The Feature Debt Index (FDI) is a DETANGLE® Architecture Health Factor metric about the modularity and extensibility of the code base with respect to new functionality.

It is based on issues selected by the “Features” issue type category. This filter is normally used to select the issue types which capture the functionality (e.g. user stories or features) as “functional” issues of your system in your issue tracker.

The Feature Debt Index measures the coupling and cohesion of the implementation of  “functional” issues across the whole codebase during the last time period.

The more “functional” issues a file contributes to, the more responsibilities the file has, the lower its Feature Cohesion and the higher the chance of upcoming bugs.

Feature Cohesion of file X

In addition to Feature Cohesion, DETANGLE® also measures Feature Coupling. The more a set of files contributes to some common“functional” issues in a chaotic and irregular way,

• the more semantically tangled the files,
• the more blurred their common responsibilities,
• thus, the higher the Feature Coupling of these files and
• the higher their error-proneness is.

Feature Coupling between files 3 and 4

The Feature Debt Index combines the coupling and cohesion values across all files to one index which represents an Architecture Health Factor Index across all functional issues and all of the code base.

The displayed icons follow certain thresholds along the Debt Index values for the system or for folders:

1. 👎 Feature Debt Index (FDI) exceeds the value of 10, i.e. the system has reached a critical level.
2. ⚠️ FDI has passed the threshold value of 5 in the last time period, i.e. the system has reached  warning level.
3. 👍 FDI value is lower than 5 and is in a normal state.

Feature Debt Index is calculated for single files, for folders or single “functional” issues and the whole system:

Take a look at DETANGLE® Metrics at a Glance for an overview of all metrics.