An examination of coupling, decoupling, uncoupling, and it's various challenges, uses, and dimensions.

1. Uncoupling
Michael T. Nygard
@mtnygard

2. About the Speaker
Developer
Architect
Operations
Blog: http://www.michaelnygard.com/
Twitter: http://twitter.com/mtnygard

3. About the Speaker
• Stewards of Clojure & ClojureScript
• Creators of Datomic
• Open source contributions
• Pedestal
• Vase
• Onto
• Simulant

6. Tensegrity
This Photo by Unknown Author is licensed under CC BY

7. No-tegrity
This Photo by Unknown Author is licensed under CC BY-NC-SA

8. Coupling

9. Scharfenberg Coupler

10. Watt’s
Linkage

11. Chebyshev
Linkage

12. Standard Model
Particle
Interactions

13. Coupling

14. Coupling
•Determines degrees of freedom.

15. Coupling
•Determines degrees of freedom.
•Enables some movements.

16. Coupling
•Determines degrees of freedom.
•Enables some movements.
•Inhibits other movements.

17. Coupling
•Determines degrees of freedom.
•Enables some movements.
•Inhibits other movements.
•Connects effects.

21. Everything is coupled to everything
• Ambient temperature
• Ambient atmosphere
• Acoustic vibrations
• Electromagnetic field
• Gravity
• Higgs field

22. We cannot attend to all things all the
time

23. Kinds of Coupling
Type Effect
Operational Consumer cannot run without the provider
Development Changes in producer and consumer must be
coordinated
Semantic Change together because of shared concepts
Functional Change together because of shared responsibility
Incidental Change together for no good reason.
(E.g., breaking API changes.)

24. Analyzing Coupling I
E-mail system
[Software System]
Microsoft Exchange
Sendsanotification that
areport isready to
Email Component
[Component: C#]
Sends emails

25. Analyzing Coupling I
E-mail system
[Software System]
Microsoft Exchange
Sendsanotification that
areport isready to
Email Component
[Component: C#]
Sends emails
Operational: Strong. SMTP is synchronous, connection-oriented, conversational

26. Analyzing Coupling I
E-mail system
[Software System]
Microsoft Exchange
Sendsanotification that
areport isready to
Email Component
[Component: C#]
Sends emails
Operational: Strong. SMTP is synchronous, connection-oriented, conversational
Development: Weak. SMTP is well-defined standard with history of interoperability

27. Analyzing Coupling I
E-mail system
[Software System]
Microsoft Exchange
Sendsanotification that
areport isready to
Email Component
[Component: C#]
Sends emails
Operational: Strong. SMTP is synchronous, connection-oriented, conversational
Development: Weak. SMTP is well-defined standard with history of interoperability
Semantic: Very strong. SMTP defines entities, attributes, and allowed values.

28. Analyzing Coupling I
E-mail system
[Software System]
Microsoft Exchange
Sendsanotification that
areport isready to
Email Component
[Component: C#]
Sends emails
Operational: Strong. SMTP is synchronous, connection-oriented, conversational
Development: Weak. SMTP is well-defined standard with history of interoperability
Semantic: Very strong. SMTP defines entities, attributes, and allowed values.
Functional: Very weak. Sender and MTA both use network connections.

30. Analyzing Coupling II-A
Reference Data System
[Software System]
Managesreference data for all
counterpartiesthe bank interacts
with
Getscounterparty
datafrom
Reference Data Importer
[Component: C#]
Imports data from the reference
datasystem
SQL connection
to RDBMS

31. Analyzing Coupling II-A
Reference Data System
[Software System]
Managesreference data for all
counterpartiesthe bank interacts
with
Getscounterparty
datafrom
Reference Data Importer
[Component: C#]
Imports data from the reference
datasystem
SQL connection
to RDBMS
Operational: Very strong. Dependent on availability of server. Must be aware of topology and failover strategy

32. Analyzing Coupling II-A
Reference Data System
[Software System]
Managesreference data for all
counterpartiesthe bank interacts
with
Getscounterparty
datafrom
Reference Data Importer
[Component: C#]
Imports data from the reference
datasystem
SQL connection
to RDBMS
Operational: Very strong. Dependent on availability of server. Must be aware of topology and failover strategy
Development: Very strong. Dependent on schema, server version, protocol version.

