# `Lockstep.Strategy.Replay`
[🔗](https://github.com/b-erdem/lockstep/blob/v0.1.0/lib/lockstep/strategy/replay.ex#L1)

Re-runs the schedule recorded in a saved trace.

PIDs in the original trace are not the same `#PID<...>` values you'll
see during replay (they're recreated by spawn). Instead the strategy
matches them positionally: each spawned process gets a 1-based
registration index in the order it calls `hello`, and the recorded
schedule says "at step K, pick whichever pid is at position P". This
works because both the original run and the replay register pids in
the same order — provided the user code is deterministic.

Divergence (a position pointing at a pid that isn't currently in the
ready set, or the trace running out before the test finishes) raises a
clear error so you know the replay isn't faithful to the original.

## Options

  * `:replay_pid_order` — list of position indexes (1-based) per step,
    as serialized into the `.lockstep` trace file by `Lockstep.Trace`.
  * `:fallback` — `:raise` (default) or `:random`. When `:random`,
    reaching the end of the recorded schedule (or hitting a position
    that isn't currently ready) switches to uniform random selection
    over the ready set instead of raising. Used by `Lockstep.Shrink`
    to test partial-prefix schedules.
  * `:fallback_seed` — RNG seed for the random tail (only meaningful
    with `fallback: :random`). Default `0`.

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