Distributed version control

In computer programming, distributed version control, also known as distributed revision control or decentralized version control, allows many software developers to work on a given project without requiring them to share a common network. The software revisions are stored in a distributed revision control system (DRCS), also known as a distributed version control system (DVCS).

Distributed vs. centralized

Distributed revision control takes a peer-to-peer approach to version control, as opposed to the client-server approach of centralized systems. Rather than a single, central repository on which clients synchronize, each peer's working copy of the codebase is a complete repository.[1] Distributed revision control synchronizes repositories by exchanging patches (sets of changes) from peer to peer. This results in some important differences from a centralized system:

Other differences include:

DVCS proponents point to several advantages of distributed version control systems over the traditional centralised model:

Software development author Joel Spolsky, the owner of a commercial DVCS, described distributed version control as "possibly the biggest advance in software development technology in the [past] ten years."[4]

A disadvantage is that initial cloning of a repository is slower as compared to centralized checkout, because all branches and revision history are copied. This may be significant if access speed is slow and the repository size is large enough. For instance, the size of the cloned git repository (all history, branches, tags, etc.) for the Linux kernel is approximately the size of the checked-out uncompressed HEAD, whereas the equivalent checkout of a single branch in a centralized checkout would be the compressed size of the contents of HEAD (except without any history, branches, tags, etc.). Another problem with DVCS is the lack of locking mechanisms that is part of most centralized VCS and still plays an important role when it comes to non-mergable binary files such as graphic assets.

Systems

Open systems

An "open system" of distributed revision control is characterized by its support for independent branches, and its reliance on merge operations. Its general characteristics include:

For a list of distributed revision control systems, see the comparison of revision control software.

Replicated systems

A replicated system of distributed revision control depends on a replicated database. A check-in is equivalent to a distributed commit. Successful commits create a single baseline, which reduces the need for merges. An example of a replicated distributed system is Code Co-op.

Work model

The distributed model is generally better suited for large projects with partly independent developers, such as the Linux kernel project, because developers can work independently and submit their changes for merge (or rejection). The distributed model flexibly allows adopting custom source code contribution workflows. The integrator workflow is the most widely used. In the centralized model, developers must serialize their work, to avoid problems with different versions.

Central and branch repositories

Every project has a central repository that is considered as the official repository, which is managed by the project maintainers. Developers clone this repository to create identical local copies of the code base. Source code changes in the central repository are periodically synchronized with the local repository.

The developer creates a new branch in his local repository and modifies source code on that branch. Once the development is done, the change needs to be integrated into the central repository.

Pull requests

Contributions to a source code repository that uses a distributed version control system are commonly made by means of a pull request. The contributor requests that the project maintainer "pull" the source code change, hence the name "pull request". The maintainer has to merge the pull request if he or she decides the contribution should become part of the source base.[5]

The developer creates a pull request to notify maintainers of a new change; a comment thread is associated with each pull request. This allows for focused discussion of code changes. Submitted pull requests are visible to anyone with repository access. A pull request can be accepted or rejected by maintainers.[6]

Once the pull request is reviewed and approved, it is merged into the repository. Depending on the established workflow, the code may need to be tested before being included into official release. Therefore, some projects contain a special branch for merging untested pull requests.[5][7] Other projects run an automated test suite on every pull request, and the reviewer checks that any new code has appropriate test coverage.

History

Closed source DVCS systems such as Sun WorkShop TeamWare were widely used in enterprise settings in the 1990s and inspired BitKeeper (1998). BitKeeper went on to serve in the early development of the Linux kernel.

The first open-source DVCS systems included Arch, Monotone, and Darcs. However, open source DVCSs were never very popular until Git and Mercurial came along. The development of Git, now the world's most popular open source DVCS, was prompted by the decision of the company that made BitKeeper to rescind the free license that Linus Torvalds and some other Linux kernel developers had previously taken advantage of.

Future

Some originally centralized systems now offer some distributed features. For example, Subversion is able to do many operations with no network.[8] Team Foundation Server and Visual Studio Team Services now host centralized and distributed version control repositories via hosting Git.

See also

References

  1. ↑ Wheeler, David. "Comments on Open Source Software / Free Software (OSS/FS) Software Configuration Management (SCM) Systems". Retrieved May 8, 2007.
  2. 1 2 O'Sullivan, Bryan. "Distributed revision control with Mercurial". Retrieved July 13, 2007.
  3. ↑ "Workflows - Mercurial". Mercurial.selenic.com. 2012-08-19. Retrieved 2013-07-22.
  4. ↑ Spolsky, Joel (2010-03-17). "Distributed Version Control is here to stay, baby". Joel on Software. Retrieved 2010-06-18.
  5. 1 2 Johnson, Mark (8 November 2013). "What is a pull request?". Oaawatch. Retrieved 27 March 2016.
  6. ↑ "Using pull requests". GitHub. Retrieved 27 March 2016.
  7. ↑ "Making a Pull Request". Atlassian. Retrieved 27 March 2016.
  8. ↑ OSDir.com. "Subversion for CVS Users :: OSDir.com :: Open Source, Linux News & Software". OSDir.com. Retrieved 2013-07-22.

External links

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