This document discusses loosely coupled architecture and how to achieve it. It defines loosely coupled architecture as one where components have little or no knowledge of other components. It recommends separating components by business processes, domain experts, data ownership, and rate of change to achieve loose coupling. Event storming workshops and dividing systems into contexts with defined communications are also presented as ways to develop loosely coupled systems.

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Loosely-Coupled
Architecture
How to get rid of the domino effect

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radblog.pl @RadekMaziarkaobjectivity.co.uk
Radek Maziarka

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Loosely-Coupled ArchitectureIn 1 sentence
... is one in which each of its components has, or makes
use of, little or no knowledge of the definitions of other
separate components.

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SHOULD
YOU
EVEN
CARE?

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Technology changes so fast...

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„A loosely coupled, well-encapsulated
architecture drives IT performance.
In the 2017 dataset, this was the
biggest contributor to continuous
delivery.”

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Architecture / deploys per developersBased on “Accelerate – The science of Lean Software and DevOps”
0.1
1
10
100
1000
1 10 50 100
Numberofdeploys/day
Number of developers
HIGHLY MODERATELY LOOSELY

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Loosely-Coupled Architecture
Large scale changes
Less coordination / communication
Deploy during normal hours
Testing on without E2E
High number of deploys
Better / faster scaling

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What is
Loosely-Coupled Architecture?

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Microservices

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Loosely-Coupled Architecture2 important factors which lead you to it
(Low) Coupling(High) Cohesion

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Payroll department
3
SALARY INFORMATION NOT
AVAILABLE OUTSIDE
DEPARTMENT
ANNEX AS A FORM OF COMMUNICATION
YOU DON’T CHANGE YOUR OWN SALARY2
1
4 NO SHARING ABOUT INTERNAL DETAILS
Good example of Loosely-Coupled Architecture

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Low Cohesion
Measures of cohesion
TemporalCoincidental Logical
Parts are grouped by when they
are processed.
The only relationship between the
parts is that they have been
grouped together.
Parts are logically categorized to
do the same thing even though
they are different by nature.
SHARED
Coupling And Cohesion https://files-cdn.cnblogs.com/files/younggift/Cohesion&Coupling.pdf
DATA ACCESS ON INIT

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High Cohesion
Measures of cohesion
FunctionalInformationalSequential
Parts all contribute to a single well-
defined task of the module.
Multiple functions represented by
entry points deal with a single data
structure.
Output from one part is the input to
another part like an assembly line.
ORDER MGMT.BUILDER
PAYMENT
CALCULATOR
Coupling And Cohesion https://files-cdn.cnblogs.com/files/younggift/Cohesion&Coupling.pdf

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PresentationDomainDataLayering https://martinfowler.com/bliki/PresentationDomainDataLayering.html

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PresentationDomainDataLayering https://martinfowler.com/bliki/PresentationDomainDataLayering.html
Vertical Slice Architecture https://jimmybogard.com/vertical-slice-architecture/

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High Coupling
Measures of coupling
ControlContent Common
One module controlling the flow of
another, by passing it information
on what to do.
One module modifies or relies on
the internal workings of another
module (e.g. data, logic).
Two modules share the same
global data.
Coupling And Cohesion https://files-cdn.cnblogs.com/files/younggift/Cohesion&Coupling.pdf
DB
1 1 2
DB
2 1 2
flag

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Low Coupling
Measures of coupling
NoData Message
Modules do not communicate at all
with one another.
One module modifies or relies on
the internal workings of another
module (e.g. data, logic).
Modules use a public interface to
exchange parameter-less
messages.
Coupling And Cohesion https://files-cdn.cnblogs.com/files/younggift/Cohesion&Coupling.pdf
1 21 2
ID
Data
Message Bus
1 2

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MicroservicesBad signs of not applying Loosely-Coupled Architecture
3
GLOBAL STATE
KNOWLEDGE OF
INTERNAL BEHAVIOR
INTEGRATION BY DATABASE2
1
4 SERVICES ARE OVERLY CHATTY

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Lease applicationModules in system to lease cars
PAYMENTSCONFIG LEASE INVOICES LEGAL

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Lease applicationModules in system to lease cars and computers
PAYMENTSCONFIG LEASE INVOICES LEGAL
CAR
COMPUTER
CAR
COMPUTER
CAR
COMPUTER
CAR
COMPUTER
CAR
COMPUTER

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Lease applicationModules in system to lease cars and computers
PAYMENTSCONFIG LEASE INVOICES LEGAL
CAR
COMPUTER
CAR
COMPUTER
CAR
COMPUTER
CAR
COMPUTER
CAR
COMPUTER

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CAR
Lease applicationModules in system to lease cars and computers
PAYMENTSCONFIG LEASE INVOICES LEGAL
COMPUTER
PAYMENTSCONFIG LEASE INVOICES LEGAL

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HOW
TO
FAIL?

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Don’t Repeat Yourself
Every piece of knowledge must have a single, unambiguous, authoritative
representation within a system.

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Cost of a new featureComparision between DRY and not-DRY
not-DRYDRY
A
B
C
A
B
C
FUN
FUN FUN
FUN
X
X
X
X
FUN

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Cost of new different featuresComparision between DRY and noT-DRY
not-DRYDRY
A
B
C
A
B
C
FUN
FUN FUN
FUN
?
X
Z
Y

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Unit.net

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RailsConf 2014 - All the Little Things by Sandi Metz https://www.youtube.com/watch?v=8bZh5LMaSmE

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Real example – notesImplementational differences within single context
PRODUCT ORDER DELIVERY USER
MGMT
PAYROLL
NOTES

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Don’t Repeat Yourself
Every piece of knowledge must have a single, unambiguous,
authoritative representation within a system.

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HOW
TO
ACHIEVE
IT?

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DOMAIN
DRIVEN
DESIGN

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How to split into modules?Few examples of good division heuristic
3
DEPARTMENTS
BUSINESS PROCESSESS
DOMAIN EXPERTS2
1
6
DATA OWNERSHIPS
RATE OF CHANGE
USER EXPERIENCE5
4

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Event StormingWorkshops of Alberto Brandolini

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Event Storming - exampleWorkshops in Objectivity

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Context mapApproaches of communications between contexts
Orders
Store
Shipping
Notifications
Down
Up
Shared Kernel
Layer
Partnership
Legacy
Reporting
Separate
Ways

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