Hello,

Both approaches have advantages and disadvantages:

1. Have a separate server for the backend and the website

2. Have both the backend and the website on the same server

Going for two different servers allows to host your backend in a DMZ. Thus, if the website is compromised, an attacker will not have access to your private data. You may have two servers of Odoo, having different data, one for the eCommerce, one for the backend. This approach is similar to having an eCommerce synchronized with an ERP (like magento and Odoo) but the interface is much easier since you sync Odoo data with Odoo data. (you can even use the base_synchro module to help you sync data)

But this approach also has a huge disadvantage: you don't get a full integration and you only interface a few data. As with any eCommerce - ERP integration, it's complex and not super efficient. (you don't get a clean inventory in real time, you may have data conflicts, it's super complex to setup, features are limited to a few synchronized documents, etc.)

As a security point of view, the solution 2 is less efficient. But having everything fully integrated is a huge advantage for the setup, maintenance and efficiency of the users. If you go for the second solution, instead of securing two servers, you can concentrate your effort into securing one Odoo server.

Note that even if your ERP is in a private network, it still need to be open to external access for lots of different reasons: emails gateway, EDI with other software, Web-Services, customer and supplier portal, etc. Most of the current ERPs tend to be online and collaborative, and not anymore hosted in a private network and accessed by employees only. DMZ are not DMZ anymore and what matters is to secure your server, not to hide it.

So, it's a trade off and every one should choose the best solution according to their need and priority. You can also opt for a solution in-between, like putting the backend in a DMZ and setting a HTTP proxy to access to website pages only.

If you need strong integration between the backend and the frontend (e.g. an advanced customer portal), the second solution is probably better. If the security is your primary concern, I would suggest the solution 1, even if it's less efficient and much more complex to setup and maintain.

Not related, but you should have a look at our Security Policy (mostly for SaaS).