feat(run-engine): flag to route getSnapshotsSince through read replica

[ericallam]

Summary

Adds RUN_ENGINE_READ_REPLICA_SNAPSHOTS_SINCE_ENABLED (default "0"). When enabled, the Prisma reads inside RunEngine.getSnapshotsSince run against the read-only replica client instead of the primary. Offloads the snapshot-polling queries fired by every running task runner off the writer.

Why

getSnapshotsSince is called from the managed runner's fetch-and-process loop (once per poll interval, plus on every snapshot-change notification). It runs four sequential reads per call — one findFirst by snapshot id, one findMany on snapshots with createdAt > X, one raw SQL against _completedWaitpoints, and chunked findMany on waitpoint. Per concurrent run, every few seconds. It's read-only, tolerates a small amount of staleness, and is an obvious candidate for the replica.

Replica-lag considerations

• Step 1 "since snapshot not found": if the runner just received a snapshot id from the primary and asks the replica before it replicates, the function throws and the caller treats the response as an error (runner falls back to a metadata refresh). Self-correcting, not silent.
• Step 2 missing newly-created snapshots: the next poll's createdAt > sinceSnapshot.createdAt filter still picks them up once the replica catches up.
• Waitpoint junction race: the riskiest path — if a latest snapshot is replicated but its _completedWaitpoints join rows aren't yet, the runner could advance past that snapshot with completedWaitpoints: []. WAL/storage-level replication replays commits in order, so in practice both should appear atomically on the reader, but the race window is why the flag ships disabled.

Aurora reader shrinks all three windows to single-digit ms in typical conditions, and its storage-level replication gives atomic visibility of committed transactions on the reader.

Test plan

• Flip the flag on in a non-prod environment, confirm snapshot polling behaves normally and getSnapshotsSince errors in Sentry stay flat.
• Verify writer query volume drops and reader query volume rises on the snapshot-polling queries.
• Keep an eye on AuroraReplicaLag (or equivalent) during rollout.

[coderabbitai]

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• .server-changes/read-replica-snapshots-since.md
• apps/webapp/app/env.server.ts
• apps/webapp/app/v3/runEngine.server.ts
• internal-packages/run-engine/src/engine/index.ts
• internal-packages/run-engine/src/engine/types.ts

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• internal-packages/run-engine/src/engine/types.ts
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🧠 Learnings (23)

📓 Common learnings

Learnt from: CR
Repo: triggerdotdev/trigger.dev PR: 0
File: internal-packages/database/CLAUDE.md:0-0
Timestamp: 2026-03-02T12:43:17.177Z
Learning: Applies to internal-packages/database/**/{app,src,webapp}/**/*.{ts,tsx,js,jsx} : Use `$replica` from `~/db.server` for read-heavy queries in the webapp instead of the primary database connection

Learnt from: ericallam
Repo: triggerdotdev/trigger.dev PR: 3417
File: apps/webapp/app/routes/realtime.v1.sessions.$session.$io.ts:37-51
Timestamp: 2026-04-20T15:06:16.910Z
Learning: In `apps/webapp/app/routes/realtime.v1.sessions.$session.$io.ts` (and all session realtime read paths), `$replica` is intentionally used for the `resolveSessionByIdOrExternalId` call — including the `closedAt` guard in the PUT/initialize path. The project convention is to use `$replica` consistently across all session realtime routes. The race window (replica lag allowing a ghost-initialize after close) is accepted as not realistic in practice (clients follow the close API response; they do not race it). If replica lag ever causes issues, the mitigation is to revisit all realtime routes together, not to swap individual routes to `prisma`. Do not flag `$replica` usage in session realtime routes as a stale-read issue.

Learnt from: CR
Repo: triggerdotdev/trigger.dev PR: 0
File: internal-packages/run-engine/CLAUDE.md:0-0
Timestamp: 2026-03-02T12:43:25.254Z
Learning: Use Prisma for data persistence with support for read-only replica queries via `readOnlyPrisma`

Learnt from: ericallam
Repo: triggerdotdev/trigger.dev PR: 3368
File: internal-packages/database/prisma/schema.prisma:666-666
Timestamp: 2026-04-16T14:21:14.907Z
Learning: In `triggerdotdev/trigger.dev`, the `BackgroundWorkerTask` covering index on `(runtimeEnvironmentId, slug, triggerSource)` lives in `internal-packages/database/prisma/migrations/20260413000000_add_bwt_covering_index/migration.sql` as a `CREATE INDEX CONCURRENTLY IF NOT EXISTS`, intentionally in its own migration file separate from the `TaskIdentifier` table migration. Do not flag this index as missing from the schema migrations in future reviews.

