Edge side includes(ESI) is a markup language specifically designed to help web servers assemble dynamic content at the web layer.
The above ESI tag is similar to tags in jsp/php/etc which allow one page to refer to another page for parts of the content on the page. By breaking up the page into smaller objects the webserver could apply different TTL settings (and user validation) to different parts of content. Twitter used to (and may still ) use â€œVarnishâ€ which supports a subset of ESI specification out of the box.
But caching on the webserver may not be the real reason why this language was invented. ESI is also supported by AkamaiÂ (CDN) on its edge caching product.Â By allowing Akamai edge nodes to do the assembling close to the user, they significantly improve perceived end user performance without giving up personalization or content freshness requirements.