Storage for a session's sequence numbers, keyed by session identity rather than by process.

A session that must resume its numbering after a reconnect, a restart, or a crash loads it from here at logon and reports every number it uses. The identity is the FIX session key — BeginString, SenderCompID, TargetCompID — so the same session finds its own numbers whichever process is running it.

Sessions have no store unless one is configured, in which case numbering starts at 1 on every connection and nothing is written.

FixAlchemy.Server.start_link(
  session_store: FixAlchemy.SessionStore.Ets,
  session_store_opts: [table: :my_fix_sequences],
  ...
)

Rules an implementation must follow

• record_outbound/2 and record_inbound/2 are called once for every message the session sends and accepts. What is written, and when, is the implementation's choice; the engine does not wait on either call.
• load/1 may return a number higher than any the session actually used. It must never return a lower one — the counterparty rejects a MsgSeqNum below what it expects, and the session ends.
• A gap between what load/1 returns and the last number used is answered by the session with SequenceReset-GapFill, and needs nothing from the implementation.
• close/1 is the only point at which anything held back is guaranteed to be flushed. It is not called when the node dies.

Implementing

defmodule MyApp.CubSessionStore do
  @behaviour FixAlchemy.SessionStore

  @impl true
  def open(key, opts), do: {:ok, {Keyword.fetch!(opts, :db), key}}

  @impl true
  def load({db, key}) do
    CubDB.get(db, key, %{next_outbound: 1, expected_inbound: 1})
  end

  @impl true
  def record_outbound({db, key} = handle, seq) do
    CubDB.put(db, key, %{load(handle) | next_outbound: seq + 1})
  end

  # record_inbound/2, reset/1 and close/1 likewise
end