73. Clean Architecture – Keeping Your Options Open

Joe drinks too much coffee, Allen spits his coffee out, and Michael feels vindicated as the discussions about Robert C. Martin’s latest book, Clean Architecture, continues.

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What is Architecture?

• Makes us think of weighty decisions and deep technical prowess
• Even though it sounds like an architect is Gandalf the Gray, it’s really still a programmer
• They still program but also focus on guiding the rest of the team towards better productivity
• They must continue to program so they can experience the problems / pains the rest of the developers feel
• Purpose of the shape of software is to facilitate the development, deployment, operation and maintenance of the software itself
  • The strategy should be to leave as many options open as possible, for as long as possible
• Software architecture is NOT about whether the software works – seems counter intuitive…but….even bad architecture can work
• There are few, if any behavioral options that architecture can leave open
• Architecture’s main goal is to support the life cycle of the system

Development

• Hard to develop software = unhealthy lifetime
• Typically small teams don’t need to think about architecture because they can work well without drawing lines
  • This is probably why most software systems aren’t architected well – they start out small and then teams / software grow
• Systems developed by multiple decently sized teams will need some sort of dividing lines / splitting up components
  • Typically this would end up with the same number of teams = same number of components
    • Probably not best for the long term deployment / architecture

Deployment

• Software systems should be deployable
• Software that is more difficult/expensive to deploy is less useful
• Good architecture means that software can EASILY be deployed with ONE action
• This is seldom a consideration during the initial phases of a new project
• Microservices were called to the floor here – typically when microservices are decided upon, folks aren’t thinking about the pain points with deployments, failures, etc

Operation

• When talking about operation, he’s talking about how efficiently the software runs
• Typically this isn’t taken into too much consideration because it’s cheaper to throw hardware at a problem than throw more resources at development, deployment and maintenance
• However, a good architecture communicates the operational needs of the system
  • The operational needs will be apparent to the developer

Maintenance

• Maintenance is the most costly aspect of software
• Primary reasons: spelunking and risk
  • Exploring to find the best place and best strategy to introduce a change
• A well thought out architecture greatly reduces these costs / risks

Keeping Options Open

• Software has two types of value: behavior and structure
• The structure is what makes it _soft_
• The key to keeping software _soft_ is by leaving as many options open for as long possible.
• Options that can be left open are those details that don’t matter. Until they do.
• All software can be broken down into two themes: policy and details.
  • Its the policy that is the true value of the system.
• The goal of the architect is to shape the system so that the policy is the most important aspect while making the details irrelevant to the policy
• The longer you wait to make these decisions, the more informed you’ll be by the time you _need_ to make that decision
• _A good architect maximizes the number of decisions not made._
• The last responsible moment: https://blog.codinghorror.com/the-last-responsible-moment/

Wrapping it up

• Good architects separate the policy from the details so throughly that the policy has no knowledge nor dependency on the details

Independence

• Recap, architect must support use cases/operation, maintenance, development, deployment of the system

Use cases

• Architecture must support the _intent_ of the system
• The most important thing architecture can do is to expose that behavior so the INTENT of the system is visible at the architectural level
• A shopping cart app with good architecture will look like a shopping cart app
• Use cases should be plainly visible
• “Screaming Architecture” – The architecture should emphasize it’s purpose, not the framework
• Frameworks are an implementation detail!

Operation

• Sometimes it can be tempting to rearrange the architecture of a system around processing requriements
• For example: A high throughput system may need to take great advantage of independent scaling and parralelism
• Ideally your system will be agnostic to it’s implementation details though, as these requirements could change over time
  * It’s better that your architecture primarily reflect it’s purpose
  * An architecture that properly isolates its components will be easier to change over time as the operational needs change

Development

• Conway’s law (Wikipedia)
  • “Any organization that designs a system will inevitably produce a design whose structure is a copy of the organization’s communication structure.”
• Goal is to partition the system into isolated, components…so should we arrange our teams that way?
• A system developed by many teams and concerns must facilitate independent actions

Deployment

• The goal is immediate deployment
• Not a hodgepodge of scripts and ftp – this is achieved by proper partitioning and isolation
• Was there life before Docker?

