Migrating existing events

Sometimes it is convenient to update existing historical events. Instead of introducing a new event in a different version (SomethingHappened-v2), we might prefer the simplicity of adding a new field to existing events via migration. Such as for example tenant_id when we introduce multi-tenancy to our application. There are various trade-offs here (you are rewriting a history after all), but we assume you understand them well if you decide to go this way.

Another valid use-case can be when you decide to migrate to a different mapper (ie from YAML to Protobuf).

Note that events are updated using upsert capabilities of your MySQL, PostgreSQL or Sqlite 3.24.0+ database.

Add data and metadata to existing events

event_store.read.each_batch do |events|
  events.each do |ev|
    ev.data[:tenant_id] = 1
    ev.metadata[:server_id] = "eu-west-2"
  end
  event_store.overwrite(events)
end

Change event type

event_store
  .read
  .of_type([OldType])
  .each_batch do |events|
    event_store.overwrite(events.map { |ev| NewType.new(event_id: ev.event_id, data: ev.data, metadata: ev.metadata) })
  end