Caching mechanism with Etag

I have to design a website architecture and work out on the caching mechanism of data.Below is the approach i was thinking on

Browser -> Web server -> Api Server -> Data services

Now i was planning to use ETAG in response headers to check for data changes.For example, Browser will ask the Web server for any data changes, the web server will ask from api server for any data changes and Api server will ask the Data source i.e. the DB or CMS for any data changes.

Now, the problem here is what I think there are many api's in data sources that are consumed by the API server which collates the data from many api's. So for example. API server api for resource A might call 3 api's in data services, so now I need to check 3 api's of data services for any change in the data. If there is any change, i need to propagate the data till the web server.

The question now is, how can I have a optimized cache design or is this design good approach?