of a project
• Not all code takes the same amount of time to write. Some code is very trivial while other
code is very hard to write
• The real money and time consumers on any project are:
• Figuring out what the customer needs
• The time it takes for the customer to figure what they really needed and the resulting rework
• Meetings
• UI, Integration work, databases, webservices
• Fix bugs
• Debugging
• Deployment
• Test
• Ship
Yes
No
Failed
?
The more
dependencies we have,
the worse this is and
the more expensive it
becomes

Where to go?
• Monoliths easily result in:
• Tight coupling
• Slow release train - All or nothing deployment
• Low reusability
• Reliability issues
• SOA done traditionally easily result in:
• A hard coupled architecture due to reusability and integration
patterns
• Both runtime (temporally) and design time (contracts)
• Performance issues
• Reliability issues
• Slow release train – requires a lot of governance
• High latency (2 way remote communication)
• High accidental complexity
• Compensations due to lack of transactions
• ESB’s to try and hide the mess

ARE MICROSERVICES THE
SOLUTION?
Next question: What are Microservices?

SOME SAY A MICROSERVICE IS THE
SAME AS A SOA SERVICE – JUST
SMALLER
That of course requires us to answer what a Service is ;-)

THE FINE-GRAINED, STATELESS, SELF- CONTAINED
NATURE OF MICROSERVICES CREATES DECOUPLING
BETWEEN DIFFERENT PARTS OF A CODE BASE AND IS
WHAT MAKES THEM
EASY TO UPDATE, REPLACE, REMOVE, OR AUGMENT.
Microservices: The resurgence of SOA principles and an
alternative to the monolith

IT SEEMS THE ARGUMENT IS THAT
JUST BY MAKING THINGS FINE
GRAINED
We by magic achieve decoupling

Let’s transform our monolith to microservices
“The” DB
UI
Logic
Microservices
Data Access
Microservices
UI-3
MS-3
MS-C
UI-2
MS-D
UI-4
MS-4MS-2
MS-B MS-E
UI-1 UI-5
MS-5MS-1
MS-A

THERE IS VALUE IN MAKING
THINGS SMALLER
For one thing it is easier to reason about them in isolation

Going small has its advantages
Monolith
Microservice Microservice
Microservice MicroserviceMicroservice
Microservice
Microservice

HOW SMALL SHOULD A
MICROSERVICE BE?

BUT I’VE HEARD THAT A
“MICROSERVICE SHOULD BE NO
LARGER THAN 100 LINES OF CODE!?”

SIZE THIS AND SIZE THAT!
BE CAREFUL
If Microservices are good, then Nanoservices must be even
better?
Why not one-liner services?

AKA SERVERLESS
39
https://www.linkedin.com/pulse/how-i-decided-use-serverlessnanoservices-architecture-benefield

AKA SERVERLESS
40
https://www.linkedin.com/pulse/how-i-decided-use-serverlessnanoservices-architecture-benefield

Nano Services
Unless we have a very good reason for doing so,
we risk building services
that are so fine-grained
that their costs outweigh their utility*
*Read Arnon Rotem-Gal-Oz’s Nano Services Anti Pattern:http://arnon.me/wp-content/uploads/2010/10/Nanoservices.pdf

IS YOUR MICROSERVICE
VALUABLE?
The value of a microservice must
exceed the cost of building &
operating it.
Microservices entail costs for serializations, deserializations,
security, communication, maintenance, configuration,
deployment, monitoring, etc.

FOR NOW ON WE WILL USE
SERVICE AND MICROSERVICE
INTERCHANGEABLY
A Service CAN be broken into smaller parts, but let’s leave
that out of the discussion for now…

Too small services
With too small services there’s a big risk that what’s left of coherence goes
out the window and we get:
• Communication-related and Layered coupling
• E.g. Logic and persistence are not the same service
• Temporal coupling
• Our service can not operate if it is unable to communicate with the services it depends upon
• Behavioral coupling
• Our services assume behaviors with regards to what and who.
• The sender/client determines what to do and knows something about how the receiver/server
should satisfy the request

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

Microservices == distributed objects?
Service star chart

Microservice characteristics
• Small
• Single Responsibility Principle (SRP)
• Own process
• Valuable
• Replaceable/Upgradeable
• Autonomous
• Independent
• Encapsulated
• Composable
• Testable
• Fast startup/shutdown
• Client friendly

SRP
This sounds really good but what about cross
Service relationships?
Customer Orders
Products

BEWARE…
When we break up big things into small
pieces we invariably push the complexity to
their interaction.
Michael Feathers
https://michaelfeathers.silvrback.com/microservices-until-macro-complexity

Let’s zoom in on the data tier
MS-A
“The” DB
MS-C
MS-D
MS-B
MS-E

IF OUR MONOLITHS CODE IS COUPLED AND MESSY
Chances are that the data(base) model is equally coupled
and messy

