# 102 — Split the single engine actor into reader, primary, and profile actors

## Summary

Today, one actor (`run()` in `src/engine/handle.rs`) does everything:
tailing/ingest bookkeeping, raw log storage, param extraction, *every*
declared metric's regex evaluation, sort-helper indexing, eviction, and
alert evaluation, all serialized through one `mpsc` channel. This is
migration step 3 (the actual split) of [[0001-actor-split]]'s resolved
plan, and depends on [[100-arc-ify-stored-log-and-metric-strings]] and
[[101-extract-metric-computation-into-separate-stage]] landing first —
this ticket is the wiring/topology change those two steps prepare for,
not a from-scratch redesign. See [[0001-actor-split]]'s "Resolved
decisions" section for the full rationale and answers to the open
questions below; this ticket's scope is that design, made concrete.

## Where

- `src/engine/handle.rs:130-365` (`run()`) — the single actor loop to
  be split. The `EngineMessage::Ingest` arm (:143-219) is the part that
  becomes the reader→primary boundary; `apply_all_metrics` /
  `apply_metric_to_log` (`src/engine/snapshot/mod.rs:616`, :499-586)
  and alert evaluation (`src/engine/alerts.rs:211`, called from
  `handle.rs:199-219`) are what moves to the new profile actor.
- `src/cli/mod.rs:95` (`spawn_engine`) and `:105-124`
  (`load_active_profile`) — the sole production call site of
  `engine::spawn`; `load_active_profile`'s existing comment ("there's
  no way to reprocess earlier lines later, so this must happen before
  `spawn_ingest`", `:104-106`) documents a constraint the new topology
  must preserve for the primary actor's startup ordering.
- `src/engine/ingest.rs` (stdin/file-tailing + Prometheus-polling
  tasks, spawned at `src/cli/mod.rs:136-145`) — today's only
  multi-task precedent, producer-side only (multiple producers into one
  actor's channel); the new reader actor sits upstream of this or
  subsumes part of it, per [[0001-actor-split]]'s three-actor design.
- `src/api/mod.rs:712-768` (`activate_profile` /
  `reload_engine_declarations`) — the runtime profile-switch path that
  currently removes-then-redeclares against the single actor; needs to
  target the new profile actor instead. See [[0004-profile-switch-reindexing-cost]]
  for the existing cost analysis of this path, which doesn't disappear
  with this split but moves and stops blocking the primary.
- `src/engine/tests.rs` (2348 lines) and the `test_handle` pattern
  (see ticket 099's removal of the duplicate `test_handle` stub) —
  built around one actor with one channel; will need real rework, not
  just extension, to cover primary alone, profile actor alone, and
  primary+profile interaction.

## Scope

Implement the three-actor topology exactly as resolved in
[[0001-actor-split]]:

- **Reader actor**: tails stdin/file, splits into lines, timestamps
  them, wraps each line in an `Arc` (per [[100-arc-ify-stored-log-and-metric-strings]]),
  sends downstream.
- **Primary actor**: ingests timestamped `Arc` lines, extracts declared
  + auto params only (no metrics, no profile logic), maintains the
  `param=value` index. Owns retention/eviction (`SetMaxLogs`) on its
  own schedule — a lagging profile actor does not block primary
  eviction; it silently misses data for the evicted range (accepted
  tradeoff, not a bug to fix here).
- **Profile actor**: holds its own `left_right` reader over the
  primary's index; builds metrics, custom params, timeseries, and
  PromQL evaluation for the one active profile. Owns alert evaluation
  (moved from the primary per the resolved decision that alerts are
  profile-scoped). Still exactly one active profile at a time — no
  concurrent multi-profile indexing; this changes *how* the active
  profile's data is computed, not the single-active-profile user model.
- **Primary→profile signaling**: a `tokio::sync::watch` (or equivalent)
  that the primary notifies right after each `left_right` sync
  completes; the profile actor wakes on that signal and re-reads
  through its own `left_right` reader — no busy-polling, no second data
  copy in transit.
- **Query fan-out**: a layer for requests that need both raw-log and
  profile-derived data in one response (e.g. `Logs`/`Timeline`
  queries, which today read both from a single `EngineSnapshot` in one
  pass — see `query.rs`'s `log_entry_params` call against
  `compiled_params`) — this becomes a real join across two
  snapshots/actors.
- **`EngineHandle` API**: split into two handle types (primary vs.
  profile), or one combined handle that internally fans out — decide
  during implementation which `api.rs`'s `ApiState` should hold; either
  is acceptable per [[0001-actor-split]], this ticket doesn't mandate
  one.
- Rework `src/engine/tests.rs` and the `test_handle` pattern to cover
  the new topology (primary alone, profile actor alone,
  primary+profile interaction) — explicitly in scope, not deferred,
  since the existing single-actor test harness cannot exercise the new
  split at all.
- Update `src/api/mod.rs:712-768`'s `activate_profile` /
  `reload_engine_declarations` to address the profile actor.

Out of scope (per [[0001-actor-split]]'s resolved decisions —
explicitly deferred, not silently dropped):

- System param suppressions — dropped from scope entirely; revisit as
  a separate idea if it becomes a real requirement.
- Full-text/regex indexing on the primary — a materially different
  investment ([[0005-additional-read-indexes]]), doesn't depend on
  actor topology.
- Profile-actor crash/restart checkpointing beyond "restart and
  rebuild from whatever the primary currently retains" — no persisted
  checkpoint required for this ticket.
- Diff-based or async profile-switch reload
  ([[0004-profile-switch-reindexing-cost]]'s options) — this ticket
  only needs `activate_profile` to target the right actor, not to
  change the switch's synchronous-rebuild cost model.