Bend, or Break

Make every effort to write code that’s as flexible as possible.

Decoupling

• Coupling is the enemy of change, because it links together things that must change in parallel.
• Software is not bridges—we don’t want components to be rigidly coupled together, or else change one component requires changing its coupled components.
• Coupling is transitive: A -> B, B -> C means A -> C.
• Decoupled code is easier to change (ETC).
• Symptoms of coupling
• dependencies between unrelated modules / libraries
• simple changes to one module propagates changes through unrelated modules (or breaks)
• developers are afraid to change code because they don’t know what will be affected
• meeting where everyone has to attend because nobody is sure who will be affected by a change

Train Wrecks

public void applyDiscount(customer, order_id, discount) {
      totals = customer
          .orders
          .find(order_id)
          .getTotals();
      totals.grandTotal = totals.grandTotal - discount;
      totals.discount   = discount;
}Code language: JavaScript (javascript)

That code transverses five levels of abstraction, from customers to total amounts. We have to know that customer exposes orders that have a .find() method, etc., all the way down. That’s a lot of implicit knowledge and things that cannot change in the future.

Tell, don’t ask. Don’t make decisions based on the internal state of an object and then update that object. This is a pattern, not a law of nature, so don’t follow it slavishly.

public void applyDiscount(customer, order_id, discount) {
      totals = customer
          .findOrder(order_id)
          .applyDiscount(discount);
}Code language: JavaScript (javascript)

The Law of Demeter expressing the following sentiment in a more detailed way (LoD probably not very relevant today): don’t chain method calls. This “one-dot rule” doesn’t apply if the things you’re chaining are very unlikely to change (like language-level features). The following Ruby code doesn’t violate the one-dot rule, because it’s language-level.

people
  .sort_by {|person| person.age }
  .first(10)
  .map {| person | person.name }

Pipelines are not method-call chains: pipelines transform data, passing it from one function to the next. We’re not relying on hidden implementation details.

• [ ] How does “tell, don’t ask” strike you?
• [ ] Do you think the “one-dot rule” is practical? Could it be helpful?

The Evils of Globalization

• Global data is coupling, as you never know what will break if you change it. Reuse should not be your primary concern when writing code, but the thinking that makes code reusable should be in your mind as you create it. Avoid global data—it slows you down.
• Singletons are global data, though at least they have intelligence behind Confg.getLogLevel() that can help you not break calling code.
• Any mutable external resource is global data. You can’t avoid using a database, so you can minimize the impact of global data by wrapping these resources behind code you control.
• If it’s important enough to be global, wrap it in an API.
• [ ] What are appropriate uses of global data?

Inheritance Adds Coupling

Subclassing just isn’t shy: it doesn’t deal with only it’s own concern. Alterations in one place (the parent class) can change the subclass elsewhere.

• [ ] How does this strike you? Do you prefer to work in an OOP mental model / language?
• [ ] Can you imagine using OOP without subclassing?

Juggling the Real World

Events represent the availability of information. They can come from external or internal sources. When we write applications that response to events, here are are few strategies:

• Finite State Machines
• The Observer Pattern
• Publish / Subscribe
• Reactive Programming and Streams

Finite State Machines

• There exist a limited number of states for your application. You can be in one state at any given time.
• Events move you from one state to another.
• Actions can be triggered upon moving.
• [ ] What do you think about using a FSM in your next application? Have you ever used one before?

The Observer Pattern

• source of events is the observable
• observers are watching the observable
• fairly simple pattern: push a function reference into a list, and call those functions when the event occurs
• because the observers have to register with the observable, you introduce coupling
• callbacks are handled synchronously, so you have more opportunity for performance bottlenecks

Publish / Subscribe (PubSub)

• generalizes the observer pattern, dealing with coupling and performance bottlenecks
• publishers and subscribers are connected via channels (the how is an implementation detail hidden from your application logic)
• subscribes register to 1 or more channels
• publishers write to channels
• good choice for decoupling the handling of asynchronous events
• observability is hard with such a distributed system
• is a good example of reducing coupling by abstracting up through a shared interface (the channel)

Reactive Programming

Reactive programming, as a paradigm, is often compared to using a spreadsheet: you change one value, and other values reactively update. Reactivity can be created with events, but streams build reactivity in. RxJS is a good example of this paradigm. Event streams unify synchronous and asynchronous processing behind a common API.