It gets more and more complicated

AND WE END UP WITH
ONE DOMAIN MODEL TO RULE THEM ALL

THE MENTAL CAPACITY REQUIRED TO
UNDERSTAND THIS DOMAIN MODEL
IS HUGE

TO UNDERSTAND ANY PART IN
ISOLATION, REQUIRES YOU TO
UNDERSTAND THE ENTIRE MODEL AS
EVERYTHING IS COUPLED TO EACH
OTHER

Side effect: Our queries get messy

Let’s refactor the data tier
UI
Logic
Microservices
Data Access
Microservices
UI-3
MS-3
MS-C
UI-2
MS-D
UI-4
MS-4MS-2
MS-B MS-E
UI-1 UI-5
MS-5MS-1
MS-A
A’s
DB
B’s
DB
C’s
DB
D’s
DB
E’s
DB
MS-C
MS-D
MS-B
MS-E
MS-A A’s
DB
B’s
DB
C’s
DB
D’s
DB
E’s
DB

THIS IS ALSO KNOWN AS A
DISTRIBUTED MONOLITH

Things that are not Services
• A Service with only functionality (and no data) is a FUNCTION
• Like: check if order is valid
• A Service that only has data is a DATABASE
• Like: Entity CRUD
• A database already has a nice API - we don’t need to bubble wrap it with REST or Asynchronous messages
• Don’t split the atom – we need cohesion as well as decoupling!
• If we want datastore abstraction (so we can swap out Postgresql with Mongo or Redis) there this little
pattern called Respository.
This is typically seen in a lot of layered SOA usages where a function calls a
function that calls a function that calls a database

4 tenets of Service Orientation
1. Services share contract and schema, not class or type
• Encapsulation improved. We need not worry about how things are implemented
(languages or platform dependent information)
2. Service interaction is controlled by a policy
• Controls the rules/form of technical communication between services take place (e.g.
using encryption incl. algorithm and keys)
3. Services have explicit boundaries
• A Service is defined by its Boundary
• What is on the inside and which is on the outside ownership wise (data and logic)
• A boundary tells us what parts make up the service and how the service interacts with
the outside world (as defined by the service contract – see point 1 and 2).
• The internal implementation and platform of a Service (what’s inside the boundary) is
irrelevant
4. Services are autonomous
• Encapsulation & Cohesion at a bigger scale.
• A service is autonomous if it doesn’t rely on other services to complete its job
• A service can be rewritten, redeployed and replaced without interfering with other
services as long as schema/contracts are upheld

This is different from Functions & Components
• Functions are very intimately linked with code that uses them
• Components form a higher modularization level than functions
• But they’re still intimate with the application they’re added to
• Services are yet another modularization level than components.
• They’re standalone, own their own data and are loosely coupled to their surroundings

Let’s refactor the “microservices”
UI
Microservices
UI-3
MS-3
UI-2 UI-4
MS-4MS-2
UI-1 UI-5
MS-5MS-1
1’s
DB
2’s
DB
3’s
DB
4’s
DB
5’s
DB

Reality rears it’s ugly head out

WHAT’S WRONG WITH USING
RPC/REST/… BETWEEN
SERVICES?

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
68
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

Also remember: REST isn’t magic!

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.

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

For a service to be autonomous is MUST own its data
Shipping
DB

For a service to be autonomous is must NOT share state

Autonomy is essential for
Scalability (scale out clustering)
Reliability (fail over clustering)

Autonomy is essential for
Reusability
Adaptability

Clarification of Autonomy vs. Authority
Definition of Autonomy
A service is autonomous if it doesn’t directly depend on another application to
complete its work. It can determine on its own what to do.
Definition of Authority
A service is the authority if other services needs to ask it for data or instruct it to
perform a task on their behalf for them to complete their job
Autonomy
Authority

IF WE KEEP UP THE SAME HABITS
THAT RESULTED IN MESSY
MONOLITHS
We will end up with microservice spaghetti!

Monolith A Monolith B
Monolith C Monolith D
Service
A.1
Service
A.2
Service
A.3
Service
A.4
Service
B.1
Service
B.2
Service
B.3
Service
B.4
Service
D.1
Service
D.4
Service
D.3
Service
D.2
Service
C.4
Service
C.3
Service
C.2
Service
C.1
DB DB
DB DB
Service transformation

Service
A.1
Service
A.2
Service
A.3
Service
A.4
Service
B.1
Service
B.2
Service
B.3
Service
B.4
Service
D.1
Service
D.4
Service
D.3
Service
D.2
Service
C.4
Service
C.3
Service
C.2
Service
C.1
DB
DB
DBDB
DB
DB DB DB
DBDBDB
DB
DB DB
DB
DB
DB DB
DB DB
Service transformation

IF WE CONTINUE DOWN
THIS PATH WE HAVE GONE
FROM BAD TO WORSE

THIS MEANS THAT WE NEED TO TAKE
CARE OF HOW WE DESIGN OUR DATA
MODELS & THEREBY OUR SERVICE
BOUNDARIES

GUIDANCE CAN BE FOUND IN
Pat Hellands
“Life Beyond Distributed Transactions? An Apostate ‘s Opinion”
Link: http://www-db.cs.wisc.edu/cidr/cidr2007/papers/cidr07p15.pdf

Life Beyond Distributed Transactions?
According to Pat Helland, we must find the solution to our problem by looking
at:
1. How do we split our data / services
2. How do we identify our data
3. How do we communicate between our services

1. How do we split our data / services
Data must be collected in pieces called aggregates. These aggregates should
be limited in size (but not smaller), so that, after a transaction they are
consistent.
Rule of thumb:
One transaction involves only one aggregate.

