---
title: Server-side vs Client-side
---

# Server-side vs Client-side



We strongly believe feature flags should be used server-side.

## Avoid layout shift and jank

When a feature flag is used on the client side, it can cause layout
shifts and jank. The application needs to wait until the feature flags
are bootstrapped over the network. In the meantime it has to take one of
two bad choices:

* Show a loading spinner
* Speculatively show one version of the page

But if the feature flag turns out to have a different value, the page
needs to be swapped out leading to jank.

When you use feature flags server-side, this problem is avoided. The
server will only send the version of the page that matches the feature
flag. No layout shift or jank.

## Keeping pages static

A big benefit using client-side feature flags is that the page itself
can stay fully static. Having static pages is great, as they can be
served by the CDN around the world which has incredibly low latency
globally.

A common misconception is that server-side usage of feature flags means
that the page can no longer be static. This is not the case. The Flags
SDK comes with multiple patterns which allow keeping the page static
without falling back to client-side usage.

These patterns are made possible by using Routing Middleware. One or
multiple feature flags can be evaluated in Routing Middleware, and the
request can then be rewritten to serve a statically generated version of
the page. This combines extremely well with Incremental Static
Regeneration (ISR).

<LearnMore href="/frameworks/next/guides/marketing-pages" icon="arrow">
  See the Marketing Pages example
</LearnMore>

## Confidentiality

Using feature flags on the client typically means the name of the
feature flag is sent to the client. Often times teams then fall back to
using cryptic alias for their feature flags in order to avoid leaking
features.

## Code size

When feature flags are used server-side, only the necessary code is sent
to the client. In contrast, when using feature flags client-side it is
common that both versions of the page are sent to the client, which
leads to an increased bundle size.