feat(observability): export database connection pool stats on /metrics

[daavoo]

Description

When Otari runs on PostgreSQL it serves requests from a fixed pool of database connections. If that pool runs out, every request fails at once, and until now there was no way to see it coming: nothing in /metrics said how many connections were in use or how many the pool would ever hand out, so the first sign of trouble was a gateway answering 503 and no way to tell from the outside whether the database or the pool was the problem.

This adds four gauges to /metrics, labeled by which pool they describe (request for request traffic, log for the usage-log writer): connections checked out, connections idle, connections open beyond the base pool size, and the ceiling the pool will never exceed. An operator can now alert on checked-out connections approaching that ceiling and act before requests start failing. docs/deployment.md says how.

Nothing about how Otari serves requests changes. The numbers are the ones SQLAlchemy already keeps, read at scrape time, so a scrape reports the live pool rather than a value a timer left behind. They do not appear on SQLite, which opens a connection per use and keeps no pool to report.

What changed after review

An earlier version of this branch also made the readiness probe answer 503 as soon as the pool was full. That is out, on review: one instantaneous reading cannot tell a pool that is briefly full from one that is stuck, so under load every replica would have gone unready together and a traffic spike would have become an outage. A readiness check that judges the pool over time is tracked separately in #1247.

Fixes #1240

How to test it locally

Requires PostgreSQL to see the gauges at all; on SQLite the endpoint correctly reports no pool series.

# Unit coverage for the stats reader and the collector
uv run pytest tests/unit/test_database_pool_stats.py tests/unit/test_gateway_metrics.py -q

# Against a running gateway with OTARI_ENABLE_METRICS=true and a PostgreSQL database
curl -s localhost:8000/metrics | grep gateway_db_pool

Already run locally and green: make lint, make typecheck, the full uv run pytest tests/unit (3654 passed), make openapi-check, make postman-check, and the OSS-edition smoke gate (uv run --frozen --no-dev python scripts/oss_edition_smoke.py). The integration suite was left to CI.

PR Type

• New Feature
• Bug Fix
• Refactor
• Documentation
• Infrastructure / CI

Relevant issues

Fixes #1240. Follow-up: #1247.

Checklist

• I understand the code I am submitting.
• I have added or updated tests that cover my change (tests/unit, tests/integration).
• I ran the Definition of Done checks locally (make lint, make typecheck, make test). make lint, make typecheck and the full unit suite are green; the integration suite was left to CI.
• Documentation was updated where necessary. docs/deployment.md documents the new metrics.
• If the API contract changed, I regenerated the OpenAPI spec (uv run python scripts/generate_openapi.py). No API surface changes now that the readiness change is out, and the spec, the Postman collection and the dashboard client are back to their main contents; make openapi-check and make postman-check pass.

AI Usage

• No AI was used.
• AI was used for drafting/refactoring.
• This is fully AI-generated.

AI Model/Tool used:

Claude Opus 5 (1M context) via Claude Code

Any additional AI details you'd like to share:

The metric names and label set were pinned into the existing test_scrape_exposes_the_pinned_families test, because a scrape is an external contract that dashboards and alerts outside this repository key on.

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Summary

• Added PostgreSQL connection-pool metrics for request and usage-log pools.
• Added unit tests for pool statistics and Prometheus output.
• Updated gateway metrics coverage and deployment guidance.
• Documented pool capacity and SQLite limitations.

These changes make connection leaks easier to detect and support operational alerts before the pool is exhausted.

Technical notes

• Metrics report checked-out, idle, overflow, and capacity connections.
• Pool capacity includes configured overflow for the request pool.
• SQLite does not expose these pooled-connection metrics.

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⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

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📥 Commits

Reviewing files that changed from the base of the PR and between 34e1092 and 4a84b0b.

📒 Files selected for processing (5)

• docs/deployment.md
• src/gateway/core/database.py
• src/gateway/metrics.py
• tests/unit/test_database_pool_stats.py
• tests/unit/test_gateway_metrics.py

Walkthrough

The gateway now reports live PostgreSQL connection-pool statistics through Prometheus gauges. Unit tests cover pool readings and metric collection. The deployment checklist documents pool capacity, alerting guidance, and SQLite behavior.

