Base behaviour and macro for building event indexers.

Fact.EventIndexer defines the callbacks and GenServer scaffolding shared by all event indexers in the Fact storage system. An indexer listens for new events and extracts zero, one, or many values from each even it processes. For each extracted value, the indexer create or appends to a file, recording the event's Fact.record_id/0.

Indexers are just event projections, that filter the event ledger, and produce keyed, ordered sets of events.

Behaviour

An indexer must implement the index_event/3 callback, which extracts values from the supplied event.

The __using__/1 macro injects the full GenServer implementation that:

• initializes and ensures the index exists
• rebuilds the index from history on startup
• subscribes to live event notifications
• updates the index as new events arrive

Custom Indexers

Custom indexers only need to implement the callback.

Examples

This would produce an index for every user, including all the events which define a user_id in the event data.

defmodule YourApp.UserIndexer do
  use Fact.EventIndexer

  @impl true
  def index_event(%{@event_data => %{"user_id" => user_id}}, _opts), 
    do: to_string(user_id)

  def index_event(_, _), do: nil
end

This would produce an index for every tenant, including all the events which define a tenant_id in the event metadata.

defmodule YourApp.TenantIndexer do
  use Fact.EventIndexer
  def index_event(%{@event_metadata => %{"tenant_id" => tenant_id}}, _opts), 
    do: to_string(tenant_id)

  def index_event(_, _), do: nil        
end