Flexible Custom AppEngine: are volumes shared between services?

[gu...@genotek.ru]

I am a bit lost as to what the volumes usage scenario could be. For example are the volumes shared/accessible between the instances of a appengine service?

I would like to use volumes for file caching -- are they feasible?

Thanks.

[George (Cloud Platform Support)]

App Engine Flex 'volumes' are in fact implemented as RAM drives. They are simply RAM sticks that are made to act like a hard drive. Some apps need what appears as a local file system to work properly, and these volumes fulfill this need (since writing to the hard drive is not allowed in App Engine). Each volume is local to one GAE instance, and can only be shared between workloads on a single instance (like a normal hard drive), and not between instances. 

It is worthwhile mentioning here that due to the specifics of the RAM implementation of volumes, the files stored this way do not enjoy permanence. RAM disks are located in your system memory, and are not automatically replicated. All data on these devices is erased in the event of a host error or a system restart. Frequently back up your data on replicated storage. More detail on RAM discs can be gathered from the "Creating in-memory RAM disks" document.