Mapping Constraints
Barry O Sullivan 2019 - @barryosull
- Software Contractor
- Jack of all trades
- Architect/Product/Legacy Dev
- PHP Dublin Organiser
- DDD IE Co-Organiser
@barryosull
Barry O Sullivan
Who am I
What this talk is about
Your System
New Feature
What are "Constraints"?
A constraint is something that limits or controls what you can do.
Tactical
Constraints
Value
Data validity
Hard rules on accepted values
- Email address
- Cost
- Mailing Address
- Flight number
Represented by "Unit" tests
Low level
Value
What values are allowed
Value
Data validity
Temporal
What changes are allowed, given previous values
Hard rules on state changes
E.g. "Cannot check out a cart that is empty"
Represented by "Unit" tests
Mid level
Value
Temporal Scope
"An itinerary cannot change once it's completed" - Entity
"Org itineraries must have a unique, incrementing ID" - Org
"All email addresses must be unique" - System
Wide scope of temporal constraints = bottlenecks
How large is the boundary
Behavioural
What can a user do
"As an agent I can create a travel plan for a client"
Represented by "Acceptance" tests
Think of them as "User stories"
High level, combination of value & temporal
Value
Modelling Constraints
Event Storming
Model Processes (Behavioural)
- Map out events/changes
- Map out temporal constraints
- Map out values in events
Implementation
- Type of app (web/desktop/mobile)
- Architecture
- Infrastructure
- Release process
A Story of Modelling
Software for Small/Medium sized Travel Agencies
Building the Product
- Event Sourced web app
- Event Storming used for modelling with domain experts
- Value/Temporal/Behavioural Constraints modelled and unit tested
Why it Didn't Work
- Product was expensive to change
- Existing constraints got in the way
(constraints became constraints) - Modelled wrong business problem
- Tried to pivot, ran out of funding
The Cost of
Constraints
Learning from experience
Cost of Implementing Constraints
- Value
- Temporal
- Behavioural
£
££
£££
Cost of Changing Constraints
Migrating to new constraints
- Model required changes
- Change existing system
- Upgrade historical data
- Ensure it's all valid
- Value
- Temporal
- Behavioural
Culling Constraints
Culling Constraints
O
X
Not all constraints are created equal
O
O
X
X
X
X
X
Categorise
Constraints
- Core/Supporting/General
- Assumptive
- Idealistic
- Imposed vs self-imposed
Constraint Rigidity
"Well actually . . . "
Another Story of Modelling
Building NoteWorthy
Constraining our Constraints
- Event stormed all processes
- Categorised by Core/Supporting/General
- Categorised by value/risk/complexity (cost)
- Culled ruthlessly
- Only built what we needed
Modelled Constraints
Auth
Idea Submission
Proposal Publishing
Crowd Funding
Story Publishing
Generic
Firebase Auth
Assumptive
Typeform + Trello
Core
Web App
Generic*
GoFundMe
Core
Existing System
Existing Systems
Legacy Constraints
The legacy system will exert control
It is a key constraint
Legacy Constraint Discovery
- Explore
- Discuss
- Define
Modelling with Legacy*
* Now with more microservices!
- Culture
- The team
- Processes
- Politics
Meta Constraints
- Constraints define your system
- Constraints are expensive
- Explore then cull your constraints
- Simplicity is king
- Legacy systems exert control
Wrap up
Q&A
@barryosull
Mapping Constraints
By Barry O' Sullivan
Mapping Constraints
How do we define the constraints of our system? A simple question, but one we often get wrong. Too often we model constraints mechanically rather than exploring and challenging them. This leads to brittle systems and bloated software. How do we move past this? This talk is an exploration of the very concept of constraints and how they affect the systems we build. We're going to look at the different types of constraints and the impact they have over time. We'll explore techniques we can use to map, define and refine these constraints, elevating what's important and dropping everything else. We'll also look at legacy, organisational and meta-system constraints and how they impact what we build and how we build it. Mapping constraints is core to what we do, let's get better at it.