DOMAIN DRIVEN DESIGN
The term Aggregate comes from DDD

Aggregates
Invoice
InvoiceLine
*
Account *
What:
• Cluster coherent Entities and Value Objects,
with complex associations into Aggregates
with well defined boundaries.
• Choose one entity to be root and control
access to objects inside the boundary
through the root.
• External objects hold references to the root
• Aggregates only refer to other aggregates
by identity (their id)
Motivation:
Control invariants and consistency through the aggregate root.
Enables: Loading schemes, coarse grained locking and…
Ensuring consistency & transactional boundaries for Distributed
scenarios
Root
*
*

THE SMALLEST SERVICE
Would be responsible for all logic and data related to a single
Aggregate

WHY?
Because consistency can only be guaranteed with in an Aggregate
It cannot span aggregates due to lack of coordinating transactions

Example of bad aggregate boundaries

In this case we can fix it
All use cases shows that
LegalEntity and Address
are created, changed
and deleted together
Parent - Child

An improved aggregate design
Address
turned out to
NOT be an
Entity
LegalEntity is an Entity
and forms the Root of our
LegalEntity aggregate

THIS GIVES US THIS RULE OF
THUMB
1 use case = 1 transaction = 1 aggregate

With this improved design
“Client” that wants
to use our
LegalEntity
Service
LegalEntity Service

2. How do we identify our data
According to Pat Helland we need to be able to uniquely identify each
Aggregate using an ID.
• This ID will usually a UUID/GUID
• Aggregates refer to each other by their ID
• they NEVER use memory pointers, join tables or remote calls
{21EC2020-3AEA-4069-A2DD-08002B30309D}
2122 (approximately 5.3×1036) combinations

WE STILL HAVEN’T CONQUERED
THE TEMPORAL COUPLING
PROBLEM

3. How we communicate between our services
• What do we do when our use case involves more than one aggregate and
therefore likely more than one service?

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

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

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

2 WAY (SYNCHRONOUS) COMMUNICATION IS THE
ROOT OF MANY PROBLEMS BECAUSE IT CREATES A
STRONG TEMPORAL COUPLING
(AS WELL AS BEHAVIORAL COUPLING)
Substituting synchronous with synchronous-
over-asynchronous doesn’t solve the core
problem

Change Address Response
Consumer
Change Address Request
Provider
RPC or Request/Response - Synchronous 2 way communication
Remote Procedure Call
Request/Reply – Asynchronous 2 way communication
Consumer Provider
Change Address Request
Change Address
Request
Change Address
Reply
Change Address
Reply
Request
Channel
Reply
Channel
VS.

THE PROBLEM ISN’T SO MUCH SYNCHRONOUS VS
ASYNCHRONOUS
The question is it two-way or one-way communication and what’s the use
case (e.g. is there a user sitting and waiting)

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

Accidental complexity from 2 way style integration
Component
C
Component
B
System A
UI
Service Service Service
Local transaction
between System B and C
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()

HOW CAN WE ENSURE CONSISTENCY
WHEN WE’RE GOING DISTRIBUTED?
Can’t we just use distributed transactions?
(XA / 2 phase commit)

Distributed Transactions…. Oh my
Sales system
Sales Delivery
system Deliveries
Customer/C
RM system Customer
SAP
Book-
keeping
Complete
Purchase
Transaction
Coordinator Transactional
Resource
Prepare
Phase
Commit
Phase
2 Phase Commit

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

THIS IS
ACCIDENTAL COMPLEXITY
DUE TO DISTRIBUTION
We’re essential handling the problem the worst possible way

THIS FORM OF COORDINATED
INTEGRATION BETWEEN
MULTIPLE SERVICES IS KNOWN
AS ORCHESTRATION

Consequence: Availability goes down
(without additional instances of each service)
Service
A
Service
B
Service
C
Availability: 99% Availability: 99% Availability: 99%
Combined availability: 97%

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

DECIDE IF YOU CAN LIVE WITH THE
CONSEQUENCES OF COUPLING
SERVICES TO EACH OTHER USING
REQUEST/RESPONSE
Different situations – different tradeoffs

HOW TO GET LOOSE COUPLING
BETWEEN SERVICES?
It highly depends on how you integrate them

