Module 13 — Production-Grade Systems

Time: 2–3 weeks (alongside a real project) · Depends on: 04, 07, 10 · Next: Compliance

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Learning objectives

• Serve models behind stable APIs with timeouts, retries, and fallbacks
• Instrument the request path with metrics, traces, and structured logs
• Version prompts and model IDs so deploys are reproducible and roll-backable
• Ship a containerized inference service with a realistic CI gate

What you can build

• FastAPI (or similar) inference service with /healthz and /v1/generate
• Latency / error / token dashboards wired to request IDs
• Blue/green or canary prompt versions with eval gates in CI

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Why this matters (CS engineer)

Friday 16:40. Support chat p95 jumps from 1.2s to “hung.” The provider is rate-limiting; your SDK default has no timeout. Workers pile up, health checks still pass (process is “up”), autoscaler adds pods that also hang, and the bill spikes from retries without jitter. Nobody can answer “what did user X see?” because logs have no shared request_id — only “the bot was weird.” Someone had also hot-edited the system prompt in the dashboard that morning; there is no version pin to roll back.

Gate 5 of the running app: everything above worked on a laptop with one user. This is the failure that forces the rest — every other Gate-5 module exists to make this incident debuggable instead of mysterious.

In class, a notebook cell that calls an LLM “works.” In production, that same call is a distributed dependency: network timeouts, rate limits, model outages, prompt regressions, secret leaks in logs, and unbounded token spend. Your job is not “call the model” — it is to put a contracted, observable, fail-soft service around stochastic generation.

Treat the LLM like any other unreliable remote system (payment gateway, search index), with three extra twists:

1. Outputs are non-deterministic — you need evals (Module 04), not only unit tests.
2. Prompts are code — they need versioning, review, and rollback.
3. Cost is continuous — every retry and every missing timeout is money.

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Mental model

flowchart LR
  Client -->|HTTP + request_id| API[App tier FastAPI]
  API -->|auth / rate limit| GW[Policy gate]
  GW -->|timeout| Primary[Primary model]
  Primary -->|429 / 5xx| Retry[Retry + jitter]
  Retry -->|still failing| Fallback[Fallback model / degrade]
  API --> Obs[Logs + metrics + traces]
  Obs --> Dash[Dashboard / alerts]
  CI[CI: lint · unit · golden eval] --> Img[Container image]
  Img --> API

Invariant: every egress call has a timeout; every public route has authz + rate limits; every request has a request_id you can grep from UI ticket → logs → provider span.

Intuition lock

Sticky picture: an LLM call is remote I/O (like a payment gateway), not a local function. Timeouts are blast-radius walls. A prompt version is deployable config (same as a feature flag). A request_id is the flight recorder that ties UI complaint → your logs → provider span.

Kill this idea: “It works in the notebook, so we just need a public HTTP route.” Without timeouts, pins, and request IDs you have a demo endpoint, not a production service.

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Execution flow, with cost and latency overlaid

The Mental model diagram above is the system view. Zoom into one request and every box is somewhere your request_id can stall, retry, or spend money — this is the shape 22 — Evaluating agentic systems formalizes into a trace schema once tool calls enter the picture:

flowchart LR
  Req[Request] --> Router[Router] --> PV[Prompt version] --> Model[Model call]
  Model --> SV{Structured validator}
  SV -->|invalid, retry budget left| Model
  SV -->|invalid, budget exhausted| Fail[Fail closed]
  SV -->|valid| Tool{Tool call requested?}
  Tool -->|no| FV[Final validator] --> Out[Response to caller]
  Tool -->|yes| Authz{Authorization}
  Authz -->|denied| Fail
  Authz -->|approved| Call[Tool] --> Result[Tool result] --> Model

Every hop on this path carries four numbers you should be logging per request_id, not just per service:

Hop	Latency	Tokens / cost	Retries	Failure mode if unmeasured
Model call	Time to first token + total	Input + output tokens	Count against budget	Silent timeout, hung worker
Structured validator	Parse time (usually negligible)	—	Counts toward the same retry budget as the model call	Invalid JSON passed downstream
Authorization	Should be near-zero	—	None — deny is terminal, not retried	A denied action silently retried until approved by accident
Tool call	Its own p95, independent of the model's	Often billed separately (API calls, compute)	Its own budget, capped below the agent's total step cap	Tool timeout mistaken for model timeout in logs

Invariant: a retry at any hop still has to respect the end-to-end deadline for the whole request — retrying the model call after the tool call already burned most of the budget is how "one retry" becomes a timeout anyway.

