Vue SDK - Stream observer

The observer is the structured view of the live stream’s lifecycle - the source to point metrics and alerting at. The logger carries human-readable text; the observer carries events you can count. Set either, both or neither.

await provideNexus(app, {
  baseUrl: '...',
  apiKey: 'wxp_...',
  streamObserver: {
    onConnected: () => metrics.increment('nexus.stream.connected'),
    onDisconnected: (cause) => metrics.increment('nexus.stream.disconnected'),
    onEvent: (name) => metrics.increment('nexus.stream.event', { name }),
    onReconnectAttempt: (attempt) => metrics.gauge('nexus.stream.attempt', attempt),
    onFallback: (reason) => metrics.increment('nexus.stream.fallback', { reason }),
    onQuarantined: (reason, until) => alerts.raise('nexus quarantined', { reason, until }),
  },
});

The callbacks

Callback	Fires when
`onConnected()`	The stream connects and the server’s response is readable
`onDisconnected(cause)`	The connection ends. `cause` is the error, or `null` for a clean disconnect
`onEvent(name)`	A recognised event arrives, before the refresh it triggers
`onReconnectAttempt(attempt)`	Before a reconnect is scheduled. `attempt` counts consecutive failures, from 1
`onFallback(reason)`	The SDK stops relying on the live stream, or the sync path degrades
`onQuarantined(reason, expiresAt)`	The server returns a quarantine

Every method is optional - { onEvent: (n) => count(n) } is a complete observer.

Fallback reasons

Constant	Value	Meaning
`FALLBACK_REASON_MAX_ERRORS`	`max-errors`	Three consecutive stream failures
`FALLBACK_REASON_RATE_LIMITED`	`rate-limited`	A `429` on the stream
`FALLBACK_REASON_TRANSPORT`	`transport`	A transport failure, or a retried sync

Treat the value as open-ended; future versions may add reasons.

Recognised events

resync, config.created, config.updated, config.deleted, flag.created, flag.updated, flag.toggled, flag.deleted, secret.created, secret.updated, secret.deleted. Anything else is ignored and not surfaced.

The contract

Calls are synchronous and must return promptly - the SDK does not defer them. Copy what you need and dispatch the work yourself. An exception raised by an observer method is absorbed and reported: it cannot break the stream lifecycle, and it is written to the logger rather than swallowed. A test in the SDK walks the source and fails the build if any observer call bypasses that guard, so the guarantee is enforced rather than intended.

What it is not

The observer reports the transport. It is not a change feed: use the framework’s reactive reads for values. And a stream that has fallen back to polling is not an outage - reads keep working, only the propagation latency changes.

Overview

Backend SDKs

Browser SDKs

OpenFeature Providers

Vue SDK - Stream observer

The callbacks

Fallback reasons

Recognised events

The contract

What it is not

​The callbacks

​Fallback reasons

​Recognised events

​The contract

​What it is not

The callbacks

Fallback reasons

Recognised events

The contract

What it is not