Degrees of coupling
UI
UI
Service
ServiceData
Data Events
Events

IF WE WANT TO DECOUPLE OUR SERVICES AS MUCH
AS POSSIBLE
THEN WE NEED TO LOOK TOWARDS
COMPOSITE UI’S
AND
EVENTS

WHAT’S A COMPOSITE UI
A Composite UI is a way to allow different services to
participate an applications UI without revealing their
internals and thereby removing the need for other
services to know the this services internal data
This helps us keep coupling low and encapsulation high

Applications
• Applications are tailored solutions for certain groups of users.
Example
• Departments will typically have UI’s tailed to the responsibility of a department (which
often cut across Service responsibilities)
• Managers which see different reports from all areas of a company
• Sales representatives which have solutions that sometimes can work in offline mode
• Customers using e.g. Android/iOS apps to access their portfolio / home-banking /
appointments / etc.

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

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 & storage
User interface
Business Logic
Data model &
storage
Service
A similar approach is available under the name Self Contained Systems http://scs-architecture.org/

If a Service doesn’t own its UI we often find the need for
a Gateway or Backend For a Frontend (BFF)
• Experience shows if you get
your Service API wrong the
first time around, it is really
expensive to fix it*
• The granularity of APIs
provided by microservices is
often different than what a
client needs*
* See http://thenewstack.io/microservices-calls-robust-api-management-tools/

Gateway or Backend For a Frontend (BFF)
• Unfortunately Gateways & BFF’s
introduce a lot of coupling
between the Gateway and the
underlying services
• If an underlying service changes
its contract in a non-backwards
compatible way the gateway
needs to change
• In these cases the clients of the
gateway may also need to change
since the changes can permeate
upwards (the gateway abstraction is
leaky)
* See http://thenewstack.io/microservices-calls-robust-api-management-tools/

Gateway/BFFGateway/BFF
GATEWAY OR
BACKEND FOR A FRONTEND
UI
Microservices
UI-3
MS-3
UI-2 UI-4
MS-4MS-2
UI-1 UI-5
MS-5MS-1
1’s
DB
2’s
DB
3’s
DB
4’s
DB
5’s
DB
Gateway/BFF

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/

ROCA style integration
Simple hyperlinks can be used to navigate between services
UI’s.

A redirection can be used to ensure
navigation works in both directions.

Hyperlinks can also support the
dynamic inclusion of content that is served by another
application into the web interface of a service.

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

A SERVICE OWNS IT UI IN ALL
CONTEXTS AND FOR ALL COMPOSITE
UI’S
Not just for HTML clients

Invoice Composite UI example
InvoiceHeader
Order:ShippingI
nfo
Invoice:
InvoiceNumber
Invoice:
Data and Due
date
Order:
RelationInformation
Order:Item-
Qty
Product:Ite
m
Product:
Description
Order:
Item-Unit-Price
Order:
Item-
Total-
Price
Order:Total
Billing:Balance
All Services
participate at
the UI level for
each individual
Item in the
Order

Coupling matrix*
* Modified version of Ian Robinson’s matrix: http://iansrobinson.com/2009/04/27/temporal-and-behavioural-coupling/
Behavioral
coupling
Temporal
coupling
Low High
Low
High
Event oriented Command oriented
Emergency services Distributed 3 layer

TO BREAK TEMPORAL COUPLING &
BEHAVIORAL COUPLING
SERVICES NEEDS TO COMMUNICATE
ASYNCHROUSLY USING BUSINESS EVENTS
MESSAGES
Services communicate facts without making
assumptions about what other services intend
to do with the events

Let’s make the implicit explicit!
Old wisdom seems to have been forgotten. Let’s introduce:
Domain Events
Which:
• Signal that something has happened
• Closely aligned to the Domain Model
• Are handled by a messaging system
• They are in the past tense:
• CustomerBilled
• ParcelShipped
• CustomerCreated
• ReviewCreated
• CommentAdded
• CommentDeleted

Event Message
An Event message is non-prescriptive of what should happen in other services. 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 Aggregate 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
• The service issuing the Event is the owner of the Event contract
“An Event describes something that HAS
happened”

Business Event as XML Message
<OrderWasAccepted>
<CustomerId>50D1F244-ABBC-4EC7-BDCA-E4934C124A89</CustomerId>
<OrderId>C199322A-01F1-4E56-918E-7A63529F8FA3</OrderId>
<ShippingAddress> ... </ShippingAddress>
<BillingAddress> ... </BillingAddress>
<Items>
<Item ProductId="4CD22C4B-600C-4477-B5BF-48ABDEE4DA61" Amount="100"
AmountUnit="Pieces" UnitPrice="100,10" UnitCurrency="EUR"/>
<Item ProductId="56E6BD19-660C-464A-9120-100DAF579855" Amount="10"
AmountUnit="Litres" UnitPrice="56,95" UnitCurrency="CHF"/>
</Items>
</OrderWasAccepted>

