Behaviour for transaction dedup stores used by MPP.Methods.Tempo.

Prevents replay attacks by tracking which transaction hashes have already been used. HMAC-bound challenges prevent cross-request replay; this store prevents a client from resubmitting the same signed transaction within the store's TTL window.

Important: The store's TTL must be ≥ your challenge expires_in to ensure a tx hash cannot be evicted and replayed while the challenge is still valid. A good default is 2× the challenge expiry.

Built-in Store

MPP.Tempo.ConCacheStore provides an ETS-based implementation with automatic TTL expiry via ConCache. It is started by the :mpp application as the default store, so replay protection is on out of the box (issue #7). For most single-node deployments this is all you need; for multi-node, configure a shared backend (Redis/Postgres) as your :store.

Custom Implementations

Implement this behaviour with your choice of backend (Redis, database, etc.) for custom needs. Store lifecycle and cleanup are the consumer's responsibility.

Deployment Strategies

The store choice depends on your deployment topology:

• Single node: ConCacheStore is sufficient — all requests hit the same ETS table.
• Multi-node with sticky routing: If your load balancer pins clients to specific nodes (e.g., Fly.io's fly-replay with cookie-based sticky sessions, or consistent hashing by session ID), per-node ConCacheStore is effectively global — the same client always hits the same node's store.
• Multi-node without sticky routing: Use a shared backend (Redis, Postgres) to ensure a tx replayed on a different node is still caught.

Keys are formatted as "mpp:charge:<lowercase_hex_value>" where the value is the transaction hash (for type="hash") or the full serialized transaction hex (for type="transaction").

Example

defmodule MyApp.PaymentStore do
  @behaviour MPP.Tempo.Store

  def get(key) do
    case :ets.lookup(:payment_dedup, key) do
      [{^key, value}] -> {:ok, value}
      [] -> :not_found
    end
  end

  def put(key, value) do
    :ets.insert(:payment_dedup, {key, value})
    :ok
  end

  def check_and_mark(key, value) do
    if :ets.insert_new(:payment_dedup, {key, value}) do
      :ok
    else
      {:error, :already_exists}
    end
  end
end

Then pass it in method_config:

plug MPP.Plug,
  method: MPP.Methods.Tempo,
  method_config: %{
    "rpc_url" => "https://rpc.moderato.tempo.xyz",
    "store" => MyApp.PaymentStore
  }