33. Analyzing Coupling II-A
Reference Data System
[Software System]
Managesreference data for all
counterpartiesthe bank interacts
with
Getscounterparty
datafrom
Reference Data Importer
[Component: C#]
Imports data from the reference
datasystem
SQL connection
to RDBMS
Operational: Very strong. Dependent on availability of server. Must be aware of topology and failover strategy
Development: Very strong. Dependent on schema, server version, protocol version.
Semantic: Very strong. Tables, columns, and joins must be known to both parties.

34. Analyzing Coupling II-A
Reference Data System
[Software System]
Managesreference data for all
counterpartiesthe bank interacts
with
Getscounterparty
datafrom
Reference Data Importer
[Component: C#]
Imports data from the reference
datasystem
Operational: Very strong. Dependent on availability of server. Must be aware of topology and failover strategy
Development: Very strong. Dependent on schema, server version, protocol version.
Semantic: Very strong. Tables, columns, and joins must be known to both parties.
Functional: Weak. Functions of data maintenance don’t overlap with retrieval into objects.
SQL connection
to RDBMS

35. Suppose we use REST instead of SQL?

36. Analyzing Coupling II-B
Reference Data System
[Software System]
Managesreference data for all
counterpartiesthe bank interacts
with
Getscounterparty
datafrom
Reference Data Importer
[Component: C#]
Imports data from the reference
datasystem
HTTPS request
to REST API

37. Analyzing Coupling II-B
Reference Data System
[Software System]
Managesreference data for all
counterpartiesthe bank interacts
with
Getscounterparty
datafrom
Reference Data Importer
[Component: C#]
Imports data from the reference
datasystem
Operational: Strong, but less than before. Dependent on availability of server.
HTTPS request
to REST API

38. Analyzing Coupling II-B
Reference Data System
[Software System]
Managesreference data for all
counterpartiesthe bank interacts
with
Getscounterparty
datafrom
Reference Data Importer
[Component: C#]
Imports data from the reference
datasystem
Operational: Strong, but less than before. Dependent on availability of server.
Development: Strong, but less. Insulated from data format changes. Open encoding can further reduce coupling
HTTPS request
to REST API

39. Analyzing Coupling II-B
Reference Data System
[Software System]
Managesreference data for all
counterpartiesthe bank interacts
with
Getscounterparty
datafrom
Reference Data Importer
[Component: C#]
Imports data from the reference
datasystem
Operational: Strong, but less than before. Dependent on availability of server.
Development: Strong, but less. Insulated from data format changes. Open encoding can further reduce coupling
Semantic: Still very strong. REST resources and C# entities must align. Concepts will still map 1:1
HTTPS request
to REST API

40. Analyzing Coupling II-B
Reference Data System
[Software System]
Managesreference data for all
counterpartiesthe bank interacts
with
Getscounterparty
datafrom
Reference Data Importer
[Component: C#]
Imports data from the reference
datasystem
Operational: Strong, but less than before. Dependent on availability of server.
Development: Strong, but less. Insulated from data format changes. Open encoding can further reduce coupling
Semantic: Still very strong. REST resources and C# entities must align. Concepts will still map 1:1
Functional: Still weak. Different languages, techniques, design patterns apply.
HTTPS request
to REST API

41. What if we invert the flow of control?

42. Analyzing Coupling II-C
Reference Data System
[Software System]
Manages reference data for all
counterparties the bank interacts
with
Reference Data Receiver
[Component: C#]
Accepts and caches data from the
reference datasystem
Broadcasts
Message Broker
[Software System]
Pub/sub hub, bub
Broadcasts

43. Analyzing Coupling II-C
Reference Data System
[Software System]
Manages reference data for all
counterparties the bank interacts
with
Reference Data Receiver
[Component: C#]
Accepts and caches data from the
reference datasystem
Broadcasts
Message Broker
[Software System]
Pub/sub hub, bub
Broadcasts
Operational: Very weak. Receiver can run with stale data when either broker or upstream are broken.