Business Event as JSON Message
{
EventType: "OrderWasAccepted",
CustomerId: "50D1F244-ABBC-4EC7-BDCA-E4934C124A89",
OrderId: "C199322A-01F1-4E56-918E-7A63529F8FA3",
ShippingAddress: { ... }
BillingAddress: { ... }
Items: [
{
ProductId: "4CD22C4B-600C-4477-B5BF-48ABDEE4DA61",
Amount: "100",
AmountUnit: "Pieces",
UnitPrice: "100,10",
UnitCurrency: "EUR"
},
{
ProductId: "56E6BD19-660C-464A-9120-100DAF579855",
Amount: "10",
AmountUnit: "Litres",
UnitPrice: "56,95",
UnitCurrency: "CHF"
}
]
}

Events are often the side effect of Commands
A Command message is prescriptive of what should happen. This is a stronger form of coupling than Events.
A Command’s primary goal is to capture USER INTENT
A Command supports a single usecase and targets a single Aggregate
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
• Therefore Commands should be handled using 2 way communication, so the exception can be communicated back to the sender
“A command describes a Task that you want someone else to
carry out for you and the recipient can reject the Command”

Commands & Events
Commands mutate Aggregate/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

EVENT MESSAGES SHOULD BE
SEND USING
One way communication

One way Messaging
• One way messaging is asynchronous and breaks
temporal coupling
• Messages can be exchanged between services over a
message channel
• The Message channel is responsible for delivering the
message(s) to the relevant parties (consumers). If
something goes wrong, the message will be put back on
the Message channel (rollback) and will be resent later*
Sender
Address Changed
Event
Receiver
Address Changed Event
Channel
Asynchronous Communication – Aka Messaging
* This is smart pipes and “dumb” endpoints – An alternative is dumb pipes and smart endpoints

SPLITTING INTO SMALLER AND MORE
FOCUSED SERVICES HAS MANY
ADVANTAGES
But it leaves us with the challenge on what
to do when processes span multiple
services

THE CHANGE OF FOCUS FROM
TECHNICAL SHORT TRANSACTIONS
To long running business transactions

BUSINESS EVENTS MESSAGES
AND BUSINESS PROCESSES
By publishing Events messages from our Services
we can communicate with each other and also
drive Business Processes
143

This form of architecture is called an
Event Driven Architecture (EDA)

SOA AND EDA ARE TWO SIDES OF
THE SAME COIN

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…

Business Events example
Sales Service
Order
Accepted
Billing Service
Order
Accepted
Customer
Billed
MessageChannel
We use the Order Accepted event message published from the Sales Service
to drive the Billing of the customer.
The billing part of the process also use Business Events, in this case Customer Billed
Event, to indicate that its part of the process is completed.
Because we use asynchronous messaging we can still accept orders in the sales service
even though the invoicing services is down. The Order Accepted event message will
remain in the Message Channel until the Billing Service is ready to process it.

What about race conditions?
• Usually they are a natural part of the business and needs to be considered as
a natural part of the process – technology and 2 Phase Commit wont solve
this*
• This challenges how we look at the process
• Based on the questions we ask ourselves
• And what we promise customers
• Example:
• Is it really not possible to purchase Taxfree even though the seating system doesn’t know
our booking or concrete seating?
• This is known as being causally consistent (a level of eventual consistent)
* See http://www.udidahan.com/2010/08/31/race-conditions-dont-exist/

Eventual consistency can be our friend
• Consistency is within each Service
• Eventual consistency is between Services
• Like in the real world
Sales Invoicing
Inventory Shipping
Order
Accepted
Invoice Customer
Checks Inventory for availability Books truck driver
Order
Packaged
Fetch Package from Inventory
Customer
Invoiced

IMPORTANT!
Figure out who owns the process

Choreographed Event Driven Processes
Sales Service
Order
Accepted
Invoicing Service
Order Fulfilment
(Saga/
Process-Manager)
Shipping Service
MessageChannel
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

Process Managers
• Process Managers are essential to the coordination and monitoring
of long running business processes/transactions
• They work as a Finite State Machines (WorkFlow) which handling the
life cycle of Process (e.g. Shipping an Order) and thereby forms a
very central new Aggregate in the System
• They can include manual steps/person intervention
• Sometimes these Process Managers belong naturally within a
specific Business capability and other times they are truly a thing by
themselves and therefore often form a new business capabilities
Many companies derive their competitive advantages from their Processes.
A Process Manager allows you coordinate Business Processes on the basis of
Events

Choreography is very different from the classical
orchestrated integration process

What should Events look like?
• Keep Events Small
• Only include relevant information (time, id’s unless you use events for
duplication)
• If possible, don’t include unnecessary id’s for aggregates in other services
• Think Inverse relationships based on what feels most natural
• Order doesn’t need to know the Shipping Id, but Shipping can know the Order Id
• Anchor Events to Time
• Include information about how long you can trust the Event’s change
• E.g. a price is valid until 8pm tomorrow night
• Reveal the intention and time anchoring as part of the Events
name

The INPAY approach to Microservices

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

Different perspectives on entities
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.

