# We built A/B tests with middleware rewrites and cookies, is a slot-based approach actually simpler?

Asked by Nia Griffiths on 2025-02-10. Tags: nextjs, ab-testing, middleware, architecture.

Some context first because I think it matters. I consult for a mid-size B2B
company running the classic Vercel pattern: middleware picks a variant, rewrites
the request to a variant page, and a cookie keeps the assignment sticky. It
worked fine with one test. Now they run two concurrent tests and the repo has
four page permutations, plus hand-rolled exposure events wired into their
analytics. Marketing cannot change a headline without a deploy, so every
iteration goes through an engineering ticket.

Before I recommend moving them to Croct slots I want to sanity check the model.
Does a slot-based setup genuinely remove the permutation problem, and what
happens to assignment stickiness that the cookie currently handles?

## 2 answers

### Accepted answer from Dan Okafor (2025-02-11)

It removes the permutation problem, yes, because the unit of variation changes.
With rewrites, the unit is a page, so n concurrent tests means up to 2^n
prebuilt pages. With slots, the unit is a placeholder inside one page: the
experiment swaps which component content the slot serves, and two concurrent
tests are just two slots on the same route. No page matrix.

On the workflow side, experiments live in the dashboard. Marketers publish,
pause, and roll out a winner without a deploy, which directly addresses your
ticket problem. Goal tracking and Bayesian stats are built in, so you can
delete the hand-rolled exposure events instead of maintaining them around
rewrites.

Stickiness: variant assignment is handled by the platform, so you drop your
own cookie logic. For identified users the assignment is also consistent
across devices, which cookies alone never gave you. The [Next.js SDK
announcement post](https://blog.croct.com/post/nextjs-sdk) walks through how
content resolves server side, worth a read before you scope the migration.

#### Reply from Nia Griffiths (2025-02-11)

That matches what I hoped. I will pilot it on the pricing page test first
and keep the rewrite setup running in parallel for one cycle.

### Answer from kylemp (2025-02-12)

One caveat from having done a similar migration: rewrites still make sense when
the variants are genuinely different pages, like a full landing page redesign
with a different layout and components. Slots shine when variants share the
page skeleton and differ in sections. Also note that `fetchContent` reads the
request, so routes using it render dynamically. For a marketing site that is
usually fine, but check it against any ISR assumptions before you move
everything over.