Distributed revision control (or Distributed Version Control (Systems) (DVCS), or Decentralized Version Control) is a fairly recent innovation in software revision control. It provides some significant advantages over the more traditional centralized approach to revision control, and it has some defining characteristics that separate it from centralized systems. However, the line between distributed and centralized systems is graying in some regards, especially since DVCSs can be used in a "centralized mode".
Main Differences From Centralized
- Each developer does work on her own local repository.
- Working model epitomizes bazaar-style development in that anyone can create their own branch.
- Repositories are cloned by anyone, and are often cloned many times.
- There may be many "central" repositories.
- Access control lists are not employed. Instead code from disparate repositories are merged based on a web of trust, i.e., historical merit or quality of changes.
- Lieutenants are project members who have the power to dynamically decide which branches to merge.
- Network is not involved in most opererations.
- A separate set of "sync" operations are available for commiting or receiving changes with remote repositories.
Advantages vs Centralized
- Allows users to work productively even when not connected to a network
- Makes most operations much faster since no network is involved
- Allows participation in projects without requiring permissions from project authorities, and thus arguably better fosters culture of meritocracy instead of requiring "committer" status.
- Allows private work, so you can use your revision control system even for early drafts you don't want to publish
- Avoids relying on a single physical machine. A server disk crash is a non-event with Distributed revision control
Disadvantages vs Centralized
- Many teams have long used and grown accustomed to the centralized model, and are reluctant to change
- Source code is considered the "crown jewels" of a software group. Centralized VCSs have been around much longer and thus perceived to be more stable
- Some projects want or need centralized control
- Distributed systems can end up with a person as the central point of control, rather than a server
- Concepts of DVCSs are slightly more difficult for developers to grasp. They become required to know more about infrastructure.
Work Model
The distributed model also impacts the traditional developer working model.
History
Arch and Monotone were some of the early systems to embrace DVCS principles. Then Darcs came on the scene, followed by a host of others, including Mercurial, Bazaar, and Git.
See the List of revision control software for a more comprehensive list.
Some of natively centralized systems are starting to grow distributed features. For example, Subversion is able to do many operations with no network, and it is planning to implement local commits.
See also
Flourishing DVCSs
- Mercurial
- git
- Bazaar-NG
- SVK
- Subversion (it is growing some distributed features, like local commits, but is really a centralized VCS)
Most, if not all, DVCSs are open source projects.
External links
- Essay on various revision control systems, especially the section "Centralized vs. Decentralized SCM"
- Linus Torvalds email describing DVCS to KDE developers
- Controverial Linus Torvalds video
- Bryan O'Sullivan video on Mercurial
- Ben Collins-Sussman article on "The Risks of Distributed Version Control"
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