Factory Builder

What the factory builder does

A factory builder sits one step before the factory itself: it collects registrations, binds concrete implementations to a factory-ready map, and gives the application a cleaner place to assemble object creation rules.

That separation matters because the builder keeps registration concerns out of consuming code. Instead of hard-coding selection logic everywhere, teams can centralise how services are prepared, which makes the resulting factory easier to read, extend, and test.

In practice, a factory builder is most useful when the set of concrete services may grow over time. New implementations can often be added by updating the builder’s registration layer rather than changing every caller that depends on the factory.

How it relates to dependency injection

The builder is typically an integration point for dependency injection primitives, not a replacement for them. It can receive already-constructed services, wire them into a registry, and then expose a factory that resolves the right implementation later.

That pattern reduces direct coupling between consumers and concrete classes. The consumer asks the factory for behaviour, while the builder handles the mapping details that would otherwise be scattered through application startup or composition code.

This also gives architecture teams a clearer boundary for change. Registration logic, construction logic, and consumption logic are separated, so refactoring one layer is less likely to break the others.

Why this pattern is used in software design

Factory builders are usually chosen when object creation is not trivial enough for a single constructor call. They help when creation depends on configuration, environment, or a set of named implementations that must be selected consistently.

They are also a maintainability tool. A well-structured builder makes the factory’s supported services visible in one place, which can improve discoverability for developers and reduce the chance of ad hoc creation code creeping into the codebase.

The trade-off is that the builder adds another layer of indirection. If the registration rules are opaque or over-abstracted, the pattern can become harder to trace than the simple code it was meant to replace.

Common implementation considerations

Good factory builders keep the registration contract explicit. Each concrete service should be registered in a way that makes its purpose, key, or selector easy to understand, especially when the factory resolves multiple implementations from the same interface.

They also work best when extension is intentional. If the builder is expected to support plugin-style additions, the registration surface should be stable enough that new services can be introduced without changing consumers or rewriting the factory’s core dispatch logic.

For teams using dependency injection containers, the builder often acts as a composition layer rather than a business layer. That keeps service wiring in infrastructure code and prevents factory concerns from leaking into domain logic.

Risk and Threat Considerations

A factory builder can create architectural risk when it becomes the hidden control point for service selection. If registrations are inconsistent, incomplete, or overwritten, the wrong implementation can be instantiated silently, which is especially damaging in security-sensitive or policy-sensitive paths.

Failure mechanism: misregistration, ambiguous keys, or uncontrolled extension can cause consumers to receive a weaker, unsafe, or unexpected concrete service, and that defect may only surface at runtime.

Impact: the result can be broken authorization paths, fragile startup behaviour, hard-to-audit object creation, and difficult incident analysis because the actual execution path depends on builder state rather than the consuming code alone.

Practitioner Guidance

What to watch for: use a factory builder when you need a deliberate composition point, not just a convenience wrapper. If the registry is small and stable, extra abstraction may add more maintenance cost than value; if the set of implementations is expected to expand, the builder can preserve order and clarity.

Governance implication: treat the builder as part of application wiring and review its registrations the same way you would review other startup or configuration code. The main question is whether the mapping stays explicit enough that future changes remain predictable.

Related resources from NHI Mgmt Group

• What is the difference between agent builder choice and agent governance?
• What is the difference between a code assistant and an autonomous code factory?
• What breaks when contractor access is not tightly governed on the factory floor?
• When should organisations choose a factory reset instead of in-place migration?