How Office Web App server request flow work?
1. Assume that user logged on Lync 2013 or Skype for Business 2015 clients and browse the Meeting URL and joined Lync / Skype for Business meeting and upload the PowerPoint presentation to Meeting.
2. Lync / Skype client uploads the PPT file to Lync front end server.
3. Lync 2013 Front end Server Encrypts and saves the PowerPoint presentation file on to the Lync file share (content store) and generates File ID and Access Token to access presentation file.
4. Lync server then retrieves WAC Server name URL (stored in CMS during WAC (OWA) Discovery process) and forms an URL that includes, WAC Presentation URL, File ID and Access Token of the PPTX File.
5. Lync Access Server sends this WAC URL formed to Presenter as well as attendees.
6. Lync Presenter/Attendee’s Client sends requests to this URL and contacts office web app server to get the PowerPoint presentation.
7. Once Office Web App Server receives this request it retrieves the Access Token, File ID and File location (file share) and access the PPTX file by contacting the Lync Content File store via Lync Front end Servers (Lync Web Services FQDN will also be present in the initial Request).
8. Office Web App server makes a temporary copy of PPTX file and creates html (dochtml) file and sends it to the Presenter/Attendee’s Client for presentation rendering.
How rendering work?
1. Lync client send WOPI App request for file (PPTX file) to Office Web Apps server (WOPI Application system).
2. Office Web Apps server check Cache for file. If file is available in cache then Office Web Apps server respond back to client.
3. If file is not in Cache then Office Web Apps server request file from Lync Server.
4. Lync Server verify that request comes from Bound WOPI server and request a file from file Store (Lync file store where file stores).
5. File store returned file to Lync Server.
6. Lync Server returned file to WOPI App server (Office Web App server).
7. Office Web App server create Cache document locally in dochtml format.
8. Then return of rendered file to Lync client in Meeting.
diagram from SharePoint WOPI to Lync. So please excuse for any error.