📚 Learning: 2026-03-02T12:43:17.177Z

Learnt from: CR
Repo: triggerdotdev/trigger.dev PR: 0
File: internal-packages/database/CLAUDE.md:0-0
Timestamp: 2026-03-02T12:43:17.177Z
Learning: Applies to internal-packages/database/**/{app,src,webapp}/**/*.{ts,tsx,js,jsx} : Use `$replica` from `~/db.server` for read-heavy queries in the webapp instead of the primary database connection

Applied to files:

• apps/webapp/app/v3/runEngine.server.ts
• .server-changes/read-replica-snapshots-since.md
• internal-packages/run-engine/src/engine/types.ts
• internal-packages/run-engine/src/engine/index.ts
• apps/webapp/app/env.server.ts

📚 Learning: 2026-04-16T14:19:16.309Z

Learnt from: CR
Repo: triggerdotdev/trigger.dev PR: 0
File: apps/webapp/CLAUDE.md:0-0
Timestamp: 2026-04-16T14:19:16.309Z
Learning: Applies to apps/webapp/app/v3/services/{cancelTaskRun,batchTriggerV3}.server.ts : When editing services that branch on `RunEngineVersion` to support both V1 and V2 (e.g., `cancelTaskRun.server.ts`, `batchTriggerV3.server.ts`), only modify V2 code paths

Applied to files:

• apps/webapp/app/v3/runEngine.server.ts

📚 Learning: 2026-04-15T15:39:31.575Z

Learnt from: CR
Repo: triggerdotdev/trigger.dev PR: 0
File: .cursor/rules/webapp.mdc:0-0
Timestamp: 2026-04-15T15:39:31.575Z
Learning: Focus on Run Engine 2.0 code in `internal/run-engine` for new development; legacy run engine code is being deprecated

Applied to files:

• apps/webapp/app/v3/runEngine.server.ts

📚 Learning: 2026-03-02T12:43:25.254Z

Learnt from: CR
Repo: triggerdotdev/trigger.dev PR: 0
File: internal-packages/run-engine/CLAUDE.md:0-0
Timestamp: 2026-03-02T12:43:25.254Z
Learning: Applies to internal-packages/run-engine/src/engine/tests/**/*.test.ts : Implement tests for RunEngine in `src/engine/tests/` using testcontainers for Redis and PostgreSQL containerization

Applied to files:

• apps/webapp/app/v3/runEngine.server.ts
• internal-packages/run-engine/src/engine/types.ts
• internal-packages/run-engine/src/engine/index.ts

📚 Learning: 2026-03-02T12:43:25.254Z

Learnt from: CR
Repo: triggerdotdev/trigger.dev PR: 0
File: internal-packages/run-engine/CLAUDE.md:0-0
Timestamp: 2026-03-02T12:43:25.254Z
Learning: All new run lifecycle logic should be implemented in the `internal/run-engine` package (`src/engine/`), not directly in `apps/webapp/app/v3/services/`

Applied to files:

• apps/webapp/app/v3/runEngine.server.ts

📚 Learning: 2026-03-22T13:26:12.060Z

Learnt from: ericallam
Repo: triggerdotdev/trigger.dev PR: 3244
File: apps/webapp/app/components/code/TextEditor.tsx:81-86
Timestamp: 2026-03-22T13:26:12.060Z
Learning: In the triggerdotdev/trigger.dev codebase, do not flag `navigator.clipboard.writeText(...)` calls for `missing-await`/`unhandled-promise` issues. These clipboard writes are intentionally invoked without `await` and without `catch` handlers across the project; keep that behavior consistent when reviewing TypeScript/TSX files (e.g., usages like in `apps/webapp/app/components/code/TextEditor.tsx`).

