Hi Ryan,
yes the environnement is a bit unusual due to scalabilty priorities:
The architecture consist of a loadbalanced set of x slaves servers and one master (load balancer/ mysql director), each time you request the website you never know which slave server is handling the response,
mysql handle that fine: one master which read/write and replicate on mysql slaves, so if you want to write and read right after, you better rw on the master due to replication delays.
this mechanism is not existing for the filesystem, don't know if it's possible/usable in session usage.
many thanks for your inputs, I think that I will do with my customs libraries afterall,
and get back on processwire for the next project! because pw looks very promising