How to break apart a monolithic system safely without destroying your team Moving from a monolith to microservices can be daunting. How do we choose the right bounded contexts? How small should services be? Which teams should get which services? And how do we keep things from falling apart? By starting with the needs of the team, we can infer some useful heuristics for evolving from a monolithic architecture to a set of more loosely coupled services. Matthew Skelton is co-founder of Skelton Thatcher Consulting / @matthewpskelton

1. How to break apart a
monolithic system safely
without destroying your team
Matthew Skelton, Skelton Thatcher Consulting
@matthewpskelton
LondonCD meetup: 11 Oct 2016
#londoncd

2. Today
Cognitive load for teams
‘Monolith’
Code Forensics
Team-first boundaries
Monolith-splitting recipe

3. For now, let’s forget:
Microservices
CQRS / Event Sourcing
Queues
(Architectural changes)

5. Continuous Delivery / etc…
30+ organisations
UK, US, EU, India, China

7. How to break apart a
monolithic system safely
without destroying your
team

8. Safer, more rapid changes
to software systems
(Business Agility)

9. A ‘team-first’ approach to
software subsystem
boundaries

10. TEAM

12. TEAM
capabilities
appetite & aptitude
understanding
responsibilities

13. Conway’s Law

15. ‘Reverse Conway’
Tobbe Gyllebring (@drunkcod)

17. homomorphic force
(#Conway  #Yawnoc)
HT @allankellynet
(same) (shape)

18. Cognitive load for teams

19. We have SCIENCE!

20. Cognitive load for teams
• Driskell et al, 1999 ‘Does Stress Lead to a Loss of Team Perspective?’
Group Dynamics: Theory, Research, and Practice 3, no. 4 (1999): 291.
• Fan et al, 2010 ‘Learning HMM-Based Cognitive Load Models for
Supporting Human-Agent Teamwork’. Cognitive Systems Research 11,
no. 1 (2010): 108–119.
• Ilgen & Hollenbeck, 1993 ‘Effective Team Performance under Stress
and Normal Conditions: An Experimental Paradigm, Theory and Data
for Studying Team Decision Making in Hierarchical Teams with
Distributed Expertise’. DTIC Document, 1993.
• Johnston et al, 2002 ‘Application of Cognitive Load Theory to
Developing a Measure of Team Decision Efficiency’. DTIC Document,
2002.

21. “stress impacts team
performance … by narrowing
or weakening the team-level
perspective required for
effective team behavior.”
– Driskell et al, 1999
Group Dynamics: Theory, Research, and Practice 1999, Vol. 3, No. 4,291-302

22. ‘Monolith’

23. “Don’t start with a monolith
when your goal is a
microservices architecture”
– Stefan Tilkov, innoQ
http://martinfowler.com/articles/dont-start-monolith.html

24. “Start monolithic and extract”
– Tammer Saleh, Pivotal
https://www.infoq.com/presentations/cloud-anti-patterns

25. A ‘team-first’ approach to
software subsystem
boundaries

26. Types of software monoliths
•Application monolith
•Joined at the DB
•Monolithic releases (coupled)
•Monolithic thinking (standardisation)
…

27. Application
monolith
Single block of code
Deployed as a unit

28. Joined at
the DB
Difficult to change
separately (but not
impossible)
Risk is (probably)
elevated
Chris Collyer, http://www.stone-circles.org.uk/stone/pentreifan.htm

29. Monolithic
releases
Smaller components
bundled together into a
‘release’

30. Monolithic
thinking
‘One-size-fits-all’ for
teams
Assumption that
minimising variation is
A Good Thing

31. Dangers of splitting a
monolith
•Reduced domain consistency (UX,
Architecture, Data)
•Data duplication (unintentional)
•Additional operational complexity due to
distributed system and async messaging
•Degraded UX across the product

32. Splitting a
monolith
Reiner Flassig - CC BY-SA 2.0 de - Wikipedia
Choose the right
technique for splitting
Understand the nature
of the monolith

34. ‘Fracture planes’ for code
•Business domain bounded context
•Regulatory compliance
•Change cadence
•Risk
•Performance isolation
•User personas
•Team location

35. Code Forensics

36. Forensics
Your Code as a
Crime Scene
Adam Tornhill

37. Adam Tornhill
Code, Crime, Complexity:
Analyzing software with
forensic psychology
Adam Tornhill
TEDxTrondheim
youtube.com/watch?v=qJ_hplxTYJw

38. ‘Code Maat’ tool

39. Adam Tornhill, http://www.adamtornhill.com/articles/crimescene/codeascrimescene.htm
Code City plus Code Maat forensics

