Coupling is the difficulty when changing things.
Tensegrity – Holds it self together by the tension among parts and the resistant tension from the beams. It’s a nest of opposing forces.
Coupling is everywhere. We are coupled to the Earth. The Earth is coupled to the Sun. The solar system is coupled to the Milky Way. The same is true if we go to the atomic level.
Coupling defines the degrees of freedom. It constrains and enables some movement. It also connects effects.
Everything is coupled to everything
- ambient temperature
- ambient atmosphere
- acoustic vibrations
- Electromagnetic field
- Gravity
- Higgs field
| Type | Effect |
|---|---|
| Operational | Consumer cannot run without the provider |
| Development | Changes in producers and consumers 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.) |
A shared concept is an idea that is exposed/implemented in one of area of the system. And then other consumers (external or internal) must embrace the concept to consume the resource. However, if the concept is flawed, that flaw proliferates throughout the system.
System Diagrams – Long Arrows
- Arrows connecting two systems is actually hiding a number of moving parts
- Extract tables to files
- Push files across network
- Load data into database
- etc…
- Architecture Qualities in Long Chains
- Latency strictly warse 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
- Digging Out – Information Hiding (Decision Hiding)
- On the Criteria to be used in Composing Systems into Modules – David Parness
Composing Interfaces
A smaller number of well defined interfaces is preferable over more interfaces and small number of concise datatypes is preferable over more datatypes.
Composiblity
- Inversely proportional to the # of interfaces
- Inversely proportional to the # of data types
Uncoupling
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
Rotate your perspective 90%
- 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
Use all your tools
- Module structure – layout of your code and libraries
- Component structure – interactions between runtime components
- Abstraction – Emphasize similar interfaces & data formats