Section Challenges

• [ ] Exercise 19 In the FSM section we mentioned that you could move the generic state machine implementation into its own class. That class would probably be initialized by passing in a table of transitions and an initial state. Try implementing the string extractor that way.
• [ ] Exercise 20 Which of these technologies (perhaps in combination) would be a good fit for the following situations:
• If you receive three network interface down events within five minutes, notify the operations staff.
• If it is after sunset, and there is motion detected at the bottom of the stairs followed by motion detected at the top of the stairs, turn on the upstairs lights.
• You want to notify various reporting systems that an order was completed.
• In order to determine whether a customer qualifies for a car loan, the application needs to send requests to three backend services and wait for the responses.

Transforming Programming

All programs transform data, yet we rarely thing about creating transformations when designing software. There’s great value in thinking about programs as being something that transforms inputs into outputs—like an industrial assembly line.

• think of the Unix philosophy
• programming is about code, but programs are about data
• break down your program into transform |> transform, then repeat
• even if your language doesn’t support pipes, you can still use the philosophy of design

const content = File.read(fileName);
const lines = findMatchingLines(content, pattern);
const result = truncateLines(lines);Code language: JavaScript (javascript)

• the reason transforms are worthwhile is that instead of hoarding state (encapsulation in objects), you pass it around—you lose a whole category of complexity and coupling
• data becomes a flow…a peer to functionality
• error handling can be done with either:
• an :ok/:error tuple (I like [error, data]), handled inside each transformation
• handle it in the pipeline (some kind of andThen function that only continues if no error)

Section Challenges

• [ ] Exercise 21 Can you express the following requirements as a top-level transformation? That is, for each, identify the input and the output.

1. Shipping and sales tax are added to an order
2. Your application loads configuration information from a named file
3. Someone logs in to a web application

• [ ] Exercise 22 You’ve identified the need to validate and convert an input field from a string into an integer between 18 and 150. The overall transformation is described by

  field contents as string
    -> [validate & convert]
      -> {:ok, value} | {:error, reason}Code language: JavaScript (javascript)

Write the individual transformations that make up validate & convert.

• [ ] Exercise 23 In _Language X Doesn’t Have Pipelines, on page 153 we wrote:

  const content = File.read(file_name);
  const lines = find_matching_lines(content, pattern);
  const result = truncate_lines(lines);Code language: JavaScript (javascript)

Many people write OO code by chaining together method calls, and might be tempted to write this as something like:

  const result = content_of(file_name)
                  .find_matching_lines(pattern)
                  .truncate_lines(lines)Code language: JavaScript (javascript)

What’s the difference between these two pieces of code? Which do you think we prefer and why?

Inheritance Tax

• two types of inheritance (from two origins):
• Simula, where inheritance was a way of combining types
• Smalltalk, where inheritance was a dynamic organization of behaviors
• both types have the issue of coupling code
• alternatives to inheritance:
• interfaces and protocols, which allow us to
• delegation
• mixins and traits
• delegate to services: has-a trumps is-a

Section Challenges

• [ ] The next time you find yourself subclassing, take a minute to examine the options. Can you achieve what you want the interfaces, delegation, and / or mixins? Can you reduce coupling by doing so?

Configuration

Parameterize your application by using external configuration. Common configurable data:

• credentials for external services
• logging levels and destinations
• ports, IP addresses, machine names, cluster names
• environment-specific validation parameters
• externally-set parameters (like tax rates)
• site-specific formatting details
• license keys

You could structure this as a flat-file off-the-shelf plain-text document (that works). You can also store it in a database table if it is likely to be changed by the customer. You can also do both!

Consider putting your configuration data behind a thin API:

• multiple applications can share configuration data (with appropriate authN and authZ)
• configuration changes can be made globally
• configuration data can be made via a specialized UI
• configuration data become dynamic (no application restart necessary)

As with all things, don’t overdo it. You can have too much configuration.