Applied to files:

• apps/webapp/app/v3/runEngine.server.ts
• internal-packages/run-engine/src/engine/types.ts
• internal-packages/run-engine/src/engine/index.ts
• apps/webapp/app/env.server.ts

📚 Learning: 2026-03-22T19:24:14.403Z

Learnt from: matt-aitken
Repo: triggerdotdev/trigger.dev PR: 3187
File: apps/webapp/app/v3/services/alerts/deliverErrorGroupAlert.server.ts:200-204
Timestamp: 2026-03-22T19:24:14.403Z
Learning: In the triggerdotdev/trigger.dev codebase, webhook URLs are not expected to contain embedded credentials/secrets (e.g., fields like `ProjectAlertWebhookProperties` should only hold credential-free webhook endpoints). During code review, if you see logging or inclusion of raw webhook URLs in error messages, do not automatically treat it as a credential-leak/secrets-in-logs issue by default—first verify the URL does not contain embedded credentials (for example, no username/password in the URL, no obvious secret/token query params or fragments). If the URL is credential-free per this project’s conventions, allow the logging.

Applied to files:

• apps/webapp/app/v3/runEngine.server.ts
• internal-packages/run-engine/src/engine/types.ts
• internal-packages/run-engine/src/engine/index.ts
• apps/webapp/app/env.server.ts

📚 Learning: 2026-03-29T19:16:28.864Z

Learnt from: nicktrn
Repo: triggerdotdev/trigger.dev PR: 3291
File: apps/webapp/app/v3/featureFlags.ts:53-65
Timestamp: 2026-03-29T19:16:28.864Z
Learning: When reviewing TypeScript code that uses Zod v3, treat `z.coerce.*()` schemas as their direct Zod type (e.g., `z.coerce.boolean()` returns a `ZodBoolean` with `_def.typeName === "ZodBoolean"`) rather than a `ZodEffects`. Only `.preprocess()`, `.refine()`/`.superRefine()`, and `.transform()` are expected to wrap schemas in `ZodEffects`. Therefore, in reviewers’ logic like `getFlagControlType`, do not flag/unblock failures that require unwrapping `ZodEffects` when the input schema is a `z.coerce.*` schema.

Applied to files:

• apps/webapp/app/v3/runEngine.server.ts

📚 Learning: 2026-03-02T12:43:25.254Z

Learnt from: CR
Repo: triggerdotdev/trigger.dev PR: 0
File: internal-packages/run-engine/CLAUDE.md:0-0
Timestamp: 2026-03-02T12:43:25.254Z
Learning: Use Prisma for data persistence with support for read-only replica queries via `readOnlyPrisma`

Applied to files:

• .server-changes/read-replica-snapshots-since.md
• internal-packages/run-engine/src/engine/index.ts

📚 Learning: 2026-04-20T15:06:16.910Z

Learnt from: ericallam
Repo: triggerdotdev/trigger.dev PR: 3417
File: apps/webapp/app/routes/realtime.v1.sessions.$session.$io.ts:37-51
Timestamp: 2026-04-20T15:06:16.910Z
Learning: In `apps/webapp/app/routes/realtime.v1.sessions.$session.$io.ts` (and all session realtime read paths), `$replica` is intentionally used for the `resolveSessionByIdOrExternalId` call — including the `closedAt` guard in the PUT/initialize path. The project convention is to use `$replica` consistently across all session realtime routes. The race window (replica lag allowing a ghost-initialize after close) is accepted as not realistic in practice (clients follow the close API response; they do not race it). If replica lag ever causes issues, the mitigation is to revisit all realtime routes together, not to swap individual routes to `prisma`. Do not flag `$replica` usage in session realtime routes as a stale-read issue.

