Is it possible to deploy Content Server (UCM earlier Stellent) and Web Server (Apache, OHS or IBM HTTP Server)  on different machines ?

Answer is Yes and No. No, If you are planning to use Site Studio with Content Server. If you are NOT using Site Studio with Content Server then Yes, it is possible to deploy Content Server on different machine than Web Server (Apache, Oracle or IBM HTTP Server).

This type of deployment “Installing Web Server and Content Server on different Machines” is called as Disassociated Installations and documented in Section F Disassociated Installation (Page 133) of Content Server Installation guide here

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Web Server and Content Server Communication
Content Server provides plug-in (RedirectUrls.so) which creates dedicated socket connection WebServer and Content Server .

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What files (content server related) are required on WebServer ?
You need to copy [content_server]/data , idcplg and weblayout directory on Web Server machine. You would also need two library files (RedirectUrls.so, IdcAuth.so) from Content Server copied to Web Server.

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Key things you should know for Disassociated Installations (Web Server and Content Server on different Machines)

1. Site Studio (more on Site Studio later) is not supported in this type of deployment
2. HTTP Caching for static requests is not supported in disassociated installations.
3. PDF byte streaming is not supported in disassociated installations.
4. Changes to content server are not automatically propagated to the Web Server.