文件夹/文件系统中的权限的有效继承

Question

I am writing a virtual file system in Java for a homework. I design the permission rules very simple which only consist of three permissions: Read, Change, Delete. Since the file system tree is expanded so that user can select any files/folders in any level. When user change the permission of a folder, all files/folders in this folder should also inherit the changes and update their own permissions. But updating all nodes under can be inefficient when tree becomes really deep or for example: father node's permission is changed after son node's permission has been changed, then seems like there may incur some redundant works. How can I make it more efficient than just updating all branches of that changed node?

Answer 1

There is a trade-off between the cost of doing the permissions update, and the the cost of calculating the effective permissions of an object.

For instance, you could make permission update faster by storing the permissions and a permission timestamp. Then you calculate the effective permissions of a node by looking at the permission / timestamp pair of the node and all of its ancestors - the one with the newest timestamp takes precedence. Of course, this means that calculating a node's effective permissions is slower.

(You could then make the effective permission calculation faster with an in-memory LRU cache of the permission / timestamp pairs for directories.)