2 Why We Need Model-Based Testing

Systems with Complex State 171
Edit("bob", "file1", Op("Change"))
Edit("alice", "file1", Op("Change"))
Commit("alice")
CommitComplete("alice", 2)
Commit("bob")
MustResolve("bob", Set("file1"))
Resolve("bob", "file1")
Commit("bob")
CommitComplete("bob", 3)
This trace shows a scenario where client alice adds a file named file1 to the
repository and commits her change. Client bob synchronizes against the repository
and begins to edit file1. Alice, in the meantime, edits file1 and commits her
changes.
When client bob commits, the system tells him that there is a conflict to resolve.
He resolves the conflict with the Resolve action and commits again. The system
responds with CommitComplete, indicating that the change has been incorporated
into the repository. The repository increments its global revision number and reports
it in the CommitComplete action.
10.4.2 State and derived state
We introduce just enough state to accurately determine which traces of the system
are possible and which are not. For ease of understanding, we can separate state into
the state of the repository and the state of the users of the repository.
Global version
namespace RevisionControl
{
partial static class Repository
{
public static int currentRevision = 0;
}
}
In C# the keyword partial may be used to break up a class definition into several
blocks. We use this here to indicate that blocks of code separated by explanatory
text all belong to the same class.
The state of the revision control system includes a global version number currentRevision.
This represents number of times any change has been committed to
the repository.
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