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 identitySOA:
Service

Billing
Product
Catalogue
Shipping
Sales
Inventory
Pricing
Retail domain split into a Macro architecture

These splits should be fairly small (but not too small)
and share as little as possible with each other
Sales Pricing InventoryShipping
✕ ✕ ✕

What’s a macro architecture
• It’s the static/stable(r) parts of your architecture
• Which are very costly to refactor and change
• Business capabilities are stable
• Therefore we should strive to align services with business
capabilities / bounded contexts (DDD)

BOUNDED CONTEXT
The setting in which a word or a
statement appears that determines its
meaning
Ensures that our
is clear and precise

DDD DOESN’T ENFORCE
PROBLEM DOMAIN AND
SOLUTION DOMAIN
ALIGNMENT FOR BOUNDED
CONTEXTS
Which is why we prefer to use the term Business Capability

Service and Business Capability alignment
“The advantage of business capabilities is their remarkable level
of stability. If we take a typical insurance organisation, it will likely
have sales, marketing, policy administration, claims management,
risk assessment, billing, payments, customer service, human
resource management, rate management, document management,
channel management, commissions management, compliance, IT
support and human task management capabilities. In fact, any
insurance organisation will very likely have many of these
capabilities.”
See http://bill-poole.blogspot.dk/2008/07/business-
capabilities.html

A Service is
• The technical authority for a given business capability
• It is the owner of all the data and business rules that support this business
capability – everywhere (including the UI)
• It forms a single source of truth for that capability
• This form of business and IT alignment ensures that we can maintain service
Autonomy & Encapsulation

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

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

Services, Applications and code
• Each Service and Application is maintained within its own Git repository
• Common IT Operations libraries and infrastructure are maintained in separate
Git repositories

So what’s inside a Service source repository?
• Autonomous Components
• Libraries
• Adapters
• Front-end UI components
• API contracts (mostly Events)
• Schemas
• Build file(s)

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

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 use-case (e.g. a workflow or a
business process)

Service
Autonomous
Component
1..*
Is implemented by
A Service is the technical authority of
a specific 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

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

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)

Where does that leave Systems?
• Systems and Services are in my
opinion the same
• For a service to be autonomous is
must be self contained – which
means it must:
• Own its UI
• Own its Business Logic
• Own its Data model & storage
• Which is similar to how Self
Contained Systems are/were build
User interface
Business Logic
Data model &
storage
Service / System

Services/Bounded Contexts and Aggregates
Sales
Service
PSP
ServiceVirtual Banking
Service
Finance
Service
Customer
customerId
…
Contract
contractId
customerId
…
VBFeeSchedule
contractId
…
PSPFeeSchedule
contractId
…
BillingTemplate
contractId
…

INPAY Autonomous Component design principles

Commands and Events
Events
Identifiers
Commands

Commands and Events
public class RegisterBankCmd extends AbstractCommand {
@TargetAggregateIdentifier
public final BankId bankId;
public final String name;
public final Country countryOfOperation;
public RegisterBankCmd(BankId bankId, String name, Country countryOfOperation) {
Objects.requireNonNull(bankId, "bankId");
Objects.requireNonNull(name, "name");
Objects.requireNonNull(countryOfOperation, "countryOfOperation");
this.bankId = bankId;
this.name = name;
this.countryOfOperation = countryOfOperation;
}
}

Commands and Events
The generic business event interface includes the
name of the topic that subscribers must use
public class BankRegistered extends AbstractEvent implements BankEvent {
@AggregateIdentifier
public final BankId bankId;
public final String name;
public final Country countryOfOperation;
public BankCreated(BankId bankId, String name, Country countryOfOperation) {
this.bankId = bankId;
this.name = name;
this.countryOfOperation = countryOfOperation;
}
}
public interface BankEvent extends Serializable {
TopicName TOPIC_NAME = BankingServiceId.ID.topicName("bank-events");
}

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

Query/Read models
• To offload the Aggregate models, i.e. the transactional
write model, we introduce the concept of Read Models
• Query/Read Models are projections of one or more
Aggregate models which store and represent data in the
best possible way for the given Query use cases
UI Application Domain Write
model
Commands – Change data
UI Application Read
models
Queries – Ask for data (no side effects)

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

CQRS Building blocks
Client
Commands
Command
Bus
Sends
Command
Handlers
Modify
Repositories
Read Write
Data
store
Event
Bus
Command
Autonomous Components
Event Handlers
Events
Read store
Query HandlersQuery Results
Queries
Query/Read Model
Autonomous Components
Events
Domain

