servicehooks

Service hooks

servicehooks runs ordered, synchronous reactions to typed service events. A registry is an in-process dispatcher owned by a service; it is not an event bus, transaction manager, retry mechanism, or background worker.

Motivation

The primary use case is cross-domain validation without leaking one domain into another. The domain performing an operation owns and invokes its lifecycle event. Application wiring can register validation from another domain without making the source domain import or depend on that domain's service.

For example, a customer service can expose a deletion event while an entitlement hook rejects deletion for customers that still own entitlements. Customer lifecycle remains owned by the customer domain, and entitlement rules remain owned by the entitlement domain.

Hooks can also produce system or audit events. The registry does not make those effects transactionally valid or durable. Application system-event publication uses the shared outbox to couple delivery to commit. That guarantee belongs to the owning service and event infrastructure, not this package.

Ownership and event boundaries

Each service defines the event type containing the lifecycle and state its hooks need. One type can represent related operations:

type CustomerLifecycleEvent struct {
    Operation CustomerOperation
    Before    *customer.Customer
    After     *customer.Customer
}

type CustomerService struct {
    servicehooks.Registry[CustomerLifecycleEvent]

    repository customer.Repository
}

Embedding promotes the registry's complete method set, including Register, Invoke, Seal, and IsSealed. If invocation and sealing must remain private to the service, store the registry in an unexported field and expose an explicit Register wrapper instead.

The registry owns named registration, registration order, sealing, synchronous invocation, context propagation, cycle detection, and hook error attribution. The service owns:

• event semantics and valid event shapes;
• the invocation point relative to persistence and commit;
• whether hook failure rejects or rolls back an operation;
• transaction separation, retries, idempotency, compensation, and observability.

A registry handles one Go event type, not necessarily one resource operation. Events can share a registry when they have the same ordering, atomicity, failure, and concurrency guarantees. Hooks with different guarantees should use separate event types and registries. In particular, do not put a transactional guard and an external after-commit notification in the same fail-fast chain.

See the example service test for create, update, and delete operations sharing one lifecycle event registry.

Registration and invocation

A hook handles one typed event:

type Hook[T any] interface {
    Handle(context.Context, T) error
}

HookFunc adapts a function when a dedicated implementation type would not clarify domain intent.

Registration names are non-empty and unique within the registry. Invocation order is exactly the order in which Register succeeds; the registry never reorders hooks. A Wire composition provider must therefore encode the order in its calls rather than relying on declaration order in wire.NewSet:

func registerCustomerHooks(
    service *CustomerService,
    entitlementValidation servicehooks.Hook[CustomerLifecycleEvent],
    audit servicehooks.Hook[CustomerLifecycleEvent],
) error {
    if err := service.Register("entitlement-validation", entitlementValidation); err != nil {
        return err
    }

    return service.Register("audit", audit)
}

Registration is a startup phase. Seal, or the first call to Invoke, makes registrations immutable; later registration returns ErrRegistrySealed. Callbacks then run without the registry lock, and invocation reuses the sealed registrations without cloning them. The zero value is ready for use, but a registry must not be copied after first use.

Invocation is sequential and fail-fast. Cancellation is checked before dispatch and before each hook, but an already-running hook must observe the propagated context itself. Hook errors are wrapped in InvocationError with the registration name while preserving errors.Is and errors.As.

The registry does not undo completed hooks. Place invocation inside an appropriate transaction or provide compensation when atomicity is required. Multiple goroutines can invoke a sealed registry concurrently, so hooks and mutable data referenced by events must provide their own synchronization.

Cycle handling

While a hook runs, its private registration identity is added to the causal context chain. Nested service calls must propagate that context for cycle detection to work.

The default CyclePolicyError rejects a nested invocation with CycleError before any hook in that invocation runs. An intentionally re-entrant hook can use WithCyclePolicy(CyclePolicySkip) when skipping only its active registration is semantically safe; unrelated hooks continue in registration order. Cycle tracking is per registration rather than suppressing the entire registry.

Hook implementation contract

Hooks should:

• treat events and referenced domain state as immutable unless the event contract explicitly permits mutation;
• propagate context, return domain errors, and keep synchronous work bounded;
• be concurrency-safe and idempotent when the service operation can be retried.

Hooks should not:

• register hooks during invocation or after application startup;
• retain events or contexts, or launch uncoordinated goroutines;
• panic for expected failures or assume the registry provides rollback, retries, logging, or compensation;
• perform irreversible external effects inside a transaction-bound registry;
• call the same registry recursively without an explicit re-entry design.

The registry intentionally does not recover panics. Programming defects may panic; expected service failures must be returned as errors.