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1. Serving skeleton (FastAPI)

Start with a thin, stateless API. Business logic and provider SDKs live behind clear boundaries so you can swap models without rewriting HTTP glue.

from fastapi import FastAPI, HTTPException, Request
from pydantic import BaseModel, Field
import os
import uuid
import time
import logging

log = logging.getLogger("llm.api")
app = FastAPI(title="AI Engineering Demo API")


class GenerateRequest(BaseModel):
    prompt: str = Field(min_length=1, max_length=20000)
    user_id: str = "anonymous"
    # pin client-visible options carefully; prefer server-side policy
    max_tokens: int = Field(default=512, ge=1, le=4096)


class GenerateResponse(BaseModel):
    text: str
    model: str
    request_id: str
    latency_ms: int


@app.middleware("http")
async def add_request_id(request: Request, call_next):
    rid = request.headers.get("x-request-id") or str(uuid.uuid4())
    request.state.request_id = rid
    started = time.perf_counter()
    response = await call_next(request)
    response.headers["x-request-id"] = rid
    elapsed_ms = int((time.perf_counter() - started) * 1000)
    log.info(
        "request done",
        extra={
            "request_id": rid,
            "path": request.url.path,
            "status": response.status_code,
            "latency_ms": elapsed_ms,
        },
    )
    return response


@app.get("/healthz")
def healthz():
    # liveness: process is up. readiness can check provider credentials separately
    return {"ok": True}


@app.post("/v1/generate", response_model=GenerateResponse)
def generate(req: GenerateRequest, request: Request):
    rid = getattr(request.state, "request_id", "unknown")
    if not os.getenv("OPENAI_API_KEY") and not os.getenv("ANTHROPIC_API_KEY"):
        raise HTTPException(503, "No model credentials configured")

    t0 = time.perf_counter()
    # call provider with timeout; catch and map errors (see §2)
    text = f"echo:{req.prompt[:200]}"  # replace with real client
    model = "demo"
    latency_ms = int((time.perf_counter() - t0) * 1000)
    return GenerateResponse(
        text=text, model=model, request_id=rid, latency_ms=latency_ms
    )

Production checklist (app tier)

• Timeouts on all egress (provider, tools, vector DB)
• Retries with jitter — idempotent paths only
• Fallback model or degraded mode (cached / templated)
• Authn/z on public routes; no open generate endpoints
• Rate limits per user / API key
• Structured JSON logging; no secrets or full PII in logs
• Horizontal scale of stateless app tier; sticky session not required

Explainer · healthz vs readiness

/healthz (liveness) answers: “Should the orchestrator kill and restart me?” Keep it cheap — process up, event loop alive. A readiness probe answers: “Can I take traffic?” That may fail closed when the provider key is missing or the circuit breaker is open. Do not make liveness depend on OpenAI being happy, or a provider outage will thrash your pods.

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2. Timeouts, retries, fallbacks

Provider SDKs default to “wait a long time.” That is wrong for interactive UX. Cap wait time; map exceptions to your HTTP status model.

import logging
import time
from typing import Callable

log = logging.getLogger("llm")


class RateLimitError(Exception):
    pass


class ProviderOutage(Exception):
    pass


def call_model_with_fallback(
    prompt: str,
    primary: Callable[[str], str],
    backup: Callable[[str], str],
    *,
    retries: int = 3,
) -> str:
    """Retry primary on rate limits; fall back on outage."""
    last_err: Exception | None = None
    for attempt in range(retries):
        try:
            return primary(prompt)
        except RateLimitError as e:
            last_err = e
            # exponential backoff + jitter in real code
            log.warning("rate_limited attempt=%s", attempt + 1)
            time.sleep(2**attempt)
        except ProviderOutage:
            log.error("primary_outage; using backup")
            return backup(prompt)
    raise RuntimeError(f"primary failed after retries: {last_err}")

Provider symptom	Your API	Client expectation
Validation / bad input	422	Fix request
Rate limited after retries	429 + Retry-After	Back off
Provider / fallback down	503	Temporary; retry later
Auth failure to your API	401 / 403	Fix credentials

Circuit breaker (concept): after N consecutive primary failures, stop calling primary for a cool-down window; serve backup or fail fast. Prevents cascading load on a dying dependency.

Think · retry safety

Reveal: when is retrying an LLM call dangerous?

Retries are safe when the call is **read-only** or **idempotent** from the product’s perspective (e.g. summarize a document already stored). They are dangerous when a successful first call already **side-effected** (sent email, charged a card, wrote a ticket) and the client timed out before seeing the response. For tool-using agents, use idempotency keys, outbox patterns, or “check then act” before retrying the whole plan.

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3. Observability: metrics, traces, logs

You cannot debug “the bot was weird yesterday” without three signals sharing a request_id.

Signal	Examples for LLM apps
Metrics	QPS, p50/p95/p99 latency, error rate by status, tokens in/out, $, cache hit %, fallback rate
Traces	Request → auth → retrieve → rerank → generate → validate
Logs	request_id, hashed user_id, template_version, model_id, tool names, latency stages

Tools to know: OpenTelemetry (vendor-neutral instrumentation), Prometheus/Grafana, Langfuse / Phoenix / Helicone (LLM-specific), provider dashboards.

