>It is also difficult if there's an ill-defined interface that exposes implementation details, or no interface at all.
I've successfully strangled a large codebase that had these issues, though we did have the benefit of a client/server application so there was a place to actually define interfaces.
We started in the middle by creating a logical service layer to group all the bits of like functionality. We left the implementations alone, just moved them to align with the new "service" layer. We slowly worked our way up the stack, including defining a new client API, and then changed the existing API methods to be a shim on top of the new methods.
We were then able to update client code to use the new interface, but the old ones stuck around for about 24 months while we sunsetted older clients. The actual strangulation took about 2-3x as long a stop-and-rewrite effort, but there were VERY few regressions because we were still in a constant test and release cycle and managed the scope of strangulation changes in each release AND all of our testing was still valid since we weren't changing input/outputs or any expected behaviors.
I've successfully strangled a large codebase that had these issues, though we did have the benefit of a client/server application so there was a place to actually define interfaces.
We started in the middle by creating a logical service layer to group all the bits of like functionality. We left the implementations alone, just moved them to align with the new "service" layer. We slowly worked our way up the stack, including defining a new client API, and then changed the existing API methods to be a shim on top of the new methods.
We were then able to update client code to use the new interface, but the old ones stuck around for about 24 months while we sunsetted older clients. The actual strangulation took about 2-3x as long a stop-and-rewrite effort, but there were VERY few regressions because we were still in a constant test and release cycle and managed the scope of strangulation changes in each release AND all of our testing was still valid since we weren't changing input/outputs or any expected behaviors.