Walkthrough: per-version isolation

Prototype: Commands and output shown reflect the current prototype and may change.

Overview: Each installed version of a program has its own storage area. After an update, the new version cannot open files the old version created. Rolling back makes the old files available again — they were never copied into the new area.

HJFS overview explains why versions are isolated; this walkthrough shows the same rule from the command line.

1. Check the installed program version

$ ./start_TinyDemo -V
TinyDemo v1.0

2. Write and read a data file under v1

The w flag writes text to a file; r reads it back. Both calls run as TinyDemo v1, so they share v1’s storage area.

$ ./start_TinyDemo w FileA
$ ./start_TinyDemo r FileA
The apple was a shiny red color.

3. Build and install a new version

$ ./make_TinyDemo-2.sh
$ ./HJFS_update_program TinyDemo TinyDemo TinyDemo.hash

List installed versions. The identifiers are install-time labels; HJFS enforces the hash.

$ ./HJFS_version_manager TinyDemo -l
260208_123022P  TinyDemo v1.0  (hash …)
260208_124510P  TinyDemo v2.0  (hash …)  [active]

4. v2 cannot read v1’s FileA

v2 is now active. FileA still exists in v1’s storage area, but v2 has its own area, and FileA is not in it.

$ ./start_TinyDemo -V
TinyDemo v2.0
$ ./start_TinyDemo r FileA
FileA: not in this version's storage area

This failed read is the isolation at work. A malicious update is also a new hash, so it too starts with an empty area and cannot open the prior version’s files. To give v2 a copy of FileA, use the file transfer utility; v2 cannot open v1’s copy itself. See Secure file transfer.

5. Write FileA under v2

v2 can create its own FileA. That is a different file in a different area. v1’s copy is unchanged.

$ ./start_TinyDemo w FileA
$ ./start_TinyDemo r FileA
The pear was a dull green color.

6. Roll back to v1

Confirm the active version, then set it back to the first. Rollback copies and merges nothing; it makes v1’s area the one open() resolves.

$ ./HJFS_version_manager TinyDemo -c
260208_124510P  TinyDemo v2.0  [active]
$ ./HJFS_version_manager TinyDemo -s 260208_123022P
$ ./start_TinyDemo -V
TinyDemo v1.0
$ ./start_TinyDemo r FileA
The apple was a shiny red color.

v2’s pear file is still in v2’s area; it is only hidden while v1 is active. The file transfer utility can copy it across if you need it. See Version rollback.