44. Analyzing Coupling II-C
Reference Data System
[Software System]
Manages reference data for all
counterparties the bank interacts
with
Reference Data Receiver
[Component: C#]
Accepts and caches data from the
reference datasystem
Broadcasts
Message Broker
[Software System]
Pub/sub hub, bub
Broadcasts
Operational: Very weak. Receiver can run with stale data when either broker or upstream are broken.
Development: Weak. Insulated from schema changes.

45. Analyzing Coupling II-C
Reference Data System
[Software System]
Manages reference data for all
counterparties the bank interacts
with
Reference Data Receiver
[Component: C#]
Accepts and caches data from the
reference datasystem
Broadcasts
Message Broker
[Software System]
Pub/sub hub, bub
Broadcasts
Operational: Very weak. Receiver can run with stale data when either broker or upstream are broken.
Development: Weak. Insulated from schema changes.
Semantic: Strong, but not as strong. Broker allows for remapping concepts.

46. Analyzing Coupling II-C
Reference Data System
[Software System]
Manages reference data for all
counterparties the bank interacts
with
Reference Data Receiver
[Component: C#]
Accepts and caches data from the
reference datasystem
Broadcasts
Message Broker
[Software System]
Pub/sub hub, bub
Broadcasts
Operational: Very weak. Receiver can run with stale data when either broker or upstream are broken.
Development: Weak. Insulated from schema changes.
Semantic: Strong, but not as strong. Broker allows for remapping concepts.
Functional: Moderate. All components must share the same messaging tech.

47. Coupling was present in all examples –
It is necessary and inescapable

48. Chains of Coupling

49. Chains of Coupling
Retek IBM PIM Informatica
Everybody
Else
Dotcom
Catalog
3rd
Party Data
Vendors
Reviews,
Ratings,
Imagery
Dotcom
Promotions
Online
Store
Faceted
Search
SKU SKU SKU
SKU
SKU SKU SKU
SKU

50. Chains of Coupling
Retek IBM PIM Informatica
Everybody
Else
Dotcom
Catalog
3rd
Party Data
Vendors
Reviews,
Ratings,
Imagery
Dotcom
Promotions
Online
Store
Faceted
Search
SKU SKU SKU
SKU
SKU SKU SKU
SKU
Price Point

51. Chains of Coupling
Retek IBM PIM Informatica
Everybody
Else
Dotcom
Catalog
3rd
Party Data
Vendors
Reviews,
Ratings,
Imagery
Dotcom
Promotions
Online
Store
Faceted
Search
SKU SKU SKU
SKU
SKU SKU SKU
SKU
Price Point Price Point
Price Point
Price Point Price Point
Price Point
Price Point

52. Chains of Coupling – Semantic Polymer
Retek IBM PIM Informatica
Everybody
Else
Dotcom
Catalog
3rd
Party Data
Vendors
Reviews,
Ratings,
Imagery
Dotcom
Promotions
Online
Store
Faceted
Search
SKU SKU SKU
SKU
SKU SKU SKU
SKU
Price Point Price Point
Price Point
Price Point Price Point
Price Point
Price Point

53. Long
Arrows

54. Each “interface” was really a chain
1. Extract tables to files
2. Push files across network
3. Load tables into “LZ”
4. Process into “cold” DB
5. Swap hot & cold DBs (hours later)
1. Send message to queue
2. Take message from queue, unwrap, inspect,
and dispatch to 1-of-N other queues.
3. Drain queue to file
4. Batch job wakes up 2 times a day, does FTP
to remote end
5. Another batch job pulls a reconciliation file,
drops file into file system
6. Parser reads the file, shreds it into
messages, puts them on another queue

55. Architecture Qualities in Long Chains
Losses accumulate:
• Latency strictly worse than the slowest link in the chain.
• Availability strictly worse than the least available link.
• Throughput strictly worse than the throughput of the worst bottleneck
• Security strictly worse than the security of the weakest link

