# `Concord.Txn`
[🔗](https://github.com/gsmlg-dev/concord/blob/main/lib/concord/txn.ex#L1)

Atomic multi-key transactions for Concord.

A transaction is a one-shot, atomic operation that:
1. Evaluates compare predicates against pre-transaction state
2. If all hold (AND), executes the `success` branch
3. Otherwise, executes the `failure` branch
4. Returns `{:ok, %Result{}}` — never `{:error, ...}` for compare failures

## Transaction Spec

    %{
      compare: [compare()],
      success: [operation()],
      failure: [operation()]
    }

## Examples

    # Atomic create-if-absent
    Concord.Txn.commit(%{
      compare: [{:exists, "/key", :==, false}],
      success: [{:put, "/key", value, %{}}],
      failure: [{:get, {:key, "/key"}, %{}}]
    })

    # Conditional update with revision check
    Concord.Txn.commit(%{
      compare: [{:mod_revision, "/key", :==, 1842}],
      success: [{:put, "/key", new_value, %{prev_kv: true}}],
      failure: [{:get, {:key, "/key"}, %{}}]
    })

# `commit`

```elixir
@spec commit(
  map(),
  keyword()
) :: {:ok, Concord.Txn.Result.t()} | {:error, term()}
```

Commits a transaction spec atomically.

## Options

- `:idempotency_key` — string key for safe retry (optional)
- `:timeout` — operation timeout in ms (default: 5000)

## Returns

- `{:ok, %Result{succeeded: true, ...}}` — success branch ran
- `{:ok, %Result{succeeded: false, ...}}` — failure branch ran (not an error)
- `{:error, {:invalid_txn, reason}}` — spec validation failed
- `{:error, reason}` — cluster error (:no_leader, :timeout, etc.)