The host's answer to which chart a new execution of a document starts on (ADR-0002, section 4).

This package keeps no publish store, so it cannot know which revision of a document is active. The host can. A resolver takes (scope, document) and answers {content_hash, machine}: the compiled machine of the chart new executions of that document start on under that scope, and that chart's content hash. StatifierRouter.Delivery calls it only when it is about to create an execution. An existing execution keeps the chart it started on and is never resolved through here: its chart is the configuration's :chart_resolver, looked up by the content hash its execution record carries (ADR-0002, the Amendment at its foot).

A resolver is a module implementing this behaviour, the documented form, or an arity-2 fun with StatifierRouter.Resolver.resolve/2's own signature, accepted wherever a module is. A host with a publish store (a blocks document store, a database table of published revisions) implements StatifierRouter.Resolver.resolve/2 over it:

defmodule MyApp.PublishedCharts do
  @behaviour StatifierRouter.Resolver

  @impl StatifierRouter.Resolver
  def resolve(scope, document) do
    case MyApp.Publishing.active_revision(scope, document) do
      {:ok, revision} ->
        machine = MyApp.Publishing.compiled_chart(revision)
        {Statifier.Machine.identity(machine).content_hash, machine}

      :error ->
        {:error, :not_published}
    end
  end
end

A host whose charts are compiled at boot can use StatifierRouter.Resolver.Static instead of writing one.

The content hash answered is the one statifier_persistence records for the execution: the content_hash of Statifier.Machine.identity/1 of the machine answered. StatifierRouter.Delivery hands the machine to StatifierPersistence.Executions.create/4, and statifier_persistence takes the hash it records from the machine itself.

An {:error, reason} answer means the host has no chart for the document. The delivery's transaction rolls back and StatifierRouter.route/3 returns {:error, {:unresolved_document, document, reason}}: nothing is created and no row is written (ADR-0003, section 1), and the error is not an outcome, so it has no ledger row (ADR-0004, section 7).