@neil
Thank-you, (begins reading).
Horizon is/was not a single computer system: it is/was a complex integration of many. Windows server platforms thru the interations of those products since 1996, Unix servers through various interations of platforms over the same period, dozens of application software components some of which were bespoke developments, some of which were turnkey products.
Recently PO attempted to migrate it to Azure & failed, hence Fujitsu continues to deliver the service.
I’ll be interested to understand how the book addresses such a complex ‘computer’.