2 Why We Need Model-Based Testing

170 Modeling Systems with Structured State
the source files in the repository. Changes made to this local copy affect the global
repository of source code only when the user commits changes. Changes are always
applied as an atomic transaction.
An important feature of a revision control system is the handling of conflicts, or
cases when two users have made simultaneous updates to the same source file. The
system provides operations that identify the conflicts and allow users to choose how
to resolve them.
In our case study, we have not modeled any particular revision control system;
instead, we have abstracted features that are common to typical revision control
systems. If you want to compare the model with an implementation, you might want
to look at the Subversion revision control system that is available as open source.
10.4.1 Vocabulary of actions
We view a revision control system as a model program with the following vocabulary
of user actions:
• Synchronize updates a given client’s local copy to include the latest revisions in
the global repository. However, changed files will not be overwritten; instead,
they will be marked as conflicts to be resolved before committing.
• Edit represents a file change in the client’s local view of the repository.
• Revert undoes a previous edit, restoring a file’s state to the version of the last
Synchronize operation.
• Commit propagates local file changes to the global repository.
• Resolve reconciles conflicts between a local version of a file and changes to the
global repository made in parallel by another user. All files with conflicts must
be resolved before a set of changes may be propagated to the global repository.
The server has two actions that represent the server’s possible repsonses to a
client’s request to commit changes:
• MustResolve occurs after a Commit action for a client with unresolved file conflicts.
No changes to the global repository are made.
• CommitComplete occurs when changes have been successfully propagated to the
repository.
Here is a trace of a sample session for two clients.
Synchronize("alice")
Edit("alice", "file1", Op("Add"))
Commit("alice")
CommitComplete("alice", 1)
Synchronize("bob")
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