Core principles for software that scales (in practice)

Lately I am thinking about this more and more:

What are the principles to write code that scales? 🤔

Leaving aside the specific language, design patterns, and theory. What should a developer focus in practice when writing large software projects?

Some (initial) ideas:

• 📖 Local reasoning
  • Modularity
  • Refactoring
• 🚩 Error handling
• 🧪 Testability
  • Configuration
• 🔬 Debugging/Observability
• 🎨 State management
  • Data structures

Sandro Maglione

@SandroMaglione

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Concepts to understand about error handling 👀 👉 Error vs Exception 👉 Stack track 👉 Error object 👉 try/catch/finally 👉 Union types (Option/Either) Error handling is at the core of every program Back to refine and master the basics 📖

9:23 PM · Apr 21, 2024

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This is how it looks like in practice 👇

Local reasoning: the secret for software that scales

You work in a car factory. You role is to attach doors to the frame of the car:

• You expect the car's frame to be delivered to you (input)
• You perform your role (attach the door)
• You produce a car with doors (output)

In this process you don't need to bother with anything else: get the frame, attach the door, move to the next frame.

This is what makes factories scale: local reasoning.

It works the same for code:

Local reasoning: property of some code wherein the correctness of the code can be inferred locally (under specified assumptions) without considering prior application state or all possible inputs

(Source)

/*
 * Can you tell what this function does, without reading any other part of the codebase?
 * - Get a `card` and `amount`
 * - Perform a payment to the given `card` for the given `amount`
 * - It requires a `PaymentService` to work (e.g. Stripe)
 * - It can fail if `card` is invalid or we are unable to check its validity
 * - When successful, it returns the details of the payment (`Payment`)
 * - It performs a side-effect (return type `Effect`)
*/
const makePayment = (params: { amount: number, card: Card }): Effect.Effect<
  Payment,
  InvalidCardError | CheckingCardError,
  PaymentService
>  => /// ...

Error handling

Error handling is not discussed enough 🤨

It's found and required in every software, but there is still a lot of confusion around it

In practice:

• Returns errors as values
• Propagate errors between functions

function getCreditCard(email: string): Effect.Effect<Card, CheckingCardError> {
  /// ...
}
 
function getIsCardValid(card: Card): Effect.Effect<true, InvalidCardError> {
  /// ...
}

/*
 * `getCreditCard` and `getIsCardValid` define their own errors (on their return type).
 *
 * Since `makePayment` calls both `getCreditCard` and `getIsCardValid`
 * those errors are propagated also in the return type of `makePayment` ☝️
*/
function makePayment(params: { amount: number, card: Card }): Effect.Effect<
  ...,
  InvalidCardError | CheckingCardError | ConnectionError
> {
  /// ...
}

Testability

Tests cannot be an afterthought 💁🏼‍♂️

Writing testable software requires thinking about the structure of your code from the start

In practice:

• Pure functions: easy to (unit) test, same input always same output
• Dependency injection (manage effects): when side effects are necessary, inject mock implementations

it("should return a valid SubscribeResponse when request successful", async () => {
  const response = await (
    await Server.main.pipe(
      Effect.provideService(
        Request,
        // 👇 API request mocked locally
        new globalThis.Request("http://localhost:3000/", {
          body: JSON.stringify({ email: "" }),
          method: "POST",
        })
      ),

      // 👇 Configuration parameters mocked locally
      Effect.provide(layerConfigProviderMock),
      
      Effect.runPromise
    )
  );

  // 👇 Check response to local mock value
  expect(response).toStrictEqual(subscribeResponseMock);
});

Debugging/Observability

One thing is making code that compiles, another thing is verifying that it does what you expect (in every case).

Debugging is necessary: you cannot foresee everything that may happen.

When something doesn't work as expected, inspecting internal state at runtime becomes necessary 🚩

In practice:

• Logging: not just text messages, but also timestamps, stack traces, relevant state, and more
• Metrics: inspect performance, count function calls, track external requests
• Tracing: complete overview of the lifetime of requests

State management

Values change: we want these changes to be performant, predictable, and easy to understand/visualize.

In practice:

• Data structures: optimize performance based on the most common operations
• State machines: define all possible states and make state transitions predictable and easy to visualize

You can read more about these topics below:

• How to implement a data structure in Typescript
• State machines step by step

Regardless of programming language or area of expertise, these principles will always follow you everywhere 👻

These are underappreciated topics, I plan to cover each of them in more details (and in practice).

Stay tuned, this can become something big 🤫

See you next 👋