A few historical additions for anybody interest:

- CASTOR at CERN had also its disk centric derivative named DPM (Disk pool manager) that helped to power the LHC computing grid for multiple decades (WLCG) before getting deprecated.

- Interestingly DPM had an architecture quite aligned with the original Google File system even if developed completely separately: (One metadata node, multiple disk node. Design to do Write-once-read-many with very partial POSIX semantics).

- The LHC computing Grid is an association of research centers with their own infrastructure. As such, they had (historically) many diffent storage systems with diffent protocols and interface.

- To unify this madness, an attempt to do a "standard" protocol was made in the 2000s: the SRM protocol (storage resources manager). In a pure XKCD fashion, it went as bad as you can imagine. It tried to rely on the tech of the time (XML, SOAP, WSDL) and is a school case of terrible protocol design (bloated, slow, weak consistency, massive server overhead, stupidly complex to implement and quite insecure). The spec are worth a read if you want a good laugh [1].

- After 20y of struggle, SRM was eventually dropped for a more pragmatic and ad hoc solution based on HTTP + xrootd [2]. EOS itself uses xrootd quite extensively. (if this did not change)

- The history of computing at CERN is globally interesting because it is a pretty good image of the evolution of computing and of the "tech fashions" associated with it.

[1]: https://sdm.lbl.gov/srm-wg/doc/SRM.spec.v2.1.1.html

[2]: https://xrootd.org/