Reacting to changes

Reads are synchronous against an in-memory rule set that a background poll keeps fresh. onChange lets you react the moment that rule set swaps — for cache busting, re-rendering, or logging a config flip.

Change listeners live on the Engine — the heavyweight client. Everyday flag reads use configure + new Client(user); here you hold the Engine itself so you can subscribe to its poll cycle.

Server

The server Engine fires registered listeners after a background poll returns new data (HTTP 200, not a 304). It returns an unsubscribe function, and never fires in test or offline mode (no polling happens there).

import { Engine } from "@shipeasy/sdk/server";

const engine = new Engine({ apiKey: process.env.SHIPEASY_SERVER_KEY! });
await engine.init();

const unsubscribe = engine.onChange(() => {
  console.log("flag rules changed — re-evaluating");
  rebuildRenderCache();
});

// on shutdown
unsubscribe();

Browser

The browser Engine exposes the equivalent subscribe(), which fires after each identify() and after any override change — exactly when a re-render is warranted.

import { Engine } from "@shipeasy/sdk/client";

const unsubscribe = engine.subscribe(() => {
  renderFlags();
});

When to use it

• Cache invalidation — drop a memoized render or a derived config when rules change.
• Long-lived servers — re-evaluate a flag that gates a background job without restarting.
• Dashboards — reflect a flag flip live without a page reload.

For one-shot serverless reads there is nothing to subscribe to — see init vs initOnce.