Kotlin SDK - Logging - Westyx Nexus

The SDK logs through SLF4J, the JVM ecosystem’s standard logging abstraction. There is no bespoke logger interface to implement.

Enabling it

Nothing to enable. NexusConfig.logger defaults to the SLF4J logger named dev.westyx.nexus, and SLF4J is a no-op unless the application binds a backend - so an application that has not asked for logging stays silent. To route SDK output somewhere specific, pass your own logger:

import org.slf4j.LoggerFactory

NexusClient.create(NexusConfig(
    baseUrl = "https://my-service.westyx.dev",
    apiKey  = System.getenv("NEXUS_API_KEY"),
    logger  = LoggerFactory.getLogger("my-app.nexus"),
))

With the Ktor plugin the default is the Ktor application’s own logger, so SDK output lands in the application log with no configuration at all.

What is logged

Level	Events
`info`	client ready (endpoint, key type, TTL, whether WIF is active); SSE stream connected; SSE stream closed by the server
`warn`	sync failed and the cache is being served; billing blocked the refresh; quarantine in force with its deadline; SSE transport error; fallback to TTL polling with the next attempt time; a config value that does not fit the requested type; an absorbed observer-callback failure
`debug`	sync applied, with entry counts; sync not modified (304); WIF provider resolution and session refresh, with the session lifetime; reconnect attempt and its backoff; `connectStream()` ignored because a stream is already running; client closed

Values are passed as structured parameters, never interpolated into the message:

log.warn("nexus: SSE fell back to TTL polling; next attempt in {}ms", wait)

A message that embeds its values produces a unique string per occurrence, which cannot be grouped or queried in a log aggregator - and it also defeats SLF4J’s level check, since the string is built whether or not the level is enabled.

Secret hygiene

No log record carries a secret value, a secret key name, an API key, a session JWT or a workload OIDC token. Where a key must be identified - a file secret that could not be materialised, for instance - only a truncated hash of the key appears. This is enforced, not just intended. LoggingSecretLeakTest drives a sync carrying both a text and a file-type secret, a secret read, a file-path read and a WIF token exchange through a capturing logger with every level enabled, then fails if any seeded value, key name, API key or token appears anywhere in the output - message, parameters or stack trace. NexusConfig.toString() also redacts apiKey, since a config object is often printed while debugging.

Observer failures

A NexusObserver callback that throws is absorbed by the SDK’s guarded dispatcher and reported here at warn, naming the callback:

nexus: observer callback onConnected threw and was absorbed

Absorbing without reporting would be the worse failure - the stream would carry on while the application’s own instrumentation silently stopped working. See Stream observer.

Overview

Backend SDKs

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OpenFeature Providers

Kotlin SDK - Logging

Enabling it

What is logged

Secret hygiene

Observer failures

​Enabling it

​What is logged

​Secret hygiene

​Observer failures

Enabling it

What is logged

Secret hygiene

Observer failures