Distributed VCS

Monotone is a Distributed VCS.

There is no Master...

Unlike other version control systems people may be accustomed to, especially centralised systems such as CVS and SVN, in monotone there is no "Master Repository" in any special privileged sense.

... even though it might look like there is

It is certainly true that for many projects (including monotone itself) there's often a central server where a repository instance lives. Developers sync with this server as an easy central point of reference, and to avoid long relay replication chains.

In the Tutorial, we saw Jim set up such a server for his work with Abe and Beth, but this is primarily a convenience. Each person's replica of the repository is as good as any other's, so long as they contain the same information.

Why do you think you need one?

Some people new to monotone have difficulty with this, and see this as a problem. To decide whether it is a problem, the question of why you think you want a master copy is important.

There are several things you might think you need a master copy to achieve, perhaps based on your established habits or expectations from other systems. For at least some of these things, you actually don't -- monotone deals with the requirement differently. In other cases, maybe you really do need a centralised rather than a distributed VCS after all.

In the following discussion, remember that there's no reason some repository instances can't be special if you want them to be, it's just that none of them really needs to be.

for backups?

You might want a place to ensure you have a safe backup.

Let's look at the central convenience server mentioned above, which seems like an obvious thing to back up, if we were to back up something.

To illustrate that it's not in any sense a master copy, this repository usually doesn't need to be backed up at all. Since nothing is committed locally on the server, and since people replicate with it frequently, it doesn't contain any revisions that ''aren't also elsewhere'' in individual developers' databases. Because all the revisions are signed, the same trust evaluations can be applied to these revisions, even if they've been sent to us by someone other than the original author (note that this is the normal case when getting revisions from such a server, because it only contains revisions that were first put there by someone else). See the TrustFoundations page for more details on cert-based trusts.

Restoring the server is simply a matter of putting up an empty server and having people feed it back from their own replicas again. Most people have a copy of most revisions, and for the very newest revisions that haven't propagated everywhere yet, everyone who cares about these revisions has a copy, at least. We saw Jim take advantage of exactly this property when setting up his server, in the tutorial.

Of course, that shouldn't prevent you taking backups of the server's, or your own, database as protection against the various gremlins that lurk in dark places -- it just means that you can generally back up any of these databases pretty much interchangably for the same effect.

for authoritative reference?

You might want a place that is considered an authoritative reference about content or history.

This might be in in case of errors or corruption, which can also be seen as a different sort of backup - and is also not needed. Since everything is signed and checked thoroughly at every opportunity, errors are found early and eliminated. Should data be corrupted (say, due to a disk error), this will be detected. The lost information will have to be restored, but it can be refreshed from any valid copy not just a master one.

There might be other kinds of authoritative reference that could be important. Time is a good example of one of those things that are just necessarily different in a distributed system. Monotone stores revision timestamps in certs. It's possible to create a date cert with an arbitrary value (and, for example, cvs_import will do so, generating certs with cvs's times). Or people's clocks can always just be wrong. So you need to decide what trust to place in them. If 'authoritative' dates are important to you for some reason, you need to establish the authority: have a trusted server add its own date certs when it receives new revisions over netsync, signed with a purpose-specific key. This server is a timestamping authority, but need not be special in any other way (and other people can choose to trust a different timestamping authority, if they really feel the need). See the TrustFoundations and UsingCerts pages for more information.

for centralised controls?

You might want a place to enforce centralised controls of various kinds.

Many of these are just not really suited or even possible to centralise in any distributed system like monotone, and need to be addressed in other, distributed ways.

Monotone has sophisticated trust evaluation mechanisms, using the information published in certs, that let each person make their own decisions about who should be trusted to commit where, and how information will be used.

Although very powerful, sometimes this is cumbersome to manage, and in a project with a common cause, people just want to agree and all follow the same set of rules. The next major emphasis of monotone feature development is centered around this TrustDiscussion, to allow users to delegate the specification of project policy to trusted authorities. Note that this will allow centralised publication of those rules, for distributed enforcement.

Monotone also has network access controls over who can copy information out of, or into, a server - and again, no server is particular or special. Everyone's database can be a server when needed. If you have confidentiality concerns over the content, you need to trust the people and protect the places that information is copied. Once someone has a copy of some information, there's nothing in particular you can do centrally to control where else the information can be copied, either with monotone or with other tools.

Some of these things you either need to let go of and accept that the distributed system and your project is better without them, or you may need to look at a centralised VCS after all.

for something else?

You might want it for some other reason; if so, please add it to this page!

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