> For the complete documentation index, see [llms.txt](https://docs.transparentedge.eu/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.transparentedge.eu/getting-started/faq/cabeceras-por-defecto/vary.md). # Vary The Vary header can be very useful, but sometimes dangerous. It is used to tell caches (either the browser cache, a proxy, or a CDN like Transparent Edge) to save different versions of the same object depending on a given header. Let’s look at some examples of how to use it well and what not do to. ### Bad use of the Vary header ``` HTTP/1.1 200 OK Vary: Cookies Cache-control: max-age=86400 Content-Type: text/html; charset=UTF-8 TSSecure: SecureLayer client-id: 4 TP-l2-Cache: MISS X-Device: desktop Date: Mon, 19 Oct 2015 15:24:05 GMT Age: 85783 Connection: keep-alive TP-Cache: HIT ``` In the response headers in this example, we can see the Vary: Cookies header. This tells the caches to save a different version of the same object—like an image or a homepage—for each different cookie received. In other words, this type of request would have a very low hit ratio, possibly below 10%, rendering the cache system ineffective. Another bad use example is the Vary: User-agent header, which would save a different version of the same object for every user-agent that visits the site. Be warned, there are quite a few user-agents out there! ### Good use of the Vary header ``` HTTP/1.1 200 OK Vary: Accept-Encoding Cache-control: max-age=86400 Content-Type: text/html; charset=UTF-8 TSSecure: SecureLayer client-id: 4 TP-l2-Cache: MISS X-Device: desktop Date: Mon, 19 Oct 2015 15:24:05 GMT Age: 85783 Connection: keep-alive TP-Cache: HIT ``` Here we are saving a different version of an object based on whether or not the object is compressed. This is done to maintain backward compatibility with old browsers and is the typical example of using this header well. Another example is the Vary: X-Device header. When we use Vary this way, we tell Transparent Edge to save a different version of the object based on the X-Device value, which can be desktop, mobile, or tablet. You can read more about detecting mobile devices [here](https://docs.transparentedge.eu/v/english/getting-started/faq/funcionalidades/deteccion-de-dispositivos). Learn more about this header[ here](https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html). --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://docs.transparentedge.eu/getting-started/faq/cabeceras-por-defecto/vary.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.