Client Feature Flags
Flag evaluation on a device you do not control, and the flicker and staleness it brings.
4 to work through
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beginner
You turn off a feature flag evaluated in the browser. Some users keep seeing the old behaviour for hours. Why, and what should the design do about it?
2 min answer -
advanced
A streaming platform delivers feature flags to clients. What happens when the flag service is slow or unreachable?
1 min answer -
advanced
A web application evaluates feature flags in the browser. A flag is turned on for 10% of users and the new code throws during application start-up, so those users see a blank page. Turning the flag off does not help them. Why not, and what is the structural fix?
4 min answer -
advanced
How do client-side feature flags differ from server-side ones, and what breaks if you ignore the difference?
2 min answer
4 terms in this topic
Bootstrap Kill Switch
A configuration value read from local storage before any application code initialises, paired with a launch-failure counter, so a client that crashes…
practiceClient Feature Flags
Flags evaluated in a browser or mobile app — where the constraints of distribution, caching and offline behaviour make them meaningfully harder than …
conceptFlag Evaluation Latency
The delay between page load and the client knowing which variant to show, which produces a visible flicker unless the flag is resolved before render.
metricFlag Propagation Time
The worst-case delay between changing a feature flag and every client acting on the new value - the number that determines whether a flag can be used…
Neighbouring topics
Frontend & Experience Architecture
General material on architecting the surface the user actually touches.
Rendering Strategies
Client, server, static and incremental rendering, and what each costs on first paint.
Edge Rendering
Running the render close to the user, and the personalisation and cache trade it implies.
Hydration Cost
The gap between visible and interactive, and the JavaScript that closes it.
Micro-Frontends
Independent deployment of UI slices, and the shared runtime that undermines it.
Module Federation
Loading code from another build at runtime, with versioning and failure to think about.
UI Monorepo Strategy
One repository for many front ends, and the build graph that makes it viable.
Design Systems
Components as a versioned internal product, with adoption and deprecation like any API.
Component Contracts
Props, slots and events as an interface, and the breaking change hidden in a style.
Client State Architecture
Server state, UI state and derived state, and why conflating them causes most bugs.
Client Caching & Data Layer
Stale-while-revalidate, invalidation and optimistic updates on the client.
BFF for Experience
One backend per experience, shaped by the screen rather than by the domain.
API Shapes for UI
REST, GraphQL and RPC judged by over-fetching, round trips and client coupling.
Web Performance Budgets
A number a build can fail against, rather than a performance sprint once a year.
Core Web Vitals
LCP, INP and CLS — what they measure, and the architecture that moves them.
Accessibility Architecture
Semantics, focus management and announcements designed in rather than audited in.
Internationalisation
Locale, direction, pluralisation and dates as structural concerns, not string tables.
Real User Monitoring
Field data from real devices and networks, against the synthetic run that looked fine.
Frontend Security
CSP, XSS, CSRF, token storage, and the trust boundary that ends at the browser.