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

Relational Database EventStore example
Time
07:39
Time
07:40
Time
07:41
Time
07:45
Time
07:46
Time
07:50
Type Aggregate
Identifier
Sequence
Number
Timestamp Event
Identifier
EventType SerializedEvent
Order 14237 0 2014-01-06 7:39 {Guid-1} OrderCreated <serialized event>…
Order 14237 1 2014-01-06 7:40 {Guid-2} ProductAdded <serialized event>…
Order 14237 3 2014-01-06 7:45 {Guid-4} ProductRemoved <serialized event>…
Order 14237 5 2014-01-06 7:50 {Guid-6} OrderAccepted <serialized event>…
Order 14238 0 2014-01-07 9:10 {Guid-X} OrderCreated <serialized event>…
DomainEvents Table

Event Replaying
Type Aggregate
Identifier
Sequence
Number
Timestamp Event
Identifier
Order
Accepted: false

Event Replaying
Type Aggregate
Identifier
Sequence
Number
Timestamp Event
Identifier
Order
Orderline
Accepted: false

Event Replaying
Type Aggregate
Identifier
Sequence
Number
Timestamp Event
Identifier
Order
Orderline
Orderline
Accepted: false

Event Replaying
Type Aggregate
Identifier
Sequence
Number
Timestamp Event
Identifier
Order
Orderline
Orderline
Accepted: true

Snapshotting
Type Aggregate
Identifier
Latest
Sequence
Number
Timestamp Serialized Aggregate
Order 14237 4 2014-01-06 7:46 <serialized aggregate>…
Product 15918 1 2014-01-07 7:40 <serialized aggregate>…
Type Aggregate
Identifier
Sequence
Number
Timestamp Event
Identifier
Snapshot Table
Domain Events Table

Full CQRS
With EventSourcing
UI Domain
Event
Store
(can e.g. be a real
EventStore DB or
a Relational DB)
Commands – Change data
Commands Events
SQL DB Document DB Graph DB
UI Data
Queries – Ask for data
Events
Query Build
Our single
source of truth

Aggregate
public class Bank extends InPayEventSourcedAggregate {
@AggregateIdentifier
private BankId bankId;
@EventSourcedMember
private Map<BankAccountId, BankAccount> bankAccounts = new HashMap<>();
public Bank(BankId bankId, String name, Country countryOfOperation) {
apply(new BankRegistered(bankId, name, countryOfOperation));
}
public void addBankAccount(BankAccountId bankAccountId, …) {
if (!bankAccounts.containsKey(bankAccountId)) {
apply(new BankAccountAdded(bankId,
bankAccountId,
…));
}
}
@EventSourcingHandler
private void on(BankRegistered e) {
bankId = e.getBankId();
}
@EventSourcingHandler
private void on(BankAccountAdded e) {
BankAccount bankAccount =
bankAccounts.put(e.getBankAccountId(), new BankAccount(e.getBankId(), e.getBankAccountId(), …));
}
}

Service project structure sales_service
sales_api
sales_contract_ac
sales_customer_ac
frontend
sales_contract_manager_adapters
contract
adapters
sales/modules
customer
psp_service
psp_api
psp_fees_ac
…
frontend
psp_contract_manager_adapters
fees
adapters
psp/modules
fx
Legend:
Service/Business-Capability
External Event/Command Contracts
Autonomous Component
Autonomous Component Adapter
Angular.js Module
Builds into a Java JAR
Uses Spring MVC/REST
All Java 8 artifacts are built
using Gradle

Autonomous Component
• Can be deployed alone or co-located
• Usually deployed together with one or more adapters
• Works transparently in a clustered environment
• Completely Spring free 
• Only depends on our Core infrastructure library
• Common types and Id’s
• Bus infrastructure
• Service lookup
• CQRS building blocks (backed by Axon framework)

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; }
}

Distributed in memory views
public class PSPFeeScheduleViewRepository extends HzBackedDataRepository<ContractId, PSPFeeScheduleView> implements Lifecycle {
private final Bus bus;
private Subscription subscription;
public PSPFeeScheduleViewRepository(HazelcastInstance hazelcastInstance, String serviceId, Bus bus) {
super(hazelcastInstance, serviceId);
this.bus = bus;
}
@Override
public void start() {
subscription = bus.subscribeTopic(TopicSubscription.FromBeginning(PSPFeeScheduleAc.SERVICE_AC_ID.subscriberId("-FeeScheduleView"),
PSPFeeScheduleRelated.TOPIC_NAME),
new BusMessageHandlerDelegator<>(this));
}
@Override
public void shutdown() {
if (subscription != null)subscription.cancel();
}
public PSPFeeScheduleView load(ContractId contractId) {
final PSPFeeScheduleView payOutFeeScheduleView = cache.get(contractId);
if (payOutFeeScheduleView == null)
throw new ValidationException(PSPFeeScheduleMessages.feeScheduleDoesNotExist.validationMsg(contractId));
return payOutFeeScheduleView;
}
@BusMessagePayloadHandler
private void on(PSPFeeScheduleCreated event) {
cache.put(event.contractId, new PSPFeeScheduleView(event));
}
@BusMessagePayloadHandler
private void on(PSPFeeScheduleDiscarded event) {
cache.remove(event.contractId);
}
...
}

The core of the infrastructure
• It’s the core infrastructure that allows services/AC’s to communicate using
messages using our Federated Bus.
• This means we don’t need to deploy any Message Brokers (such as
RabbitMQ, ActiveMQ) – the Bus supports:
• Broadcast
• Notifications
• Non durable Queues
• Durable Queues
• Non-durable Topics

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

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

Non-durable Topics and Client handled subscriptions
• Highly resilient pattern for an Event Driven Architecture that’s backed by AC’s that
use EventSourcing.
• 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 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.