40. Beware of badly-named
subsystems

41. "information-poor abstract
names are magnets for extra
[unwanted] responsibilities"
– Adam Tornhill
p.185, Your Code as a Crime Scene

42. Team-first boundaries

43. DevOpsTopologies.com

44. Team types
Component team
Platform / ’substrate’ team
Supporting / ‘productivity’ team
Product/Feature team

45. Team types
devopstopologies.com
Component team
Platform / ’substrate’ team
Supporting / ‘productivity’ team
Product/Feature team

46. Code repositories
Align repositories to subsystem
boundaries
Avoid monolithic-y repos like TFS*
* Don’t get me started on TFS, grrr…

47. Code repositories
Repo 1 Build Test Deploy Run
Repo 2 Build Test Deploy Run
Repo 3 Build Test Deploy Run

48. “You can use a monorepo only if
your organisation has published a
scientific paper on Computer
Science. Otherwise, use one repo
per separate deployable thing.”
– Matthew Skelton
LondonCD meetup group, 11 Oct 2016 

49. Find natural or available
‘fracture planes’ for splitting
a monolith
(with the team in mind)

50. Monolith-splitting recipe
Tried and tested!

51. How to break apart a monolith
without destroying your team
1. Instrument the monolith – logging
2. Grok data flows and fault responses
3. Align teams to available segments
4. Split off segments one-by-one

52. Instrument the monolith

53. Instrument the monolith

54. Instrument the monolith

55. Instrument the monolith
Correlation ID Logs
Event ID

57. use logging as a
channel/vector to
make distributed systems
more testable
use logging as a
channel/vector to
make distributed systems
more testable

58. Grok data flows and fault responses

59. Grok data flows and fault responses
Correlation ID
Event ID
Unexpected
collaborating
subsystems
Undetected
fault condition

60. Grok data flows and fault responses
Correlation ID
Event ID
Adjust
subsystem
boundaries
Fix poor fault
responses

61. runbooktemplate.info
Run Book dialogue sheets

62. Align teams to available segments

63. Align teams to available segments

64. Align teams to available segments
Map to
business
domain

65. Align teams to available segments
Identify likely
components
or ‘platform’
elements

66. Split off segments one-by-one

67. Split off segments one-by-one

68. Split off segments one-by-one
Separate:
- Builds
- Infrastructure
- Deployments
- Versions
- Lifecycle

69. Team needs / responsibilities /
capabilities come first

71. How to break apart a monolith
without destroying your team
1. Instrument the monolith – logging
2. Grok data flows and fault responses
3. Align teams to available segments
4. Split off segments one-by-one

72. How to break apart a monolith
without destroying your team
1. Instrument the monolith – logging
2. Grok data flows and fault responses
3. Align teams to available segments
4. Split off segments one-by-one
* plus a few other things

73. Further material

75. Books & articles
• Working Effectively with Legacy Code, by Michael Feathers
• Building Microservices by Sam Newman
• ‘Managing Cognitive Load for Team Learning’ by Jo Pearce
http://12devsofxmas.co.uk/2015/12/day-3-managing-cognitive-
load-for-team-learning/

76. Training
• From Monolith to Microservices (online training) – Sam Newman,
author of Building Microservices
http://www.oreilly.com/live-training/from-monolith-to-
microservices.html

77. Talks & slides
• What is cognitive load theory and why should you care?, by Jo
Pearce - http://www.slideshare.net/JoPearce5/what-is-cognitive-
load-theory-5mins
• Building Microservices by Sam Newman

78. Research papers
• Driskell, James E., Eduardo Salas, and Joan Johnston. ‘Does Stress Lead to a Loss of Team
Perspective?’ Group Dynamics: Theory, Research, and Practice 3, no. 4 (1999): 291.
• Fan, Xiaocong, Po-Chun Chen, and John Yen. ‘Learning HMM-Based Cognitive Load Models
for Supporting Human-Agent Teamwork’. Cognitive Systems Research 11, no. 1 (2010):
108–119.
• Ilgen, Daniel R., and John R. Hollenbeck. ‘Effective Team Performance under Stress and
Normal Conditions: An Experimental Paradigm, Theory and Data for Studying Team Decision
Making in Hierarchical Teams with Distributed Expertise’. DTIC Document, 1993.
http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA284683.
• Johnston, Joan H., Stephen M. Fiore, Carol Paris, and C. A. Smith. ‘Application of Cognitive
Load Theory to Developing a Measure of Team Decision Efficiency’. DTIC Document, 2002.
http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA525820.

79. thank you
Matthew Skelton
@matthewpskelton
skeltonthatcher.com