The host's answer to which bindings an event is routed by under a scope (ADR-0001, the Amendment of 2026-09-25).

A configuration's :bindings is one list for every scope. A host whose bindings differ by scope - each scope routing its own sources to its own documents - gives the configuration a :bindings_resolver instead. It takes the event's scope and answers the list of %StatifierRouter.Binding{} structs that scope routes by, in the order StatifierRouter.route/3 returns its outcomes in. The two keys are exclusive: StatifierRouter.Config.new/1 refuses a configuration that gives both.

A bindings resolver is a module implementing this behaviour, the documented form, or an arity-1 fun with StatifierRouter.BindingsResolver.resolve/1's own signature, accepted wherever a module is. A host that keeps its bindings in its own tables builds each one with StatifierRouter.Binding.new/1 and answers the structs:

defmodule MyApp.ScopedBindings do
  @behaviour StatifierRouter.BindingsResolver

  @impl StatifierRouter.BindingsResolver
  def resolve(scope) do
    MyApp.Routing.bindings_for(scope)
  end
end

It is called with a scope in hand, each time one is: once per StatifierRouter.route/3 call, once per StatifierRouter.Broadway partition, and once per StatifierRouter.subscribe/3 of a subscribing execution's scope. Nothing here caches the answer; caching is the host's, and a host that builds its bindings from rows will want to, because StatifierRouter.Binding.new/1 compiles two predicator programs per binding.

Each answer is checked as :bindings is at StatifierRouter.Config.new/1, each time it is given: a binding whose id is the reserved name StatifierRouter.SendHandler.execution_target/0 is refused as {:reserved_binding_id, name}, and a duplicated id as {:duplicate_binding_id, id}. An answer that is not a list of %StatifierRouter.Binding{} structs raises ArgumentError, as a malformed StatifierRouter.Resolver answer does. StatifierRouter.Broadway's partitioner rescues that raise and hashes the message id, and route/3 raises it again where the message fails.

What each caller does with a refused answer:

• StatifierRouter.route/3 returns the refusal as {:error, reason} before any binding is evaluated, so nothing is written and a front does not acknowledge the event;
• StatifierRouter.Broadway's partitioner hashes the message id, as it does for any message it cannot address, and route/3 then fails the message;
• StatifierRouter.subscribe/3 raises ArgumentError naming the refusal, because its return names no such refusal.

The publish-time checks and the address reaper take no scope and never call a bindings resolver: StatifierRouter.Contracts.check/3 reads the configuration's bindings: [], so a host checks its bindings per scope with StatifierRouter.Contracts.undeclared_binding_events/2, and StatifierRouter.Addresses.reap/3 computes its horizons from the bindings the host hands it.