Beyond Ruby Scripts: Rewriting Checkout Logic with Shopify Functions & Rust

Introduction

For the last eight years, Shopify Scripts (written in Ruby) were the "secret weapon" of Shopify Plus. They allowed us to intercept the cart to discount products, hide payment methods, or reorder shipping rates based on complex rules.

But if you are a CTO managing a high-volume store today, you know the anxiety that comes with them. Scripts are fragile, run on a shared environment, and critically, they are being deprecated.

Shopify has moved the ecosystem to Shopify Functions.

This is not just a syntax change from Ruby to JavaScript or Rust. It is a fundamental architectural shift from "Hosted Server Scripts" to "Edge-Native WebAssembly."

At Redlio Designs, we see this migration not as a "chore," but as a strategic opportunity. Rewriting your logic in Functions allows you to execute complex business rules in under 5 milliseconds—speeds that Ruby could never touch.

1. Why Shopify is Killing Scripts

To understand the future, we must diagnose the failure of the past.

The Scaling Problem

When you have a Flash Sale with 10,000 users checking out per minute, your Ruby script has to run 10,000 times inside Shopify's central servers. If your script is inefficient, it creates latency. Shopify imposes strict CPU limits, meaning your complex logic might simply "time out."

The "Black Box" Problem

Scripts are often "pasted" into the Script Editor app. They lack version control, unit testing, and CI/CD pipelines. A junior developer can paste a bad line of code on Friday at 4 PM and crash your checkout.

The Redlio Verdict: Continuing to run your business logic on Ruby Scripts is like running a Formula 1 car on 87-octane gas. It works, until you push the pedal.

2. The New Architecture: WebAssembly (Wasm)

Shopify Functions are built on WebAssembly (Wasm). This is the same technology that allows complex software like Figma to run in a web browser.

How the Architecture Changes:

• Deployment: Instead of pasting code into a text box, we build an "App." The logic is compiled locally into a binary (.wasm file) and deployed to Shopify's edge network.
• Execution: When a user checks out, Shopify executes this binary. Because it is pre-compiled, it runs in nanoseconds.
• Configurability: We can build UI Dashboards for your marketing team using Shopify App Bridge. Marketers can update discount thresholds without touching code.

3. Why Rust? The Economics of Type Safety

While you can write Shopify Functions in JavaScript, at Redlio, we architect high-performance enterprise logic using Rust.

Why choose a systems-programming language for e-commerce? Financial Safety.

Checkout logic handles money. JavaScript is "loosely typed," meaning it is prone to silent math errors (e.g., treating "100" as text).

The Rust Advantage:

• Strict Typing: Rust forces the developer to define exactly what data is moving where. You cannot accidentally add a "User Name" to a "Price."
• Zero Garbage Collection: Rust manages memory manually, ensuring predictable performance (latency) for every single customer.
• Unit Testing: We can write rigorous test suites that simulate thousands of cart combinations before deployment.

4. Strategic Capabilities (Things Scripts Couldn't Do)

1. Complex Bundling (Cart Transform)

• Old Way: Hacky frontend JavaScript to group items visually.
• The Functions Way: A Cart Transform Function. If a user adds "Shampoo" and "Conditioner," the Function dynamically merges them into a single "Hair Kit" line item with a special price, while inventory decrements correctly for components.

2. Validation & Gating

• Old Way: Waiting for the shipping step to fail.
• The Functions Way: Cart Validation Functions. We can validate zip codes against a custom database instantly and block checkout with a localized error message ("Sorry, we cannot ship Lithium batteries to Alaska") before the user tries to pay.

3. Payment Method Hiding

• Old Way: Unstable JavaScript hacks in checkout.liquid.
• The Functions Way: A Payment Customization Function can securely hide "Cash on Delivery" for orders over $500 or force PayPal for high-risk customers.

5. Performance at Scale: The Black Friday Test

Latency kills conversion. Amazon found that every 100ms of latency cost them 1% in sales.

• Shopify Scripts: Soft 50ms execution budget, often spiked under load.
• Shopify Functions: Hard 5ms execution budget—and they stick to it.

Because Functions run on Shopify's global edge network, the logic executes geographically close to the user, reducing total checkout latency from ~200ms to <50ms.

6. The Redlio Migration Protocol

Migrating from Scripts to Functions is dangerous if done poorly. You are swapping the engine while the car is moving.

Our 4-Step Protocol:

1. Logic Audit: Map out every conditional statement in your current Ruby Scripts.
2. Rust Porting: Rewrite logic in modular Rust functions.
3. Shadow Testing: Run automated scripts creating 10,000 carts to ensure math matches the old logic to the penny.
4. The "Strangler" Rollout: Use Shopify to roll out the Function to 10% of traffic first to validate stability.

Conclusion: Engineering Discipline for Revenue

The era of the "Hacked Checkout" is over. Shopify is maturing into a true Enterprise platform.

At Redlio Designs, we are one of the few agencies with a dedicated Backend Engineering Team specializing in Rust. We help CTOs sleep at night knowing their checkout logic is typed, tested, and compiled for speed.

Is your checkout infrastructure leaking revenue?

Contact Redlio Designs today for a comprehensive Logic Audit. Let’s turn your legacy scripts into a competitive advantage.

Frequently Asked Questions

When are Shopify Scripts officially dying? 

Shopify has announced the deprecation of checkout.liquid and is aggressively pushing Functions. We recommend migrating before Q3 2026 to avoid holiday code freeze risks.

Can my marketing team still edit discounts? 

Yes. We build a Metafield-driven UI. Your marketing team fills out a simple form in the Admin ("Spend X, Get Y"), and the Function reads those Metafields. It separates code from configuration.

Is Rust mandatory? 

No, but highly recommended for enterprise. Rust file sizes are smaller (faster load), memory usage is lower, and type safety prevents "undefined" errors during checkout.