refactor(signals): route emit_signal through backend facade

[webjunkie]

Problem

Part of a broader effort to cut django.setup() import time (which took it from ~11.8s to ~5s overall). emit_signal lived in products/signals/backend/api.py, which imported the signals temporal workflows at module scope. That formed a latent circular import — api -> temporal package -> reingestion -> api.emit_signal — that only held together by import-order luck and dragged the whole signals temporal workflow stack onto the import path of anything that touched emit_signal (which is a lot: other products, core temporal workers, the scout harness). Any shift in import order would both expose the cycle as a hard ImportError and re-pay that startup cost.

It's also a cross-boundary API that should sit behind a facade per the product-isolation guidelines, so the cleanup and the isolation step are the same move.

Changes

Move emit_signal to products/signals/backend/facade/api.py and route all callers through it. The temporal workflow imports it needs are deferred into the function body, so the facade imports nothing temporal at module scope — the cycle is gone, importing the facade is cheap, and the signals workflow stack no longer rides onto startup via this path. Pure rename plus deferred imports; no behaviour change.

How did you test this code?

I'm an agent (Claude Code). Automated only: products/signals/backend/test/test_api.py (9 passed), ruff/ty clean via lint-staged, and I verified the facade, signals.views, and the temporal package all import without the old circular-import error. No manual testing; relying on CI for the full suite.

Docs update

No user-facing change.

🤖 Agent context

Surfaced while profiling Django startup import cost (python -X importtime plus an __import__-hook tracer to find what drags heavy modules onto the startup path). This latent cycle was masked by import order and would bite the moment the order shifted — deferring the temporal imports behind the facade removes the cycle and completes a planned isolation step in one go. Kept separate from the larger startup-perf PR (the lazy API router and friends) because it's a self-contained refactor the signals owners can review on its own. Agent-assisted, human-reviewed — not self-merged.

[assign-reviewers-posthog]

👥 Auto-assigned reviewers

Skipped a review request for @PostHog/team-data-modeling (products/endpoints/**), @PostHog/team-warehouse-sources (posthog/temporal/data_imports/**) because they only have minor changes here. These are soft owners (CODEOWNERS-soft / each product's product.yaml), so nothing blocks merge — self-assign if you'd like a look. (Generated files and lockfiles are ignored when deciding ownership.)

[greptile-apps]

_{Reviews (1): Last reviewed commit: "refactor(signals): route emit_signal thr..." | Re-trigger Greptile}

[Copilot]

Pull request overview

Refactors Signals emission so emit_signal is accessed via the Signals backend facade and no longer pulls Signals Temporal workflow modules into the import graph at module import time, eliminating a latent circular import and reducing Django startup/import cost.

Changes:

• Updated all call sites and tests to import/patch emit_signal from products.signals.backend.facade.api.
• Deferred Signals Temporal workflow imports inside emit_signal (instead of importing workflows at module scope).
• Updated Temporal activities/workflows and misc utilities (mgmt command, scout harness, endpoints) to use the facade entrypoint.

Reviewed changes

Copilot reviewed 16 out of 17 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
products/signals/backend/views.py	Switches emit_signal import to the facade entrypoint.
products/signals/backend/test/test_scout_harness_tools.py	Updates patch target to facade path for harness tool tests.
products/signals/backend/test/test_scout_harness_api.py	Updates patch target to facade path for API tests.
products/signals/backend/test/test_api.py	Updates imports/patch targets to facade path for emit_signal unit tests.
products/signals/backend/temporal/reingestion.py	Routes re-ingestion emissions through facade emit_signal.
products/signals/backend/temporal/emit_eval_signal.py	Updates local import to facade emit_signal.
products/signals/backend/temporal/backfill_error_tracking.py	Updates local import to facade emit_signal.
products/signals/backend/scout_harness/tools/emit.py	Uses facade emit_signal for harness emission (kept deferred import).
products/signals/backend/management/commands/ingest_signals_json.py	Updates management command import to facade emit_signal.
products/signals/backend/facade/api.py	Removes module-scope workflow imports; adds lazy imports inside emit_signal.
products/signals/backend/facade/init.py	Ensures facade is a package for stable import path.
products/endpoints/backend/tests/test_endpoint_execution.py	Updates patch target to facade path for endpoint failure-signal test.
products/endpoints/backend/api.py	Uses facade emit_signal for endpoint failure signal emission.
posthog/temporal/session_replay/session_summary/activities/video_based/a7b_emit_session_problem_signals.py	Updates import to facade emit_signal.
posthog/temporal/data_imports/signals/pipeline.py	Updates import to facade emit_signal.
posthog/temporal/ai_observability/eval_reports/test/test_emit_signal.py	Updates patch/import of _SIGNAL_VARIANT_LOOKUP and emit_signal to facade path.
posthog/temporal/ai_observability/eval_reports/emit_signal.py	Updates local import to facade emit_signal.

Comments suppressed due to low confidence (1)

products/signals/backend/facade/api.py:92

• The deferred Temporal workflow imports still run before the early-return gates (AI data processing approval + source enabled). This means callers that are immediately skipped will still pay the cost (and side effects) of importing the full signals Temporal stack. Move these imports to after the gate checks so Temporal modules are only imported when we actually intend to start workflows.

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[deployment-status-posthog]

Deploy status

Environment	Status	Deployed At	Workflow
dev	✅ Deployed	2026-06-01 15:42 UTC	Run
prod-us	✅ Deployed	2026-06-01 15:57 UTC	Run
prod-eu	✅ Deployed	2026-06-01 16:00 UTC	Run