Applied to files:

• .server-changes/read-replica-snapshots-since.md
• internal-packages/run-engine/src/engine/index.ts

📚 Learning: 2026-03-02T12:43:17.177Z

Learnt from: CR
Repo: triggerdotdev/trigger.dev PR: 0
File: internal-packages/database/CLAUDE.md:0-0
Timestamp: 2026-03-02T12:43:17.177Z
Learning: New code should always target Prisma RunEngineVersion V2 (run-engine + redis-worker), not V1 (legacy MarQS + Graphile)

Applied to files:

• .server-changes/read-replica-snapshots-since.md
• internal-packages/run-engine/src/engine/index.ts

📚 Learning: 2026-03-02T12:43:25.254Z

Learnt from: CR
Repo: triggerdotdev/trigger.dev PR: 0
File: internal-packages/run-engine/CLAUDE.md:0-0
Timestamp: 2026-03-02T12:43:25.254Z
Learning: Applies to internal-packages/run-engine/src/engine/systems/**/*.ts : Integrate OpenTelemetry tracer and meter instrumentation in RunEngine systems for observability

Applied to files:

• internal-packages/run-engine/src/engine/types.ts
• internal-packages/run-engine/src/engine/index.ts

📚 Learning: 2025-11-27T16:26:37.432Z

Learnt from: CR
Repo: triggerdotdev/trigger.dev PR: 0
File: .github/copilot-instructions.md:0-0
Timestamp: 2025-11-27T16:26:37.432Z
Learning: Applies to internal-packages/database/**/*.{ts,tsx} : Use Prisma for database interactions in internal-packages/database with PostgreSQL

Applied to files:

• internal-packages/run-engine/src/engine/index.ts

📚 Learning: 2026-04-13T21:44:00.032Z

Learnt from: ericallam
Repo: triggerdotdev/trigger.dev PR: 3368
File: apps/webapp/app/services/taskIdentifierRegistry.server.ts:24-67
Timestamp: 2026-04-13T21:44:00.032Z
Learning: In `apps/webapp/app/services/taskIdentifierRegistry.server.ts`, the sequential upsert/updateMany/findMany writes in `syncTaskIdentifiers` are intentionally NOT wrapped in a Prisma transaction. This function runs only during deployment-change events (low-concurrency path), and any partial `isInLatestDeployment` state is acceptable because it self-corrects on the next deployment. Do not flag this as a missing-transaction/atomicity issue in future reviews.

Applied to files:

• internal-packages/run-engine/src/engine/index.ts

📚 Learning: 2026-04-15T15:39:06.868Z

Learnt from: CR
Repo: triggerdotdev/trigger.dev PR: 0
File: CLAUDE.md:0-0
Timestamp: 2026-04-15T15:39:06.868Z
Learning: Use Prisma 6.14.0 from `internal-packages/database` for database operations. This is the pinned version for the monorepo.

Applied to files:

• internal-packages/run-engine/src/engine/index.ts

📚 Learning: 2025-07-12T18:06:04.133Z

Learnt from: matt-aitken
Repo: triggerdotdev/trigger.dev PR: 2264
File: apps/webapp/app/services/runsRepository.server.ts:172-174
Timestamp: 2025-07-12T18:06:04.133Z
Learning: In apps/webapp/app/services/runsRepository.server.ts, the in-memory status filtering after fetching runs from Prisma is intentionally used as a workaround for ClickHouse data delays. This approach is acceptable because the result set is limited to a maximum of 100 runs due to pagination, making the performance impact negligible.

Applied to files:

• internal-packages/run-engine/src/engine/index.ts

📚 Learning: 2026-03-02T12:42:47.652Z

Learnt from: CR
Repo: triggerdotdev/trigger.dev PR: 0
File: apps/supervisor/CLAUDE.md:0-0
Timestamp: 2026-03-02T12:42:47.652Z
Learning: Applies to apps/supervisor/src/env.ts : Environment configuration should be defined in `src/env.ts`

Applied to files:

• apps/webapp/app/env.server.ts

📚 Learning: 2026-04-15T15:39:31.575Z

Learnt from: CR
Repo: triggerdotdev/trigger.dev PR: 0
File: .cursor/rules/webapp.mdc:0-0
Timestamp: 2026-04-15T15:39:31.575Z
Learning: Applies to apps/webapp/**/*.{ts,tsx} : Access environment variables through the `env` export of `env.server.ts` instead of directly accessing `process.env`

Applied to files:

• apps/webapp/app/env.server.ts

📚 Learning: 2026-04-16T14:19:16.309Z

Learnt from: CR
Repo: triggerdotdev/trigger.dev PR: 0
File: apps/webapp/CLAUDE.md:0-0
Timestamp: 2026-04-16T14:19:16.309Z
Learning: Applies to apps/webapp/**/*.server.ts : Access environment variables via `env` export from `app/env.server.ts`. Never use `process.env` directly

