> For the complete documentation index, see [llms.txt](https://docs.tumbler.app/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.tumbler.app/after-first-launch/caching-and-304.md).

# Caching and 304

## Who this is for

Teams that tune subscription performance and want to reduce origin load.

## What you can do

* Enable conditional requests and save traffic.
* Configure correct `Cache-Control`, `ETag`, and `Last-Modified` headers.
* Define the `version_id` update policy.

## Examples

* `ETag: "v2025-01-15T12:00:00Z"`
* `Cache-Control: public, max-age=300`
* `304 Not Modified` response when `If-None-Match` matches.

## See also

* [subscription/response-formats.md](/subscription-contract/response-formats.md)
* [subscription/examples.md](/subscription-contract/examples.md)
* [operations/reliability-and-timeouts.md](/after-first-launch/reliability-and-timeouts.md)

## Header recommendations

* Set `Cache-Control` with public max-age if the response is stable.
* Generate a strong `ETag` from the body or `version_id`.
* Support `If-None-Match` and return `304` without a body.
* Alternative: `Last-Modified` + `If-Modified-Since` if `ETag` is not used.

## ETag and Last-Modified

* ETag must be deterministic: compute it from the canonical response body/`version_id` (normalized JSON with fixed field and node order, no random spaces), not from request time. Same content means same `ETag`.
* Node order matters: clients may choose the first suitable node or cache nodes in received order, so reordering elements is a new version. Change `version_id`/`ETag` to clear caches with stale order even if field values are identical.
* For mirrors and CDNs, return the same `ETag` for the same content to avoid extra downloads.
* `Last-Modified` is optional. Set the actual data modification time (not request time) and update it together with `ETag`.

## 304 response

* When `If-None-Match` is present, compare it with the current `ETag`. If it matches, return `304 Not Modified` without a body.
* If you use `Last-Modified`, check `If-Modified-Since` and also return `304` when content has not changed.
* In a `304` response, repeat current `Cache-Control` and `ETag` (if used) so the cache policy remains known.
* If the original `200` had `Last-Modified`, include it in `304`; if you do not use this header, omit it.

## Update policy

* Change `version_id` and `ETag` for any meaningful content change.
* Do not change `ETag` for decorative edits; only payload changes should affect it.
* When adding/removing/reordering nodes, create a new version: old clients get 200 with the new body, and clients that already cached data get 304 only after `version_id` is updated.


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