Leaving Options Open

• Good architecture balances these concerns
• Sounds easy. But the reality is that is hard.
  • Why? Here are the things we don’t know up front:
    • All of the use cases
    • The operational constraints
    • The team structure
    • Deployment requirements
• Worse, those things _will_ change
• We’re trying to hit a moving target

Decoupling Layers

• The goal is to support all necessary use cases…even the ones we don’t know about yet
• The architect DOES know about the basic intent of the system – apply the Single Responsibility Principle and the Common Closure Principle to isolate functionality, and promote composability
• The database, query, and schema are technical details that have nothing to do with the business rules or UI – they change at different rates so keep them in separate layers
• Good starting point: UI, App / Biz Rules, Core Biz Rules, Database

Decoupling Use Cases

• Use cases are narrow vertical slices that cut through horizontal layers of the system
• For example: The uses cases for adding an order and deleting an order will likely change for different reasons, at different rates – so why tie them together?
• Try to keep use cases separated, for example – having a separate UI to delete orders and a separate one to add orders
• But…what about UX? If it’s common for the user to delete orders and add orders at the same time – why punish them?
• Great podcast from Eat Sleep Code (Pocket Casts), with Sara Ford (Twitter) talking about how software developers tend to design systems in such a way that it’s easier to work on…but not necessarily easier to use!
• The benefit is that decoupling the features makes them easier to add and change

Decoupling Mode

• Having your components divided up into layers makes it easier to scale those layers separately
• You can add more back-end servers, or more front-end servers as necessary
• Decoupling what we did for use cases is now aiding our operations!
• Good architecture is about leaving your options open

Independent Developability and Deployability

• Horizontal layers make it easier for teams to be organized based on what makes sense for the biz (a la feature, layer, etc)
• Horizontal layers buy you some deployment flexibilty

Duplication

• Architects often fall into the trap of the “fear of duplication”
• Duplication is generally a bad thing
• And when there is duplication, we are honor-bound to reduce it
• But there are different kinds of duplication
  • True duplication – every change to one instance requires the same change to the other instances
  • False or accidental duplication – seems to be duplicated, but changes at different rates and/or for different reasons
• When separating use cases, there’s a temptation to couple use cases that have similar screens, algorithms, or database queries/schemas
  • Resist the urge to eliminate this duplication until you know that it is true.
  • But don’t get lazy either and simply pass a entire row from the DB straight to the UI. View models are cheap/easy and decouple these layers.

Decoupling Mode (revisited)

• We can decouple at the many levels: source code, deployment, and execution level.
• Source control: Packages (changes in one, don’t force a recompile)
• Deployment level: Similar, but independently deployed
• Service level: Separated by network, completely independent deployment
• Which is best? Difficult to know during the early phases of a project.
• Microservices are (were?) all the rave but service level decoupling is expensive both in terms of dev time and system resources.
• Uncle Bob’s preference? Push the decoupling to the point where a service _could_ be formed. _Should_ it become necessary. In the meantime, leave it in the same address same for as long as possible.
• Start at the source code level, but keep things nice and isolated so you could easily split it off
• Good architecture will allow a system to start as a monolith, and then grow to independently deployable units, and all the way back to monolith if need be down the line.
• Good architecture protects the majority of the source code from these changes – the decoupling is left open

Resources We Like

• Clean Architecture by Robert C. Martin (Amazon)
• Feature Toggles, Package Management and Versioning in Microsoft TFS and VSTS 2017 by Marcel de Vries (Pluralsight)

Tip of the Week

• Selectively stage chunks of a file using git add -p [<pathspec>] – https://git-scm.com/docs/git-add
• Sql Operations Studio is now available
• Use Flow for static type checking your next JavaScript project – https://flow.org/
• Application Security Weekly, listen and learn how to do security right.

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