Sommerville-Software-Engineering-10ed

25.1 ■ Version management 739
Definitive project
repository
1
Alice’s public
repository
3
3
Bob’s public
repository
4
Charlie’s private
repository
2
Alice’s private
repository
2
Bob’s private
repository
Figure 25.7 Opensource
development
Charlie
Alice
Bob
own private repository for testing (3). Once he is satisfied that the changes are
acceptable, he then updates the definitive project repository (4).
A consequence of the independent development of the same component is that
codelines may branch. Rather than a linear sequence of versions that reflect changes
to the component over time, there may be several independent sequences, as shown
in Figure 25.8. This is normal in system development, where different developers
work independently on different versions of the source code and change it in different
ways. It is generally recommended when working on a system that a new branch
should be created so that changes do not accidentally break a working system.
At some stage, it may be necessary to merge codeline branches to create a new version
of a component that includes all changes that have been made. This is also shown in
Figure 25.8, where component versions 2.1.2 and 2.3 are merged to create version 2.4. If
the changes made involve completely different parts of the code, the component versions
may be merged automatically by the version control system by combining the code
changes. This is the normal mode of operation when new features have been added. These
code changes are merged into the master copy of the system. However, the changes made
by different developers sometimes overlap. The changes may be incompatible and interfere
with each other. In this case, a developer has to check for clashes and make changes
to the components to resolve the incompatibilities between the different versions.
When version control systems were first developed, storage management was one
of their most important functions. Disk space was expensive, and it was important to
Codeline 2
V2.0
Codeline 2.1
<branch>
V2.1.1 V2.1.2
<merge>
V2.1 V2.4
<branch>
V2.2 V2.3
Figure 25.8 Branching
and merging
V1.0 V1.1 V1.2
Codeline 1