56. Digging Out

57. Information Hiding
“On the Criteria To Be Used in
Decomposing Systems into Modules",
David Parnas,
CACM, 1972

58. A KWIC Example
• Input
Software comprises an endless supply of structures.
• Output
an endless supply of structures. Software comprises
comprises an endless supply of structures. Software
endless supply of structures. Software comprises an
of structures. Software comprises an endless supply
Software comprises an endless supply of structures.
structures. Software comprises an endless supply of
supply of structures. Software comprises an endless

59. Modularization 1
1. Input
Read EBCDIC characters, store them in core. 6-bit characters packed 4 per word. EOL is a
special character.
2. Circular shifter
Prepare index; pair of addr of first char of shift, original index of line in input array
3. Alphabetizer
Take arrays from 1 & 2, produce new array of pairs like in 2, but in alphabetical order.
4. Output
Using arrays from 1 & 3, format output
5. Control
Allocate memory, call operations in1 - 4, report errors.

60. Consider the Effect of Changes
For each change case listed here, how many modules have to be changed?
• Read and print ASCII instead of EBCDIC.
• Stop using packed characters, store one character per word.
• Write index for circular shifts to offline storage instead of core to support larger
input documents.

61. Modularization 2
1. Line Storage
Offers functional interface: SETCH, GETCH, GETW, DELW, DELLINE
2. Input
Reads EBCDIC chars, calls line storage to put them into lines.
3. Circular Shifter
Offers same interface as line storage. Makes it appear to have all shifts of all lines.
4. Alphabetizer
Offers sort function INIT, and access function iTH that gets a line.
5. Output
Repeatedly call iTH on alphabetizer, printing the line.
6. Control
Similar to first approach, call each module in sequence.

62. Consider the Effect of Changes
For each change case listed here, how many modules have to be changed?
• Read and print ASCII instead of EBCDIC.
• Stop using packed characters, store one character per word.
• Write index for circular shifts to offline storage instead of core to support larger
input documents.

63. Why is the second one better?
• It hides decisions inside modules.
• Functional interfaces provide an abstract representation of the underlying data.
• Information hiding

64. Line
Line
Line
Line
Line
Ordered
Ordered
Storage
Input
Circular Shifter
Alphabetizer
Output
"Needs an interface"
"Offers an interface"
Why did the second
modularization survive
change better?
• Very few data types
• Small number of well
defined interfaces
• Highly composable

65. LineLine
LineLine
Line
Ordered Storage
Input
Circular ShifterOutput
Ordered
Alphabetizer
Controller run
run
Creates all
components
Composing Interfaces

66. LineLine
LineLine
Line
Ordered Storage
Input
Circular ShifterOutput
Ordered
Alphabetizer
Controller run
run
Creates all
components
Composing Interfaces
Consider each connector with respect to
Operational, Development, Semantic, and Functional coupling

67. Relating Composability
Inversely proportional to # of interfaces
Inversely proportional to # of data types

68. Uncoupling

69. Orthogonality in Software
• Separation of concerns
• High cohesion within a module or component
• Low coupling between modules or components
• Little overlap in functionality between modules
• Information hiding / decision hiding

70. Batch Process
File System
[Container: Network File Share]
Stores risk reports
Report Distributor
[Component: C#]
Publishes the report for the web
application
Publishesrisk reportsto
Reference Data System
[Software System]
Manages reference data for all
counterparties the bank interacts
with
Central Monitoring Service
[Software System]
The bank-wide monitoringand
alertingdashboard
Trade Data System
[Software System]
The system of record for trades of
type X
E-mail system
[Software System]
Microsoft Exchange
Sendsanotification that
areport isready to
Getstrade datafrom
Sendscritical failure alertsto
[SNMP]
Getscounterparty
datafrom
Email Component
[Component: C#]
Sends emails
Trade Data Importer
[Component: C#]
Imports data from the trade data
system
Reference Data Importer
[Component: C#]
Imports data from the reference
datasystem
Report Checker
[Component: C#]
Checks that the report has been
generated by 9 a.m. Singapore
time
Alerter
[Component: C# with SNMP
library]
Sends SNMPalerts
Sendsalerts using
Orchestrator
[Component: C#]
Orchestrates the risk calculation
process
Sendsemail
using
Importsdata
using
Importsdata
using
Risk Calculator
[Component: C#]
Does math
Report Generator
[Component: C# and
Microsoft.Office.Interop.Excel]
Generates an Excel compatible
risk report
Generatesthe risk
report using
Calculatesrisk
using
Scheduler
[Component: Quartz.net]
Starts the risk calculation process
at 5 p.m. New York time
Starts
Starts
Publishesthe risk
report using

