File Coupling

Before performing this analysis, you will want to have a active file to target

From the previous step, we decided to target the file src/mongo/db/repl/replication_coordinator_impl.cpp and see how it is coupled to other files in the repository.

We can analyze the file coupling with the following aggregation:

[
    { 
        "$match": { 
            "date": {"$gt": ISODate("2020-01-01")}, 
            "files.filename": "src/mongo/db/repl/replication_coordinator_impl.cpp" 
        } 
    },
    { 
        "$facet": { 
            "total_commits": [{ "$count": "commit" }], 
            "seen_with": [
                { 
                    "$unwind": { "path": "$files" } 
                }, 
                { 
                    "$match": { 
                        "files.filename": { "$ne": "src/mongo/db/repl/replication_coordinator_impl.cpp" } 
                    } 
                }, 
                { 
                    "$sortByCount": "$files.filename"
                },
            ] 
        } 
    },
]

We can also use the file-coupling subcommand to perform this aggregation:

mdb-code-insights --config-file config.yml file-coupling --since "2020-01-01" --filename src/mongo/db/repl/replication_coordinator_impl.cpp

src/mongo/db/repl/replication_coordinator_impl.cpp: 309 instances

 - src/mongo/db/repl/replication_coordinator_impl.h: 91: 29.45%
 - src/mongo/db/repl/replication_coordinator_mock.cpp: 63: 20.39%
 - src/mongo/embedded/replication_coordinator_embedded.cpp: 62: 20.06%
 - src/mongo/db/repl/replication_coordinator_mock.h: 61: 19.74%
 - src/mongo/embedded/replication_coordinator_embedded.h: 60: 19.42%
 - src/mongo/db/repl/replication_coordinator_noop.h: 60: 19.42%
 - src/mongo/db/repl/replication_coordinator_noop.cpp: 60: 19.42%
 - src/mongo/db/repl/replication_coordinator.h: 59: 19.09%
 - src/mongo/db/repl/topology_coordinator.cpp: 57: 18.45%
 - src/mongo/db/repl/replication_coordinator_impl_test.cpp: 51: 16.50%
 - src/mongo/db/repl/replication_coordinator_impl_heartbeat.cpp: 47: 15.21%
 ...

From the output, a lot of the files look to be related to the file we are analyzing. Most of them look like tests or header files that we would expect to be closely related. However, the src/mongo/db/repl/topology_coordinator.cpp file seems to stand out. This file seems to change along with the given file more frequently than the unit tests for that file. That might be something we would want to look into.

Now that we have analyzed file coupling, let’s try another analysis to see file ownership