# `SuperCache.Cluster.TxnRegistry`
[🔗](https://github.com/ohhi-vn/super_cache/blob/main/lib/cluster/tnx_registry.ex#L1)

In-memory transaction log for the three-phase commit protocol.

Each in-flight transaction is stored as a map entry keyed by its
`txn_id` binary.  The log is held in an ETS table owned by this
GenServer so that:

- Reads are lock-free (`:public` + `:read_concurrency`).
- The table survives coordinator or participant crashes as long as the
  owning VM is alive (the ETS table is linked to this GenServer's
  lifetime, not to the caller's).
- On node restart, `SuperCache.Cluster.ThreePhaseCommit.recover/0` can
  iterate the table to resolve in-doubt transactions.

## Transaction states

| State           | Meaning                                                   |
|-----------------|-----------------------------------------------------------|
| `:prepared`     | PREPARE accepted; waiting for PRE_COMMIT or ABORT        |
| `:pre_committed`| PRE_COMMIT acknowledged; commit is safe even after crash |
| `:committed`    | COMMIT applied; entry removed immediately after          |
| `:aborted`      | ABORT received; entry removed immediately after          |

## Memory management

Committed and aborted entries are deleted synchronously by
`ThreePhaseCommit` after each successful/failed transaction.
Stale `:prepared` entries (e.g. from a coordinator crash before
PRE_COMMIT) are cleaned up by `recover/0` on the next startup.

# `child_spec`

Returns a specification to start this module under a supervisor.

See `Supervisor`.

# `count`

```elixir
@spec count() :: non_neg_integer()
```

Return the number of in-flight transactions (useful for tests).

# `get`

```elixir
@spec get(binary()) :: map() | nil
```

Look up a transaction by id.  Returns the entry map or `nil`.

# `list_all`

```elixir
@spec list_all() :: [{binary(), map()}]
```

Return all transaction entries as `[{txn_id, entry}]`.

# `mark_pre_committed`

```elixir
@spec mark_pre_committed(binary()) :: :ok
```

Transition `txn_id` to `:pre_committed`.

# `register`

```elixir
@spec register(binary(), non_neg_integer(), list(), [node()]) :: :ok
```

Register a new transaction in state `:prepared`.

`replicas` is the list of participant nodes (may be empty on participants
themselves — they only need `ops` and `partition_idx`).

# `remove`

```elixir
@spec remove(binary()) :: :ok
```

Remove a transaction entry (called after commit or abort).

# `start_link`

---

*Consult [api-reference.md](api-reference.md) for complete listing*