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

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

Logical Architecture Building Blocks

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
psp_api
psp_fees_ac
psp_contract_manager_adapters
frontend
sales_api
app
libs
contract
customer
fees

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 String getApplicationName() {
return "ContractManager";
}
@Override
protected Collection<AutonomousComponent> getAutonomousComponentsHostedInThisApplication() {
CurrencyExchangeRateAc currencyExchangeRateAc = new CurrencyExchangeRateAc();
return list(
new IDMCoreAc(),
new PSPFeeScheduleAc(currencyExchangeRateAc.getCurrencyConverter()),
new VBFeeScheduleAc(currencyExchangeRateAc.getCurrencyConverter()),
new ContractAc(),
new CustomersAc(),
currencyExchangeRateAc
);
}
public static void main(String[] args) {
SpringApplication.run(Application.class, args);
}
}

What about Security/Identity management (IDM)?
• That’s an AC as well, belonging to IT operations
• Technical security belongs with IDM
• May this caller call this endpoint
• Is this User authenticated
• Simple Authorization: Does this User have Privilege A, B and C
• Business related Authorization belongs in the designated Business ACs
• May this user approve a Contract with Fees below X, Y and Z

Security and Applications
@ComponentScan(basePackages = { "com.inpay.contractmanager", "com.inpay.adapters", "com.inpay.itops.spring" })

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

50 shades of 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!

Page
Controller
Page
Controller
Page
Controller
Page
Controller Page
Template
Page
Template
Page
Template
Page
Template
ServiceService
Scope
Back-
end
HTTP
Back-
end
Back-
end
Back-
end
Back-
end
• Coordinate back-end communication
• Avoid UI coupling
CommonAngular Application

Service A Service B Service C
IT-OPS
Application
App
Application
App logic
(Layered, CQRS,…)
lication
Storage
UI Components
Services & Composite UI’s

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 and the view model
it binds to is delivered by the
underlying Service.
Page layout

AngularJS as Composite UI Framework
• Misko Hevery – Don’t fight HTML, extend it!
• For our purpose each widget maps naturally to a directive.
• AngularJS is modular in nature which aligns nicely with
our needs having multiple Services taking part
in an application.

Angular Controllers & Services (1 of 2)
• An angular service is normally responsible for gathering
and posting data to the back-end.
• There will be at least one angular service per Service
that participates in the application
• In angular a service is injected in a controller. Not vice
versa.
• Yet application in/output must be coordinated in some
way through the services
• In angular controllers are NOT singletons.
• Each directive get its own instance of a controller,
meaning there are numerous controller instances at
playWhat are the consequences of this with regards to a composite UI?
• An angular service is normally responsible for gathering and
posting data to the back-end.
• There will be at least one angular service per Service that participates in
the application
• In angular a service is injected in a controller. Not vice versa.
• Yet application in/output must be coordinated in some
way through the services
• In angular controllers are NOT singletons.
• Each directive get its own instance of a controller, meaning there are
numerous controller instances at play

Angular Controllers & Services (2 of 2)
• Gathering and posting data through angular services
needs to happen in a coordinated manner.
• Client side events
• The controllers instantiated through directives needs to be
kept in sync and caution must be made to avoid
numerous round-trips to the backend Services
• There is a need to have a controller that drives the overall
use-case for a single page. The application page
controller.
• The application page controller must not know the data
structure of any of the participating angular services.
• Gathering and posting data through angular services needs to
happen in a coordinated manner.
• Client side events
• The controllers instantiated through directives needs to be kept in
sync and caution must be made to avoid numerous round-trips to the
backend Services
• There is a need to have a controller that drives the overall use-case
for a single page. The application page controller.
• The application page controller must not know the data structure of
any of the participating angular services.

Application Startup
• Client-side event subscription
• Enrolling for client-side distributed post operations

Page Rendering Example
Page Ctrl
Sales
Service
UI Broker
Customer
Service
Pricing
Service
Product
Service
Page Widget
Ctrl
Widget
Ctrl
Widget
Ctrl
Widget
Ctrl
Widget
Ctrl
Widget
Ctrl
Widget
Ctrl
Widget
Ctrl
1.
Backend
API
Backend
API
Backend
API
Backend
API
2.
3.
4.
5.
5.
6.
7.
7.
7.
7.
5.
8.
8.
8.
8.

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