A system which tracks versions, usually of SourceCode, but potentially any digital content. It allows multiple people to work on a project, automatically coordinating everyone's changes with everyone else's — much like the wiki. Having a project versioned makes it possible to roll back erroneous changes, find out when bugs or errors were introduced and by whom, maintain multiple branches of the same thing simultaneously without conflicts, and many things more. Different VersionControlSystems have different sets of features in terms of multiple file handling, MetaData versioning, distributed storage, security goals, etc.
The main stages of development can be summarized as follows:
The design of the earliest systems revolved around versioning a single working copy, directly edited by all users. To prevent attempts at simultaneous modification of a single file, editing was not allowed without checking files out, which only one user at a time could do for any given file.
The next evolutionary step was to decouple the repository from the working copy, so that there may then be many working copies. The exemplar in this class of systems, known as centralised VCSs, is CVS. It lifts the obvious restrictions of earlier systems with a design in which the repository is mediated by a server. Multiple users can collaborate by each checking out a private working copy of the project.
Note that in CVS, “checking out” no longer implies locking. (In other centralised VCSs, it may; eg. VisualSourceSafe. In some, such as PerForce, it is optional.) Checking in changes is simply blocked if someone else has already checked in other changes in the meantime. Before the latecomer is allowed to check in their own changes, they have to update their working copy with the upstream changes, resolving any conflicts manually.
This works reasonably well. CVS ended up as the de facto standard for a decade.
However, its single-repository nature, subsequently adopted by most following major systems, perpetuates problems harking back to the earlier model – and adds new ones:
The solution to all this was to not only give each collaborator a separate working copy, but a separate repository also. This class of system, whose pioneering solid implementation was BitKeeper, is known as DistributedVersionControlSystems. The technical basis that allows this is algorithmic merging: 3-way merging allows combining non-overlapping changes automatically, and merge point tracking allows repeatedly merging branches without unnecessary conflicts.
Since each collaborator has their own repository and can make commits, the effect is that everyone has their own private branch, with full versioning for local changes, and these branches can be published at the discretion of their author and can be merged by others easily. Actually, each collaborator often has several local branches – since merging is easy and branches never need be published, it is painless to create short-lived branches for experiments or tests, to use them as a general workflow aspect (eg. start a new branch for every separate bug fix), or for any other purpose, whether intended for public consumption or not.
Everyone has full offline access to the project history, and all repository operations (except pushing or pulling changes, obviously) take place at full local disk speed.
From this point on, the evolution of VCSs has basically been about developing increasingly sophisticated architectures for handling merging. (Classic 3-way merging is only the simplest possibility.)
Each development took a long time, much of which was spent simply coming to recognize that there was a problem that needed to be solved. Many of the new developments remained controversial to adherents of older ways of doing things, even to the present day.
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