71. Batch Process
File System
[Container: Network File Share]
Stores risk reports
Report Distributor
[Component: C#]
Publishes the report for the web
application
Publishesrisk reportsto
Reference Data System
[Software System]
Manages reference data for all
counterparties the bank interacts
with
Central Monitoring Service
[Software System]
The bank-wide monitoringand
alertingdashboard
Trade Data System
[Software System]
The system of record for trades of
type X
E-mail system
[Software System]
Microsoft Exchange
Sendsanotification that
areport isready to
Getstrade datafrom
Sendscritical failure alertsto
[SNMP]
Getscounterparty
datafrom
Email Component
[Component: C#]
Sends emails
Trade Data Importer
[Component: C#]
Imports data from the trade data
system
Reference Data Importer
[Component: C#]
Imports data from the reference
datasystem
Report Checker
[Component: C#]
Checks that the report has been
generated by 9 a.m. Singapore
time
Alerter
[Component: C# with SNMP
library]
Sends SNMPalerts
Sendsalerts using
Orchestrator
[Component: C#]
Orchestrates the risk calculation
process
Sendsemail
using
Importsdata
using
Importsdata
using
Risk Calculator
[Component: C#]
Does math
Report Generator
[Component: C# and
Microsoft.Office.Interop.Excel]
Generates an Excel compatible
risk report
Generatesthe risk
report using
Calculatesrisk
using
Scheduler
[Component: Quartz.net]
Starts the risk calculation process
at 5 p.m. New York time
Starts
Starts
Publishesthe risk
report using
Risk calculator produces a data structure that the
report generator must consume.
Example from c4model.com

72. Batch Process
File System
[Container: Network File Share]
Stores risk reports
Report Distributor
[Component: C#]
Publishes the report for the web
application
Publishesrisk reportsto
Reference Data System
[Software System]
Manages reference data for all
counterparties the bank interacts
with
Central Monitoring Service
[Software System]
The bank-wide monitoringand
alertingdashboard
Trade Data System
[Software System]
The system of record for trades of
type X
E-mail system
[Software System]
Microsoft Exchange
Sendsanotification that
areport isready to
Getstrade datafrom
Sendscritical failure alertsto
[SNMP]
Getscounterparty
datafrom
Email Component
[Component: C#]
Sends emails
Trade Data Importer
[Component: C#]
Imports data from the trade data
system
Reference Data Importer
[Component: C#]
Imports data from the reference
datasystem
Report Checker
[Component: C#]
Checks that the report has been
generated by 9 a.m. Singapore
time
Alerter
[Component: C# with SNMP
library]
Sends SNMPalerts
Sendsalerts using
Orchestrator
[Component: C#]
Orchestrates the risk calculation
process
Sendsemail
using
Importsdata
using
Importsdata
using
Risk Calculator
[Component: C#]
Does math
Report Generator
[Component: C# and
Microsoft.Office.Interop.Excel]
Generates an Excel compatible
risk report
Generatesthe risk
report using
Calculatesrisk
using
Scheduler
[Component: Quartz.net]
Starts the risk calculation process
at 5 p.m. New York time
Starts
Starts
Publishesthe risk
report using
Risk calculator produces a data structure that the
report generator must consume.
Data importers probably have similar
implementation needs
Example from c4model.com

