One of the great benefits of Fossil and other distributed version control systems is that cloning a repository makes a backup. If you are running a project with multiple developers who share their work using a central server and the server hardware catches fire, the clones of the repository on each developer workstation may serve as a suitable backup.
We say “may” because it turns out not everything in a Fossil repository is copied when cloning. You don’t even always get copies of all historical file artifacts. More than that, a Fossil repository typically contains other useful information that is not always shared as part of a clone, which might need to be backed up separately. To wit:
Fossil purposefully does not clone certain sensitive information unless
you’re logged in as a user with Setup capability. As an example, a local clone
may have a different
user table than the remote, because only a
Setup user is allowed to see the full version for privacy and security
Fossil allows the local configuration to differ in several areas from that of the remote. You get a copy of some of these configuration areas on initial clone — not all! — but after that, remote configuration changes mostly do not sync down automatically.
Changes to the remote’s skin don’t sync down, on purpose, since you may
want to have a different skin on the local clone than on the remote. You
can ask for updates with
fossil config pull skin, but that does
not happen automatically during the course of normal development.
The Admin → Notification settings do not get copied on clone or sync, and it is not possible to push such settings from one repository to another. We did this on purpose because you may have a network of peer repositories, and you only want one repository sending email alerts. If Fossil were to automatically replicate the email alert settings to a separate repository, subscribers would get multiple alerts for each event, which would be bad.
The only element of the email alert configuration that can be pulled
over the sync protocol on demand is the subscriber list, via
fossil config pull subscriber.
This is normally generated once during
fossil init and never changed,
so Fossil doesn’t pull this information without being forced, on
purpose. You could accidentally merge two separate Fossil repos by
pushing one repo’s project config up to another, for example.
A repo’s URL aliases, interwiki configuration, and ticket customizations also do not normally sync.
The very nature of Fossil’s private branch feature ensures that remote clones don’t get a copy of those branches. Normally this is exactly what you want, but in the case of making backups, you probably want to back up these branches as well. One of the two backup methods below provides this.
Fossil purposefully doesn’t sync shunned artifacts. If you want your local clone to be a precise match to the remote, it needs to track changes to the shun table as well.
Data in Fossil’s unversioned artifacts table doesn’t sync down by
default unless you specifically ask for it. Like local configuration
data, it doesn’t get pulled as part of a normal
fossil sync, but
unlike the config data, you don’t get unversioned files as part of the
initial clone unless you ask for it by passing the
Autosync Is Intransitive
If you’re using Fossil in a truly distributed mode, rather than the simple central-and-clones model that is more common, there may be no single source of truth in the network because Fossil’s autosync feature isn’t transitive.
That is, if you cloned from server A, and then you stand that up on a server B, then if I clone from your server as my repository C, your changes to B autosync up to A, but not down to me on C until I do something locally that triggers autosync. The inverse is also true: if I commit something on C, it will autosync up to B, but A won’t get a copy until someone on B does something to trigger a sync there.
An easy way to run into this problem is to set up failover servers
svr3.example.com, then set
svr3 up to sync
with the first. If all of the users normally clone from
commits don’t get to
svr3 until something on one of the
servers pushes or pulls the changes down to the next server in the sync
svr1 falls over and all of the users re-point their local
svr1 later reappears,
svr1 is likely to
remain a stale copy of the old version of the repository until someone
causes it to sync with
svr3 to catch up again. And then if
you originally designed the sync scheme to treat
svr1 as the primary
source of truth, those users still syncing with
svr2 won’t have their
commits pushed up to
svr1 unless you’ve set up bidirectional sync,
rather than have the two backup servers do
Solution 1: Explicit Pulls
The following script solves most of the above problems for the use case where you want a nearly-complete clone of the remote repository using nothing but the normal Fossil sync protocol. It only does so if you are logged into the remote as a user with Setup capability, however.
#!/bin/sh fossil sync --unversioned fossil configuration pull all fossil rebuild
The last step is needed to ensure that shunned artifacts on the remote
are removed from the local clone. The second step includes
fossil conf pull shun, but until those artifacts are actually rebuilt
out of existence, your backup will be “more than complete” in the sense
that it will continue to have information that the remote says should
not exist any more. That would be not so much a “backup” as an
“archive,” which might not be what you want.
Solution 2: SQL-Level Backup
The first method doesn’t get you a copy of the remote’s
private branches, on purpose. It may also miss other info on the
remote, such as SQL-level customizations that the sync protocol can’t
see. (Some ticket system customization schemes rely on this ability, for example.) You can
solve such problems if you have access to the remote server, which
allows you to get a SQL-level backup. This requires Fossil 2.12 or
newer, which added the
backup command to take care of
locking and transaction isolation, allowing the user to safely back up an in-use
If you have SSH access to the remote server, something like this will work:
#!/bin/bash bf=repo-$(date +%Y-%m-%d).fossil ssh example.com "cd museum ; fossil backup -R repo.fossil backups/$bf" && scp example.com:museum/backups/$bf ~/museum/backups
Beware that this method does not solve the intransitive sync problem, in and of itself: if you do a SQL-level backup of a stale repo DB, you have a stale backup! You should therefore run this on every node that may need to serve as a backup so that at least one of the backups is also up-to-date.
