Streamed Lines: Branching Patterns for Parallel Software Development

Copyright © 1998 by Brad Appleton, Stephen Berczuk, Ralph Cabrera, and Robert Orenstein.
Permission is granted to copy for the PLoP '98 conference.

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Introduction to Branching

The following is a brief introduction to the concepts of file checkin/checkout, and to branching and merging. If you are already familiar with these concepts you may safely skip this section.

The checkout/checkin model

Most version control tools in widespread use employ the checkout-edit-checkin model to manage the evolution of version-controlled files in a repository or codebase. Developer's checkout a file from the codebase into their workspace where they perform and then test their modifications. When they are finished, they checkin their modified source file revisions back into the codebase. No files may be modified unless they are first checked-out.

Figure 1: The checkout-checkin model

Serial development using exclusive checkouts

In a strictly sequential development model, when a developer checks-out a file, the file is write-locked: No one may checkout the file if another developer has it checked-out. Instead, they must wait for the file to be checked-in (which releases or removes the write-lock). This is considered a pessimistic concurrency scheme which forces all work to take place on a single line of development.

Figure 2: Serial development line

Concurrent development using branches

Branching is a common mechanism used by many version control (VC) tools to support concurrent software development. In its most basic form, branching allows development to take place along more than one path for a particular file or directory. If one developer wants to checkout a file that another developer has already checked-out, she merely creates a branch and then checks-out the file on the new branch. This is an optimistic concurrency scheme that creates a parallel line of development for the file.

Figure 3: Branching off a new development line

Branching can be likened to a Unix fork that creates a new thread of execution with copy-on-write semantics. When a branch is created, the contents of the revision serving as the branchpoint are the same as the contents of the initial revision on the newly created branch. When the revision on the new branch is modified and checked-in, the two lines of development will have different contents and will evolve separately, in isolation from one another.

Synchronizing concurrent development lines using merges

Merging is the means by which one development line synchronizes its contents with another development line. When merging from a child branch back to a parent branch, the contents of the latest version on the child branch are reconciled against the contents of the latest version on the parent branch (preferably using a 2-way or 3-way file differencing or comparison tool). The contents of the two files are merged together into a new revision which must correctly resolve any conflicting changes made along each development line since the branchpoint was created (or since the last time the two development lines were synchronized).

Figure 4: Merging back to the parent branch

Logical and Physical Configuration Elements

So far we have discussed checkout/checkin (and branching) of files and directories. Files and directories are physical configuration elements for software. Most VC tools that support branching let you checkout/checkin files, and perhaps directories. Some VC tools are integrated with programming environments and let you checkout/checkin logical configuration elements, typically programming language elements like classes and methods (or modules and functions). The discussion throughout Streamed Lines assumes that files and directories are the minimal configuration elements, but the ideas and strategies presented here are applicable to VC tools that allow branching of methods and classes. They also apply equally well to most any kind of software artifact (not just source-code files).
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