Describe polling with ETag cachingsh-expand-on-definition-of-done

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+# Polling with ETag caching
+
+Polling for changes (repeatedly asking server if there are any new changes)
+introduces high load on a GitLab instance, because it usually requires
+executing at least a few SQL queries. This makes scaling large GitLab
+instances (like GitLab.com) very difficult so we do not allow adding new
+features that require polling and hit the database.
+
+Instead you should use polling mechanism with ETag caching in Redis.
+
+## How to use it
+
+1. Add the path of the endpoint which you want to poll to
+   `Gitlab::EtagCaching::Middleware`.
+1. Implement cache invalidation for the path of your endpoint using
+   `Gitlab::EtagCaching::Store`. Whenever a resource changes you
+   have to invalidate the ETag for the path that depends on this
+   resource.
+1. Check that the mechanism works:
+   - requests should return status code 304
+   - there should be no SQL queries logged in `log/development.log`
+
+## How it works
+
+1. Whenever a resource changes we generate a random value and store it in
+   Redis.
+1. When a client makes a request we set the `ETag` response header to the value
+   from Redis.
+1. The client caches the response (client-side caching) and sends the ETag as
+   the `If-None-Modified` header with every subsequent request for the same
+   resource.
+1. If the `If-None-Modified` header matches the current value in Redis we know
+   that the resource did not change so we can send 304 response immediately,
+   without querying the database at all. The client's browser will use the
+   cached response.
+1. If the `If-None-Modified` header does not match the current value in Redis
+   we have to generate a new response, because the resource changed.
+
+For more information see:
+- [RFC 7232](https://tools.ietf.org/html/rfc7232)
+- [ETag proposal](https://gitlab.com/gitlab-org/gitlab-ce/issues/26926)