Changes

Database pool observability

Layer / File(s)	Summary
Pool statistics and lifecycle tracking src/gateway/core/database.py, tests/unit/test_database_pool_stats.py	Adds typed pool statistics, capacity calculation, support for request and log pools, handling for unsupported pools, lifecycle cleanup, and unit coverage.
Prometheus pool metric collection src/gateway/core/database.py, src/gateway/metrics.py, tests/unit/test_gateway_metrics.py, tests/unit/test_database_pool_stats.py	Registers gauges for checked-out, idle, overflow, and capacity connections. The metric contract and collector behavior tests cover pool labels and scrape-time values.
Connection-pool monitoring guidance docs/deployment.md	Documents PostgreSQL pool metrics, capacity formulas, alerting guidance, and the absence of these metrics on SQLite.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Feature · Severity of issue fixed: Medium

Merge Risk: 🟡 Moderate · up to 34e10

A regression that leaks inference-route database connections would not be detected by these tests. Add the intended PostgreSQL integration coverage before merging.

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	Issue #1241 requires an integration test that uses the real PostgreSQL pool on a data-plane inference route. The reviewed changes add unit tests with _QueuePool stand-ins and test pool metrics. They…	Add a PostgreSQL integration test for a data-plane inference request. Observe the actual startup pool used by that route. Assert connection release at the teardown point in the same task, before orphaned-session or runtime cleanup can run. …
Docstring Coverage	⚠️ Warning	Docstring coverage is 36.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 30 functions across 4 files. (1 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Out of Scope Changes check	✅ Passed	The added pool counters, collector, unit tests, metrics exports, and deployment documentation all concern PostgreSQL connection-pool visibility. They support the pool-state test objective in issue #12…
Title check	✅ Passed	The title uses the Conventional Commit prefix "feat(observability):", clearly describes exporting database connection-pool statistics through metrics, uses imperative wording, and is approximately the…
Description check	✅ Passed	The description is complete and follows the repository template. It explains the user impact, testing steps, PR type, linked issues, checklist status, documentation update, API contract status, and AI…

Full details: Linked Issues check

Explanation

Issue #1241 requires an integration test that uses the real PostgreSQL pool on a data-plane inference route. The reviewed changes add unit tests with _QueuePool stand-ins and test pool metrics. They do not show a PostgreSQL request test, an assertion on the startup engine used by inference routes, or a same-task teardown assertion before runtime cleanup. The existing integration metrics test does not provide this lifecycle check.

Resolution

Add a PostgreSQL integration test for a data-plane inference request. Observe the actual startup pool used by that route. Assert connection release at the teardown point in the same task, before orphaned-session or runtime cleanup can run. Keep the check PostgreSQL-specific and exclude SQLite.

Full details: Docstring Coverage

Explanation

Docstring coverage is 36.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 30 functions across 4 files. (1 skipped: 1 unsupported.)

✨ Finishing Touches 💡 1

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• Commit unit tests in branch feat/db-pool-observability

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch feat/db-pool-observability

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[peteski22]

Drafted with Claude Code (Claude Opus 5), then checked by a human before posting.

The pool metrics cover the first gap in #1240. Four things before merge:

1. Move the pool gauges from metrics.py to core/database.py, so they follow the pattern from #1205.
2. Drop the readiness change. One reading cannot tell a busy pool from a stuck one. #1247 tracks the design.
3. Take pool capacity from config, not from a private SQLAlchemy field.
4. The PR description is for a different change. It says test-only and Fixes #1241, so merging would close #1241 without its integration test. The code here fixes #1240.

The older log and raise in the SELECT 1 branch is #1246, so it stays out of this PR.

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Actionable comments posted: 1

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Inline comments:
In `@tests/unit/test_database_pool_stats.py`:
- Around line 120-142: Add integration coverage for the PostgreSQL-backed
inference route that sends a real POST /completions request through provider
dispatch and verifies release_session(db) runs at request teardown. Keep the
existing pool-statistics tests unchanged, and assert the connection is returned
after the inference request completes.

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Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

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📥 Commits

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📒 Files selected for processing (5)

• docs/deployment.md
• src/gateway/core/database.py
• src/gateway/metrics.py
• tests/unit/test_database_pool_stats.py
• tests/unit/test_gateway_metrics.py

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