Applied to files:

• apps/webapp/app/env.server.ts

📚 Learning: 2026-04-15T15:39:31.575Z

Learnt from: CR
Repo: triggerdotdev/trigger.dev PR: 0
File: .cursor/rules/webapp.mdc:0-0
Timestamp: 2026-04-15T15:39:31.575Z
Learning: Applies to apps/webapp/**/*.test.{ts,tsx} : Do not import `env.server.ts` directly or indirectly into test files; instead pass environment-dependent values through options/parameters to make code testable

Applied to files:

• apps/webapp/app/env.server.ts

📚 Learning: 2026-04-16T14:19:16.309Z

Learnt from: CR
Repo: triggerdotdev/trigger.dev PR: 0
File: apps/webapp/CLAUDE.md:0-0
Timestamp: 2026-04-16T14:19:16.309Z
Learning: Applies to apps/webapp/**/*.test.{ts,tsx} : For testable code, never import `env.server.ts` in test files. Pass configuration as options instead (e.g., `realtimeClient.server.ts` takes config as constructor arg, `realtimeClientGlobal.server.ts` creates singleton with env config)

Applied to files:

• apps/webapp/app/env.server.ts

📚 Learning: 2026-03-27T18:11:57.032Z

Learnt from: nicktrn
Repo: triggerdotdev/trigger.dev PR: 3114
File: apps/supervisor/src/index.ts:83-98
Timestamp: 2026-03-27T18:11:57.032Z
Learning: In `apps/supervisor/src/index.ts`, `RESOURCE_MONITOR_ENABLED` (env var in `apps/supervisor/src/env.ts`) defaults to `false`. As a result, the local `ResourceMonitor`-based `maxResources`/`skipDequeue` gating in `preDequeue` is inactive in compute mode deployments. Do not flag local resource monitor usage in compute mode as a live bug; it has no practical impact unless `RESOURCE_MONITOR_ENABLED` is explicitly set to `true`.

Applied to files:

• apps/webapp/app/env.server.ts

🔇 Additional comments (5)

apps/webapp/app/env.server.ts (1)

838-838: LGTM — default-off flag is wired into the env schema.

The new variable is validated through EnvironmentSchema and defaults to "0", matching the rollout-safe behavior described in the PR.

apps/webapp/app/v3/runEngine.server.ts (1)

22-23: LGTM — option wiring is straightforward and scoped.

The flag is converted once at the RunEngine boundary and uses the existing $replica client already provided to the engine.

internal-packages/run-engine/src/engine/types.ts (1)

148-151: LGTM — option contract documents the important precedence rule.

The comment makes it clear that explicit tx clients override replica routing, which is important for transaction-scoped reads.

internal-packages/run-engine/src/engine/index.ts (1)

1636-1637: LGTM — replica routing preserves transaction override semantics.

tx still takes precedence, and the primary client remains the default unless the feature flag is enabled.

.server-changes/read-replica-snapshots-since.md (1)

1-6: LGTM — rollout note captures the flag and default behavior.

The server-change entry clearly documents the operational purpose and disabled-state behavior.

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Walkthrough

This PR introduces a configuration flag that allows RunEngine.getSnapshotsSince to use a read-only replica database client instead of the primary client for Prisma read operations. The change is controlled by a new environment variable RUN_ENGINE_READ_REPLICA_SNAPSHOTS_SINCE_ENABLED (default: disabled). When enabled, snapshot polling queries are offloaded from the primary database to a replica. The implementation adds the flag to the environment schema, passes it through the RunEngine configuration, extends the RunEngine options type, and modifies the getSnapshotsSince method to conditionally select between the read-only replica and primary client.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main feature being added: a flag to route getSnapshotsSince queries through a read replica.
Description check	✅ Passed	The PR description is comprehensive and follows the template structure with all key sections populated: summary, reasoning, replica-lag considerations, and a detailed test plan.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/read-replica-snapshots-since

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[devin-ai-integration]

Devin Review found 1 potential issue.