73. Batch Process
File System
[Container: Network File Share]
Stores risk reports
Report Distributor
[Component: C#]
Publishes the report for the web
application
Publishesrisk reportsto
Reference Data System
[Software System]
Manages reference data for all
counterparties the bank interacts
with
Central Monitoring Service
[Software System]
The bank-wide monitoringand
alertingdashboard
Trade Data System
[Software System]
The system of record for trades of
type X
E-mail system
[Software System]
Microsoft Exchange
Sendsanotification that
areport isready to
Getstrade datafrom
Sendscritical failure alertsto
[SNMP]
Getscounterparty
datafrom
Email Component
[Component: C#]
Sends emails
Trade Data Importer
[Component: C#]
Imports data from the trade data
system
Reference Data Importer
[Component: C#]
Imports data from the reference
datasystem
Report Checker
[Component: C#]
Checks that the report has been
generated by 9 a.m. Singapore
time
Alerter
[Component: C# with SNMP
library]
Sends SNMPalerts
Sendsalerts using
Orchestrator
[Component: C#]
Orchestrates the risk calculation
process
Sendsemail
using
Importsdata
using
Importsdata
using
Risk Calculator
[Component: C#]
Does math
Report Generator
[Component: C# and
Microsoft.Office.Interop.Excel]
Generates an Excel compatible
risk report
Generatesthe risk
report using
Calculatesrisk
using
Scheduler
[Component: Quartz.net]
Starts the risk calculation process
at 5 p.m. New York time
Starts
Starts
Publishesthe risk
report using
Risk calculator produces a data structure that the
report generator must consume.
Data importers probably have similar
implementation needs
Report checker doesn’t appear to connect with
the file system that holds the reports. FS location
is latent coupling that will be a nasty surprise later.
Example from c4model.com

74. Batch Process
File System
[Container: Network File Share]
Stores risk reports
Report Distributor
[Component: C#]
Publishes the report for the web
application
Publishesrisk reportsto
Reference Data System
[Software System]
Manages reference data for all
counterparties the bank interacts
with
Central Monitoring Service
[Software System]
The bank-wide monitoringand
alertingdashboard
Trade Data System
[Software System]
The system of record for trades of
type X
E-mail system
[Software System]
Microsoft Exchange
Sendsanotification that
areport isready to
Getstrade datafrom
Sendscritical failure alertsto
[SNMP]
Getscounterparty
datafrom
Email Component
[Component: C#]
Sends emails
Trade Data Importer
[Component: C#]
Imports data from the trade data
system
Reference Data Importer
[Component: C#]
Imports data from the reference
datasystem
Report Checker
[Component: C#]
Checks that the report has been
generated by 9 a.m. Singapore
time
Alerter
[Component: C# with SNMP
library]
Sends SNMPalerts
Sendsalerts using
Orchestrator
[Component: C#]
Orchestrates the risk calculation
process
Sendsemail
using
Importsdata
using
Importsdata
using
Risk Calculator
[Component: C#]
Does math
Report Generator
[Component: C# and
Microsoft.Office.Interop.Excel]
Generates an Excel compatible
risk report
Generatesthe risk
report using
Calculatesrisk
using
Scheduler
[Component: Quartz.net]
Starts the risk calculation process
at 5 p.m. New York time
Starts
Starts
Publishesthe risk
report using
Risk calculator produces a data structure that the
report generator must consume.
Data importers probably have similar
implementation needs
Report checker doesn’t appear to connect with
the file system that holds the reports. FS location
is latent coupling that will be a nasty surprise later.
Orchestrator might end need
to do lots of data
transformation to bridge
interfaces.
Example from c4model.com