Minimal structured log shape

import json
import time


def log_generation(
    *,
    request_id: str,
    model: str,
    template_version: str,
    tokens_in: int,
    tokens_out: int,
    latency_ms: int,
    status: str,
) -> None:
    print(
        json.dumps(
            {
                "ts": time.time(),
                "event": "generation",
                "request_id": request_id,
                "model": model,
                "template_version": template_version,
                "tokens_in": tokens_in,
                "tokens_out": tokens_out,
                "latency_ms": latency_ms,
                "status": status,
            }
        )
    )

Redaction rule: prefer hashes or truncated previews of prompts over full bodies in default logs. Store full transcripts only in access-controlled stores with retention policy (Module 14).

Explainer · the three questions

On-call should answer in under five minutes:

1. Is it broken? (error rate / SLO burn)
2. Is it slow? (p95 latency, which span?)
3. Is it expensive or wrong? (token spike, fallback rate, eval drift)

If your dashboard only has “number of requests,” you will guess. Instrument the stages you own: retrieve, generate, validate.

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4. CI/CD for prompts and models

PR → lint / typecheck / unit tests
   → golden eval subset (Module 04)
   → build image (git SHA tag)
   → deploy staging → smoke (/healthz + 1 generate)
   → prod (canary or blue/green)

Pin prompt template version and model id in config (env, feature flag, or config service) — not buried in ad-hoc strings across files.

# config example — load from env or remote config
PROMPT_VERSION = "support_reply@v3"
PRIMARY_MODEL = "gpt-4o-mini"      # placeholder ids — see Setup
FALLBACK_MODEL = "claude-haiku-xxxxxx"

Canary idea: route 5% of traffic to support_reply@v4; compare eval score + human thumbs + error rate; promote or roll back by flipping config, not redeploying code if the template is externalized.

Think · prompt as release artifact

Reveal: what must travel with a prompt version when you roll back?

Rollback is not “paste the old string into the dashboard.” You need the **template id/version**, the **model id**, decoding defaults (temperature, max tokens), and any **tool allowlist / policy version** that template assumed. If v4 added a new tool and you only revert the prose, the agent still has the wider blast radius. Treat the pin as a small **config bundle**, canaried and rolled back together.

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5. Docker sketch

FROM python:3.11-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt
COPY src ./src
ENV PORT=8000
# do not bake secrets into the image
CMD ["uvicorn", "src.api:app", "--host", "0.0.0.0", "--port", "8000"]

• Run as non-root in real images; pin base digests when you harden.
• Inject OPENAI_API_KEY / ANTHROPIC_API_KEY at runtime (K8s secrets, cloud secret manager).
• Tag images with git SHA so “what is in prod?” is answerable.

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Failure modes

Failure	Symptom	Fix
No timeout	Hung workers, cascade	Bound every HTTP client
Retry storms	Amplified outages	Jitter, circuit breaker, bulkheads
Prompt edit in prod by hand	Mystery regressions	Versioned templates + PR + eval
Logs contain prompts + secrets	Compliance incident	Redact; separate secure transcript store
Single model provider	Total outage	Fallback model / degrade mode
Eval only on laptop	Silent quality drop	Golden suite in CI (subset) + nightly full

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Lab

Lab · ship a production-shaped endpoint

1. Put one module feature behind FastAPI with /healthz and /v1/generate (or your path).
2. Add request_id middleware and JSON structured logs (no raw secrets).
3. Implement call_model_with_fallback (can stub primary/backup).
4. Write a Dockerfile; build and run locally.
5. Add a 20-case golden eval that can run in CI (nightly if costly; PR subset of 5).

Capture: p95 latency under a small load script, and a greppable request_id from log line → response header.

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Quizzes

Quiz · 25 XP

What is the single most important default for every LLM provider call in production?

Temperature = 0 so outputs are deterministic An explicit client timeout (and mapped failure handling) Logging the full prompt body for every request Retrying infinitely until the provider responds

One attempt grades immediately. Correct answers award XP.

Quiz · 25 XP

Why pin prompt template versions and model IDs in config rather than hardcoding them only in scattered call sites?

Providers require config files for billing So you can reproduce, canary, and roll back behavior without archaeology Config files make the model more accurate It removes the need for evals in CI

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OSS & further materials

Resource	Why
FastAPI	Async-friendly API skeleton used above
OpenTelemetry	Traces/metrics/logs standard
Langfuse / Arize Phoenix	LLM tracing & eval UIs
Twelve-Factor App	Config, logs, disposability habits
Course Modules 04, 10	Evals and unit economics

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Checkpoint

• Timeouts + fallback path exist on the critical generate path
• You can diagnose a slow request from logs/traces via request_id
• Deploy is reproducible from a git SHA (image + config pins)
• At least a small golden eval runs before promote

Mark Module 13 complete when the lab and checkpoint are honest — not when you only skimmed.

Complete module · +120 XP

Exercise

• Catalog: EX-13 — Production endpoint
• Prove: /healthz plus a generate/triage path with a deadline and a mapped fallback. Prefer growing capstone-starter/ over a greenfield app.
• Test: cd capstone-starter && pytest tests/test_api.py -v

Next: Module 14 — Compliance & governance