Launch gateway hardening: probes, retries, sidecar CPU, DB pools, rate limiting

[lorenzo-norcini-scale]

Consolidated hardening from the 2026-08-13 gateway incident (per-tenant rate limiting tracked in MLI-8236, sync-forwarder timeout in MLI-8206). Supersedes #864, #865, #866 and #867.

Changes

Readiness (app + chart). healthcheck is now async def, so the /readyz probe no longer queues behind blocked threadpool requests; the probe gets an explicit timeoutSeconds (default 5, gateway.readinessProbeTimeoutSeconds). This removes the mechanism that ejected saturated-but-healthy pods (11/60 Ready during the incident). No livenessProbe is added: restarting a saturated pod discards in-flight work.

Async-task polls off the shared threadpool (app). GET /v1/async-tasks/{task_id} does a blocking result-backend read (S3 on AWS) and previously ran in the shared anyio threadpool, so a growing backlog's poll load starved every other sync route. The handler is now async and dispatches the read through a dedicated CapacityLimiter(40).

Retry policy (chart). VirtualService retries no longer include 503 (retryOn: connect-failure,unavailable,502,504); retrying the overload signal roughly 4x-ed offered load at saturation. Policy configurable under gateway.retries; perTryTimeout unset by default because the route carries streaming requests.

Sidecar CPU request (chart). sidecar.istio.io/proxyCPU: 250m on gateway pods. Without it, sidecars starved on CPU-packed nodes during scale-out and new pods never became Ready.

Lazy DB engines (app). DBManager built five engines per worker process eagerly (sync/async x RW/RO + NullPool); the gateway uses only the async pair. Engines are now built per kind on first use, cutting idle Postgres pools per pod by more than half and making scale-out stop storming the reader's connection limit.

Per-pod proxy rate limits (chart, default off). Values-gated EnvoyFilter installing local_ratelimit on the gateway sidecars: per-pod token buckets driven by a rateLimits.routes list (overflow 429s at the proxy without consuming a gateway worker) plus a throttledTenants list that clamps named callers on the async-task routes across both accepted Authorization forms, replacing hand-authored fault-injection during incidents.

Per-user rate limiting (app, default off). Redis-backed fixed-window limiter as a per-route FastAPI dependency (user_rate_limit(route_class), composed with authentication), using the existing aioredis pool with a cached client; counter increment and TTL are one atomic Lua eval; 429 + Retry-After. Configured via user_rate_limits in the service config with a log-only rollout mode. Fails open on any Redis error or a check exceeding 100ms; outage and log-only warnings are sampled.

Sync forwarder timeout (from #864). Explicit, configurable timeout on the sync forwarder.

Docs. Async-task polling guidance using the guide's existing tenacity idiom (jittered exponential backoff, bounded outstanding set).

Known limitation

Per-user limits key on the resolved user_id. While prod authentication falls back to FakeAuthenticationRepository (plugins package unimportable in the internal image), any string authenticates, so a caller could rotate credentials past user-keyed limits; the identity-blind per-pod buckets are the backstop. Fixing the plugins packaging is a separate security work item in the internal repo.

Verification

• tests/unit: all passing (one pre-existing test requires the WORKSPACE env var). Parametrized unit tests for the limiter: enforce vs log-only, unconfigured routes, fail-open on error and on timeout.
• Chart rendered with default and override values: probe timeout, retry block, proxyCPU annotation, and EnvoyFilter (route buckets, tenant clamp path-scoped, rendered Basic prefix matches the incident-derived value) all emit as expected; EnvoyFilter absent when disabled.
• black / ruff / isort / mypy pass on all touched files.
• Reviewed by 4 parallel cleanup agents plus 3 rounds of external review; all findings applied or explicitly deferred (security item above).

🤖 Generated with Claude Code

Greptile Summary

This PR consolidates gateway incident hardening across readiness, retry behavior, proxy and application rate limiting, async polling isolation, database engine lifecycle, and forwarding deadlines.

• Makes health probes and async-task polling independent of the shared request threadpool.
• Adds configurable Envoy and Redis-backed rate limits, proxy CPU requests, retry policy, and forwarding deadlines.
• Adds deployment configuration, documentation, and unit coverage for the new behavior.

Confidence Score: 2/5

The PR does not appear safe to merge while expired database sessions leak async pools and forged credentials can exhaust legitimate tenants' proxy rate-limit buckets.

Credential refresh still invokes asynchronous engine disposal without awaiting it, leaving superseded pools alive, while the tenant clamp still selects and consumes buckets from an unauthenticated raw Authorization header before FastAPI verifies the caller.

Files Needing Attention: model-engine/model_engine_server/db/base.py; charts/model-engine/templates/istio-ratelimit-envoyfilter.yaml

Important Files Changed

Filename	Overview
charts/model-engine/templates/istio-ratelimit-envoyfilter.yaml	Adds per-pod route and tenant local-rate-limit descriptors to inbound gateway sidecars.
model-engine/model_engine_server/api/rate_limits.py	Implements a fail-open Redis fixed-window limiter with enforcement and log-only modes.
model-engine/model_engine_server/api/tasks_v1.py	Moves blocking async-result reads to a dedicated limited threadpool and attaches per-user rate-limit dependencies.
model-engine/model_engine_server/inference/forwarding/celery_forwarder.py	Applies the configured forwarding deadline as a Celery hard limit to both current and legacy task registrations.
model-engine/model_engine_server/db/base.py	Restores eager database-engine construction and the original credential-refresh lifecycle.
charts/model-engine/templates/gateway_deployment.yaml	Configures the gateway readiness timeout and Istio sidecar CPU request.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  C[Caller] --> E[Gateway Envoy sidecar]
  E --> A[FastAPI gateway]
  A --> R[(Redis rate-limit state)]
  A --> D[(PostgreSQL)]
  A --> Q[Celery queue]
  Q --> W[Forwarder worker]
  W --> U[User model service]

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