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Event store using PostgreSQL for persistence.
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Files
lib/event_store/publisher.ex
defmodule EventStore.Publisher do
@moduledoc """
Publish events ordered by event id
"""
use GenServer
require Logger
alias EventStore.{Publisher,Storage,Subscriptions}
defmodule PendingEvents do
defstruct [
initial_event_id: nil,
last_event_id: nil,
stream_uuid: nil,
events: [],
]
end
defstruct [
last_published_event_id: 0,
pending_events: %{},
serializer: nil,
]
def start_link(serializer) do
{:ok, latest_event_id} = Storage.latest_event_id()
GenServer.start_link(__MODULE__, %Publisher{
serializer: serializer,
last_published_event_id: latest_event_id,
}, name: __MODULE__)
end
def notify_events(pid, stream_uuid, events) do
GenServer.cast(pid, {:notify_events, stream_uuid, events})
end
def init(%Publisher{} = state),
do: {:ok, state}
def handle_cast(:notify_pending_events, %Publisher{last_published_event_id: last_published_event_id, pending_events: pending_events, serializer: serializer} = state) do
next_event_id = last_published_event_id + 1
state = case Map.get(pending_events, next_event_id) do
%PendingEvents{stream_uuid: stream_uuid, events: events, last_event_id: last_event_id} ->
:ok = Subscriptions.notify_events(stream_uuid, events, serializer)
state = %Publisher{state |
last_published_event_id: last_event_id,
pending_events: Map.delete(pending_events, next_event_id),
}
:ok = notify_pending_events(state)
state
nil ->
state
end
{:noreply, state}
end
def handle_cast({:notify_events, stream_uuid, events}, %Publisher{last_published_event_id: last_published_event_id, pending_events: pending_events, serializer: serializer} = state) do
expected_event_id = last_published_event_id + 1
initial_event_id = first_event_id(events)
last_event_id = last_event_id(events)
state = case initial_event_id do
^expected_event_id ->
# events are in expected order, immediately notify subscribers
:ok = Subscriptions.notify_events(stream_uuid, events, serializer)
%Publisher{state |
last_published_event_id: last_event_id,
}
initial_event_id ->
# events are out of order, track pending events to be later published in order
pending = %PendingEvents{
initial_event_id: initial_event_id,
last_event_id: last_event_id,
stream_uuid: stream_uuid,
events: events
}
state = %Publisher{state |
pending_events: Map.put(pending_events, initial_event_id, pending)
}
:ok = notify_pending_events(state)
state
end
{:noreply, state}
end
# Attempt to publish any pending events by sending a message to self.
defp notify_pending_events(%Publisher{pending_events: pending_events})
when pending_events == %{}, do: :ok
defp notify_pending_events(%Publisher{}),
do: GenServer.cast(self(), :notify_pending_events)
defp first_event_id([first | _]), do: first.event_id
defp last_event_id(events), do: List.last(events).event_id
end