Architects have to balance providing clear guidance for important decisions with the need to let people get on and build their aspects of the system without interference. In this talk Eoin Woods explores how architecture principles can help achieve this by making constraints and priorities clear without being unnecessarily prescriptive about how they are to be implemented. Presented at O'Reilly Software Architecture Conference in London during October 2016.

1. Using Software
Architecture Principles
in Practice
Eoin Woods - Endava
@eoinwoodz

2. BACKGROUND
• Eoin Woods
• CTO at Endava (technology services, 3300 people)
• 10 years in product development - Bull, Sybase, InterTrust
• 10 years in capital markets applications - UBS and BGI
• Software engineer, then architect, now CTO
• Author, editor, speaker, community guy

3. COMMON CHALLENGES

4. COMMON CHALLENGES
• Understanding rationale for architecture & design
• Maintaining knowledge over time
• Relating contexts across enterprise / system / code
• Guiding design without making the decisions
• Minimal effective documentation

5. COMMON SOLUTIONS
• Lots of documentation
• always out of date
• “Listen to the code”
• doesn't contain all of the information
• Oral history
• Fragile and easily lost or distorted

6. UNCOMMON SOLUTIONS
• Architecture design decisions
• Architecture principles
These practices are elegant solutions to capturing and communicating
architectural knowledge
Today we’ll talk about principles

7. ARCHITECTURE PRINCIPLES

8. ARCHITECTURE PRINCIPLES
• What is a “principle” ?
• a fundamental truth or proposition serving as the foundation for belief or action
[OED]
• An architecture principle is ….
• a declarative statement made with the intention of guiding architectural
design decisions in order to achieve one or more qualities of a system

9. A (VERY) SIMPLE EXAMPLE
• Use Tiered Architectures:
Use application tiers to separate presentation, logic & data.
Our rationale for this is:
• data lives longer than applications; business logic lives longer than UIs
• tiers separate aspects of the system to allow different rates of evolution
• tiers allow variation of technology and qualities for different parts of systems (e.g. scalable
servers, secure databases)
• while more complex initially than two tier systems, this is outweighed by the benefits for
all but the simplest cases

10. REQUIREMENTS AND DECISIONS
• Requirement - the desired system quality
• normally measurable
• Principle - a constraint or guide for decisions, reflecting the requirement(s)
• applicable to a broad context (guides many decisions)
• Decision - how something is to be achieved
• bound to a specific narrow context

11. PRINCIPLES ALIGNTEAMS
Goal
Requirement
Principle
Decision

12. PRINCIPLES ALIGNINGTEAMS
Goal: allow product to be sold into regulated industries (unknown regulations)
Requirement: allow deployment on premise and to “cloud” platforms
Principles:
1. Do not use cloud specific services
2. Only assume key/value storage for data stores
Decisions:
1. Initially deploy to AWS
2. Do not use unique AWS services (e.g. SQS)
3. Use Aerospike for DB accessed via LibStore
Guidance
Rationale

13. PRINCIPLES ALIGNINGTEAMS
Requirement: add new trading partner 2 in weeks, <= 50 days effort
Principles:
1.We prefer industry protocols, then standard in-house ones,
then ad-hoc point-to-point ones
2. Partner specific detail must not pollute domain model -
where necessary then isolate in specific areas
Decisions:
1. Integrate using “Trader” model with partner extensions
2. Integrate using RPC and pub/sub messaging
3. For pub/sub use RabbitMQ, RPC use HTTP/JSON REST
Guidance
Rationale

14. ARCHITECTURE PRINCIPLES IN PRACTICE

15. USEFULTYPES OF PRINCIPLE
• Define a goal
• “single visitor logon for all web sites”
• Indicate a preference or constraint
• “prefer 3rd party data formats, over
in-house, over custom”
• Avoid a specific technical problem
• “use tiers to avoid UIs becoming
bound to databases”
• Encourage a practice
• “our software must always be
deployable at the end of a sprint”
• Remind people of useful knowledge
• “abstractions live longer than
details” [Hunt &Thomas]

16. DEFINING A PRINCIPLE
• A good name
• A clear description
• A rationale
• An example
• and a counter-example
• Applicability
• if necessary for clarity
• Implications
• if not obvious

17. ANOTHER EXAMPLE
Name Prefer Standardised Message Protocols
Description
We prefer to use industry protocols for messaging, where this isn’t
possible we use well defined in-house ones, only in the last resort do
we use local ad-hoc protocols.
Rationale
Minimise concept (re)definition, maximise future integration options,
allow flexible organisational structure
Example
Use FIX if that’s not possible, Equities wide standard transaction
messages, otherwise, point-to-point ad-hoc fmt
Applicability All inter-system business transactions across Markets IT
Implications
Teams need to understand the options; time needs to be taken to
understand new protocols if needed

18. PROPERTIES OF GOOD PRINCIPLES
Constructive stated for a definite purpose, useful guide for decision making
Reasoned rational, logical, consistent
Well Articulated comprehensible by all of the necessary stakeholders
Testable can check if you’ve followed it and where the exceptions are
Significant not just a truism; would the opposite ever be the case?
(Nick Rozanski)

19. VIOLATING PRINCIPLES
• Principles can’t always be followed
• but when broken must be broken for justifiable reasons
• i.e. benefits have to outweigh the costs
• This doesn’t (necessarily) reduce their usefulness
• reason for breaking a principle is valuable design information
• many violations signal the need to revisit the principle
• capturing the violation signals a non-standard decision

20. PRACTICAL COMPLICATIONS
Identification identifying meaningful principles, avoiding truisms
Quantity avoiding excessive number (too many authors?)
Description being clear, complete, succinct & understandable
Validation difficult to validate and measure the value
Communicating often difficult to understand & internalise
Retrofitting difficult to apply principles usefully after the fact

21. TO CONCLUDE

22. CONCLUSIONS
• Principles provide “laws” to guide the design process
• useful at different levels of scale, help to create informed design decisions
• Principles can provide traceability
• link back to more abstract principles, requirements or goals
• justifies decisions by reference to a particular context
• Address some common challenges
• maintain knowledge, capture rationale, provide context, …
• elegant and minimal solution avoid lots of documents

23. FINDING OUT MORE
(Yes - really!)

24. THANKYOU … QUESTIONS?
Eoin Woods
Endava
eoin.woods@endava.com
@eoinwoodz