Offline feature flag evaluation

Featurevisor evaluates flags and variables locally after you supply a datafile. An unavailable CDN does not stop evaluation of the datafile already held by an SDK instance.

Start with a known configuration#

Use a datafile produced by the Featurevisor CLI as a bundled startup configuration. In Node.js, a file packaged with your application can be loaded directly:

import { readFileSync } from 'node:fs'
import { createFeaturevisor } from '@featurevisor/sdk'
const bundledDatafile = JSON.parse(
  readFileSync(new URL('./datafile.json', import.meta.url), 'utf8'),
)
const f = createFeaturevisor({ datafile: bundledDatafile })
const enabled = f.isEnabled('checkout', { userId: 'user-123' })
const supportEmail = f.getVariable('supportEmail', { country: 'nl' })

The bundled datafile should contain those definitions. Missing flags evaluate as disabled, while missing variables can return undefined. Choose application defaults where appropriate.

Refresh when the network is available#

Fetch outside the evaluation path. A failed fetch should leave the current instance in place:

async function refresh() {
  try {
    const response = await fetch(DATAFILE_URL)
    if (!response.ok) throw new Error('Datafile request failed')
    const datafile = await response.json()
    f.setDatafile(datafile, true)
  } catch (error) {
    reportRefreshFailure(error)
  }
}

Define DATAFILE_URL and reportRefreshFailure in your application. Publish CLI generated datafiles, and connect SDK diagnostics to your monitoring for invalid datafile reports. Do not recreate the instance with empty configuration after a failed refresh.

The base SDK does not fetch, retry, or persist datafiles automatically. Schedule refreshes using your application's lifecycle, and avoid overlapping requests that could apply an older response after a newer one.

Plan for restarts and stale configuration#

An in memory datafile lasts only as long as the process or application instance. A cold start during an outage needs a bundled copy or application managed persistent cache.

Record the active revision with f.getRevision() and monitor refresh failures. A cached configuration can keep a retired flag enabled until a refresh succeeds. For actions that require current authorization, use server checks independently of feature flags.

See the Node.js SDK, browser SDK, and runnable examples for your platform. Pair this with a configuration rollback plan.