How We Handled a 10x Traffic Spike During Black Friday

Black Friday is the test that separates robust Shopify stores from fragile ones. For a mid-market lifestyle brand planning their biggest promotional campaign to date — a 30% off sitewide event supported by a £120,000 paid media budget — the stakes were high. A site crash or checkout failure during peak hours could cost more than the entire media spend. We were brought in six weeks before the event to prepare.

The Pre-Event Audit

Six weeks out, we ran a full performance and infrastructure audit. Three critical risks were identified: a third-party review app that loaded synchronously and would fail under high API load, a custom checkout script that hadn't been tested under concurrent sessions, and a Klaviyo integration that could trigger a cascade of emails simultaneously at event launch, creating a feedback loop on the server.

Preparation Work

1. 1Replaced the synchronous review app load with an asynchronous deferred load after page interaction.
2. 2Load tested the custom checkout script using k6 with simulated concurrent sessions — found and fixed a race condition.
3. 3Configured Klaviyo campaign sends to throttle at 10,000 emails per hour rather than instant blast.
4. 4Implemented a virtual queue app pre-configured for the event window.
5. 5Pre-loaded sale pricing and collection changes in Shopify Launchpad for atomic go-live.
6. 6Set up a real-time monitoring dashboard with alerts for checkout error rate and page load time.
7. 7Briefed the client's fulfilment partner on anticipated order volume.

The Load Test

We ran a simulated Black Friday load test three weeks before the event: 2,000 concurrent virtual users browsing, adding to cart, and completing checkout. The test identified two issues in the checkout script and one in the cart drawer that only manifested under concurrent load. Both were fixed before the event. Shopify Plus itself handled the load without issue — it's the custom code layer that almost always fails first.

“We had done Black Friday before and it had gone badly. Having someone prepare the site properly made all the difference. It was the smoothest event we've ever had.”

The Event Results

• Peak traffic of 10,800 concurrent sessions — 10x their normal daily peak.
• Zero checkout failures throughout the entire 24-hour event.
• Page load time maintained at 2.1 seconds average even at peak load.
• £1.2m in revenue over the Black Friday weekend — the brand's best trading period ever.
• Virtual queue handled 3,200 users during the first 15 minutes without any customer-facing errors.