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Unison file sync intro

Primary LanguageJavaScript

Synchronizing data with unison

Unison File synchronizer written in Ocaml (and not Unison-Code something-something implemented in haskell that performs register allocation and instruction scheduling).

Works for OSX, Unix and Windows. It allows two replicas to be stored on different hosts, modified separately then brought up to date by propagating the changes.

History

Started in 1999, by Benjamin C Pierce based on his research paper. It is written in Ocaml and uses rsync algorithm to reconcile the changes.

Overview

Unison is a bi-directional synchronizer that can resolve conflicts when both replicas were modified.

In practice it means that the software uses client-server model that reconcile changes between server and any number of clients one at a time.

Server profile

Setup

Configure profile

Packages: unison -- client/server for synchronization unison-gtk -- gui utility

Running Unison

Create a configuration file ~/.unison/default.prf

  • Configure remote and local roots

Local root

root = /home/alex

Remote root

Uses ssh protocol

root = ssh://unison@unison.flamy.ca//home/unison/sync

The files are copied to /home/unison/sync folder over ssh synchronization information is stored in ~/.unison folder, but no configuration is kept on the server

Folders to synchronize

path = Documents path = vagrant path = Desktop path = Photos path = .config

Full configuration file

root = /home/alex
root = ssh://unison@unison.flamy.ca//home/unison/sync

path = Documents
path = vagrant
path = Desktop
path = Photos

Synchronization via command line

$ unison --auto

Command output

$ unison -auto
Contacting server...
Connected [//sync//home/unison/sync -> //workstation//home/alex]
Looking for changes
  Waiting for changes from server
Reconciling changes

local          sync
changed  ---->            .config/sublime-text-3/Packages/TypeScript/typescript/TS.log
changed  ---->            .config/sublime-text-3/Packages/User/Package Control.cache/01524fae79697630d0454ba3fabd9414
changed  ---->            .config/sublime-text-3/Packages/User/Package Control.cache/01524fae79697630d0454ba3fabd9414.info
changed  ---->            .config/sublime-text-3/Packages/User/Package Control.last-run

Proceed with propagating updates? []

Other synchronization options

  • batch -- batch mode, don't ask any questions
$ unison -batch

Synchronizing via socket

specify remote root as a socket protocol

socket://remotehost:portnum//absolute/path/of/root

Using multiple unison configurations

default shortcut

unison default -auto

where default is the default.prf file in ~/.unison directory

Other interesting options

  • backups/backup -- keep backup copies of all files
  • fastckeck false -- disable fastcheck
  • prefer -- choose this replica version for conflicting changes

fastcheck

Default behaviour -- fast checking on unix and slow checking on windows (Examining the files)

prefer

Automatic preference resolution

prefer <root> newer/older

Batch output options

$ unison -batch
Contacting server...
Connected [//sync//home/unison/sync -> //workstation//home/alex]
Looking for changes
  Waiting for changes from server
Reconciling changes
changed  ---->            .config/sublime-text-3/Packages/TypeScript/typescript/TS.log
local        : changed file       modified on 2018-04-07 at 12:16:44  size 82        rw-r--r--
sync         : unchanged file     modified on 2018-04-05 at 19:54:25  size 82        rw-r--r--
changed  ---->            .config/sublime-text-3/Packages/User/Package Control.cache/01524fae79697630d0454ba3fabd9414
local        : changed file       modified on 2018-04-07 at 12:16:49  size 3262924   rw-r--r--
sync         : unchanged file     modified on 2018-04-05 at 19:54:36  size 3262924   rw-r--r--
changed  ---->            .config/sublime-text-3/Packages/User/Package Control.cache/01524fae79697630d0454ba3fabd9414.info
local        : changed file       modified on 2018-04-07 at 12:16:49  size 91        rw-r--r--
sync         : unchanged file     modified on 2018-04-05 at 19:54:36  size 91        rw-r--r--
changed  ---->            .config/sublime-text-3/Packages/User/Package Control.last-run
local        : changed file       modified on 2018-04-07 at 12:16:48  size 10        rw-rw-rw-
sync         : unchanged file     modified on 2018-04-05 at 19:54:35  size 10        rw-rw-rw-
Propagating updates
UNISON 2.48.4 started propagating changes at 21:33:08.34 on 09 Apr 2018
[BGN] Updating file .config/sublime-text-3/Packages/TypeScript/typescript/TS.log from /home/alex to //sync//home/unison/sync
[BGN] Updating file .config/sublime-text-3/Packages/User/Package Control.cache/01524fae79697630d0454ba3fabd9414 from /home/alex to //sync//home/unison/sync
[BGN] Updating file .config/sublime-text-3/Packages/User/Package Control.cache/01524fae79697630d0454ba3fabd9414.info from /home/alex to //sync//home/unison/sync
[BGN] Updating file .config/sublime-text-3/Packages/User/Package Control.last-run from /home/alex to //sync//home/unison/sync
[END] Updating file .config/sublime-text-3/Packages/TypeScript/typescript/TS.log
[END] Updating file .config/sublime-text-3/Packages/User/Package Control.last-run
[END] Updating file .config/sublime-text-3/Packages/User/Package Control.cache/01524fae79697630d0454ba3fabd9414.info
[END] Updating file .config/sublime-text-3/Packages/User/Package Control.cache/01524fae79697630d0454ba3fabd9414
UNISON 2.48.4 finished propagating changes at 21:33:08.55 on 09 Apr 2018
Saving synchronizer state
Synchronization complete at 21:33:08  (4 items transferred, 0 skipped, 0 failed)

Reconciliation

To be done manually, mention the other projects Harmony (Synchronize hierarchical XML structure), Boomerang -- a programming language for well-behaved bidirectional transformations on text data.

Caveats

  • Files compared by inode number and modtime
  • No understanding of hard links

More interesting setup

IONODE sync (look at the python demo file in the project)

Issue

  • Anecdotally, synchronization for different versions of unison is iffy. (though it is possible to run multiple versions of unison side-by-side)
  • Uses a lot of ram for synchronizing lots of changes. I had to dedicate a VM with 2GB of RAM, otherwise the program would crash when things haven't been synchronized for a while.

References

Unison home page -- https://www.cis.upenn.edu/~bcpierce/unison/ unison debian page -- https://wiki.debian.org/Unison unison arch page -- https://wiki.archlinux.org/index.php/unison

unison wiki article that's been deleted for some reason -- https://web.archive.org/web/20151125022816/https://en.wikipedia.org/wiki/Unison_%28file_synchronizer%29