# Loom Logging Observability

> Logging and observability patterns for production systems.

- Skill: `cosmix/loom-logging-observability` (Agent Skill)
- Install (CLI): `npx skillmds@latest add cosmix/loom-logging-observability`
- Raw SKILL.md: https://api.skillmd.com/api/skills/cosmix/loom-logging-observability/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: cosmix (https://skillmd.com/u/cosmix)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/cosmix/loom-logging-observability

---


# Logging and Observability

## Overview

Understand system behavior through the three pillars — logs, metrics, traces — correlated by shared IDs. This skill covers structured logging, OpenTelemetry tracing, Prometheus metrics, aggregation backends, and alerting, with emphasis on the cost/cardinality traps and sampling decisions that separate a working setup from an expensive broken one.

## Three Pillars — what each answers, and its cost model

| Pillar      | Answers                              | Cost driver                         | Use for                                        |
| ----------- | ------------------------------------ | ----------------------------------- | ---------------------------------------------- |
| **Metrics** | "Is it broken? how much?" (aggregate) | Label **cardinality** (# series)    | Dashboards, SLOs, alerting — always-on, cheap  |
| **Traces**  | "Where in the request path?" (causal) | