A Software Product Line needs a place for behavior that belongs to one product. A salon appointment rule, a restaurant table flow, or a specialized real estate workflow may be essential to that business without being useful to every Holism application. Keeping niche behavior close to its runnable product prevents the shared platform from becoming a catalogue of unrelated exceptions. It also makes ownership and testing clearer.
Ask who uses the behavior
Begin with the customer outcome, not the code shape. If a capability serves many products with similar rules, a general part may fit. If it exists for one product's domain and would confuse others, keep it in that product's repository. A niche module can still use shared accounts, settings, and UI components. Location reflects business reuse, not whether its implementation uses generic libraries.
Define a clean contract with shared parts
Product-specific behavior often depends on general parts such as orders or content. Use their public APIs and events rather than reaching into private collections or business modules. Keep the product's rules in its own process or specific part, with inputs and outputs that the rest of the application can understand. This makes it possible to upgrade a shared part without searching for hidden product-specific imports across repositories.
Keep data ownership explicit
A reservation may need customer details and a payment state, but the salon or restaurant workflow should not silently become the owner of account identity or order storage. Decide which module creates, changes, and reads each record. Cross-part data can be embedded in the consuming item when the architecture requires a self-contained view. Avoid embedding same-part relations just for convenience. Clear ownership reduces contradictory updates and makes exports easier to reason about.
Use settings for variation within a domain
Two salons may differ in booking windows or cancellation rules. Those choices can belong in the salon-specific capability as validated tenant settings. A restaurant's table turnover setting does not belong in a universal scheduling core merely because both features involve time. Keep shared primitives small and let the niche module define its business vocabulary. This prevents an abstract model from becoming harder to use than the real workflow.
Promote only proven common behavior
A feature may start in one product and later prove useful elsewhere. Before moving it, compare the workflows and identify what is stable across them. Extract a public contract and migration path, then remove duplicated ownership. Avoid promoting the entire first implementation with its original product's assumptions. Reuse should reduce work for the second product without imposing the first product's terminology or constraints.
Test the assembled product
Repositories are composed into a runnable application, so product-specific tests must cover the actual combination of core, parts, settings, and locale. Check the niche workflow's failure paths and its interaction with general parts. A shared part's own tests cannot prove that a restaurant order or salon booking still works after an update. Keep a focused regression path for each product's critical task.
Let business value guide placement
The right boundary helps teams change a unique workflow quickly while keeping shared foundations stable. For SMB products, this is a cost decision as much as an architecture decision. Product-specific code is acceptable when it is honest about its scope and built on supported platform contracts. It becomes a problem only when it leaks into shared layers or duplicates foundations that the platform already provides. Document the reason for a niche boundary so a future developer does not generalize it accidentally. When requests from another product appear, compare the business rules before moving code. Similar screens can conceal very different obligations and failure modes. That comparison should include support and operations.