Calling external API by timer

We need to call external API by timer (every 5 seconds) and update DB. Project's architect proposes to make requests directly from our website by timer, I think, that it should be as background service, which makes requests by timer to external API independently with website. He does not want, that separately service calls DB, he wants, that only web calls DB.

I think, much more better way even make API on our Web, call external API by timer using background service and then call API on our Web, than call something by timer from Website.

I would like to have any links on documentation or on books, where described in details, why approach to do calls by timer from web is bad approach