75. Batch Process
File System
[Container: Network File Share]
Stores risk reports
Report Distributor
[Component: C#]
Publishes the report for the web
application
Publishesrisk reportsto
Reference Data System
[Software System]
Manages reference data for all
counterparties the bank interacts
with
Central Monitoring Service
[Software System]
The bank-wide monitoringand
alertingdashboard
Trade Data System
[Software System]
The system of record for trades of
type X
E-mail system
[Software System]
Microsoft Exchange
Sendsanotification that
areport isready to
Getstrade datafrom
Sendscritical failure alertsto
[SNMP]
Getscounterparty
datafrom
Email Component
[Component: C#]
Sends emails
Trade Data Importer
[Component: C#]
Imports data from the trade data
system
Reference Data Importer
[Component: C#]
Imports data from the reference
datasystem
Report Checker
[Component: C#]
Checks that the report has been
generated by 9 a.m. Singapore
time
Alerter
[Component: C# with SNMP
library]
Sends SNMPalerts
Sendsalerts using
Orchestrator
[Component: C#]
Orchestrates the risk calculation
process
Sendsemail
using
Importsdata
using
Importsdata
using
Risk Calculator
[Component: C#]
Does math
Report Generator
[Component: C# and
Microsoft.Office.Interop.Excel]
Generates an Excel compatible
risk report
Generatesthe risk
report using
Calculatesrisk
using
Scheduler
[Component: Quartz.net]
Starts the risk calculation process
at 5 p.m. New York time
Starts
Starts
Publishesthe risk
report using
Risk calculator produces a data structure that the
report generator must consume.
Data importers probably have similar
implementation needs
Report checker doesn’t appear to connect with
the file system that holds the reports. FS location
is latent coupling that will be a nasty surprise later.
Orchestrator might end need
to do lots of data
transformation to bridge
interfaces.
Example from c4model.com

76. Let’s explore options to uncouple these

77. Problem: Risk calculator
produces a data structure
that the report generator
must consume.
Solutions depend on architectural style
Here we’re in a Windows service so we
might use a shared library to define the interface.
Orchestrator
[Component: C#]
Orchestrates the risk calculation
process
Risk Calculator
[Component: C#]
Does math
Report Generator
[Component: C# and
Microsoft.Office.Interop.Excel]
Generates an Excel compatible
risk report
Generatesthe risk
report using
Calculatesrisk
using

78. Problem: Redundant implementation details
This would be a good place to use a
shared library for common implementation.
Trade Data Importer
[Component: C#]
Imports data from the trade data
system
Reference Data Importer
[Component: C#]
Imports data from the reference
datasystem
Orchestrator
[Component: C#]
Orchestrates the risk calculation
process
Importsdata
using
Importsdata
using

79. File System
[Container: Network File Share]
Stores risk reports
Report Distributor
[Component: C#]
Publishes the report for the web
application
Publishesrisk reportsto
Report Checker
[Component: C#]
Checks that the report has been
generated by 9 a.m. Singapore
time
Alerter
[Component: C# with SNMP
library]
Sends SNMPalerts
Sendsalerts using
Scheduler
[Component: Quartz.net]
Starts
Publishesthe risk
Problem: Latent coupling about filesystem
layout.
Solution: A module to hide the decision
about filesystem layout from
both the Report Distributor and the
Report Checker

80. Just Used Libraries in 3 Ways

81. Just Used Libraries in 3 Ways
• Define meeting point (interface)

82. Just Used Libraries in 3 Ways
• Define meeting point (interface)
• Extract repeated functionality

83. Just Used Libraries in 3 Ways
• Define meeting point (interface)
• Extract repeated functionality
• Isolate shared concept/decision

84. Find solutions by rotating your perspective
• When looking at components, think about modules
• When looking at modules, think about components
• When looking at data, think about code
• When looking at code, think about data

85. Use all your tools
1. Module structure – layout of your code and libraries
2. Component structure – interactions between runtime components
3. Abstraction – Emphasize similar interfaces & data formats

86. Coupling
• Determines degrees of freedom.
• Enables some movements.
• Inhibits other movements.
• Connects effects.

87. Kinds of Coupling
• Operational
• Development
• Semantic
• Functional

88. Summary
• Hide decisions
• Choose degrees of freedom that matter
• Avoid semantic polymers
• Use static and dynamic structures
• Find more instances of fewer, more general interfaces
• Prefer explicit to tacit

89. Uncoupling
Michael T. Nygard
@mtnygard