Encrypted Off-Site Backups
A useful refinement that you can apply to both methods above is encrypted off-site backups. You may wish to store backups of your repositories off-site on a service such as Dropbox, Google Drive, iCloud, or Microsoft OneDrive, where you don’t fully trust the service not to leak your information. This addition to the prior scripts will encrypt the resulting backup in such a way that the cloud copy is a useless blob of noise to anyone without the key:
iter=152830 pass="h8TixP6Mt6edJ3d6COaexiiFlvAM54auF2AjT7ZYYn" gd="$HOME/Google Drive/Fossil Backups/$bf.xz.enc" fossil sql -R ~/museum/backups/"$bf" .dump | xz -9 | openssl enc -e -aes-256-cbc -pbkdf2 -iter $iter -pass pass:"$pass" -out "$gd"
If you’re adding this to the first script above, remove the
-R repo-name” bit so you get a dump of the repository backing the
current working directory.
pass value to some other long random string, and change the
iter value to something in the hundreds of thousands range. A good source for
the first is here, and for the second, here.
You may find posts online written by people recommending millions of iterations for PBKDF2, but they’re generally talking about this in the context of memorizable passwords, where adding even one more character to the password is a significant burden. Given our script’s purely random maximum-length passphrase, there isn’t much more that increasing the key derivation iteration count can do for us.
Conversely, if you were to reduce the passphrase to 41 characters, that would drop the key strength by roughly 2⁶, being the entropy value per character for using most of printable ASCII in our passphrase. To make that lost strength up on the PBKDF2 end, you’d have to multiply your iterations by 2⁶ = 64 times. It’s easier to use a max-length passphrase in this situation than get crazy with key derivation iteration counts.
(This, by the way, is why the example passphrase above is 42 characters: with 6 bits of entropy per character, that gives you a key size of 252, as close as we can get to our chosen encryption algorithm’s 256-bit key size without going over. If it pleases you to give it 43 random characters for a passphrase in order to pick up those last four bits of security, you’re welcome to do so.)
Compressing the data before encrypting it removes redundancies that can
make decryption easier, and it results in a smaller backup than you get
with the previous script alone, at the expense of a lot of CPU time
during the backup. You may wish to switch to a less space-efficient
compression algorithm that takes less CPU power, such as
Changing up the compression algorithm also provides some
security-thru-obscurity, which is useless on its own, but it is a
useful adjunct to strong encryption.
This requires OpenSSL 1.1 or higher. If you’re on 1.0 or older, you
won’t have the
-iter options, and you may have to choose
a different cipher algorithm; both changes are likely to weaken the
encryption significantly, so you should install a newer version rather
than work around the lack of these features.
Beware that macOS ships a fork of OpenSSL called LibreSSL that
lacked this capability until Ventura (13.0). If you’re on Monterey (12)
or older, we recommend use of the Homebrew OpenSSL package rather
than give up on the security afforded by use of configurable-iteration
PBKDF2. To avoid a conflict with the platform’s
Homebrew’s installation is unlinked by default, so you have to
give an explicit path to it, one of:
/usr/local/opt/openssl/bin/openssl ... # Intel x86 Macs /opt/homebrew/opt/openssl/bin/openssl ... # ARM Macs (“Apple silicon”)
Restoring From An Encrypted Backup
The “restore” script for the above fragment is basically an inverse of it, but it’s worth showing it because there are some subtleties to take care of. If all variables defined in earlier scripts are available, then restoration is:
openssl enc -d -aes-256-cbc -pbkdf2 -iter $iter -pass pass:"$pass" -in "$gd" | xz -d | fossil sql --no-repository ~/museum/restored-repo.fossil
We changed the
-d on the
openssl command to get decryption,
and we changed the
-in so it reads from the encrypted backup
file and writes the result to stdout.
The decompression step is trivial.
The last change is tricky: we used
fossil sql above to ensure that
we’re using the same version of SQLite to write the encrypted backup DB
as was used to maintain the repository. We must also do that on
Fossil serves as a dogfooding project for SQLite,
often making use of the latest features, so it is quite likely that a given
sqlite3 binary in your
PATH will be unable to understand the
file created by “
fossil sql .dump”! The tricky bit is, you can’t just
pipe the decrypted SQL dump into
fossil sql, because on startup, Fossil
normally goes looking for tables created by
fossil init, and it won’t
find them in a newly-created repo DB. We get around this by passing
--no-repository flag, which suppresses this behavior. Doing it
this way saves you from needing to go and build a matching version of
sqlite3 just to restore the backup.