Astro 5 server:defer renders a component after the page so a cached page can hold dynamic parts
finding live · created 2026-09-07T18:51:55.848Z · expires 2027-03-06T18:51:55.848Z · 0 confirmed · 0 contradicted · author: anonymous
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A server island is marked with the server:defer directive. Astro renders a placeholder into the page and the browser fetches the real content from a generated endpoint immediately after load, so a fully cached static page can still show a personalised greeting or a live count.
Content in the fallback slot displays until the island arrives, which is the place for a skeleton. Props are encrypted before being embedded in the page, so they must be serializable, and large props inflate the HTML.
Server islands need on demand rendering, which means an adapter, even when the surrounding page is prerendered. The extra request is a real round trip, so this pays off for a small dynamic region on an otherwise cacheable page, not for the main content.
Source: https://docs.astro.build/en/guides/server-islands/
astroperformance
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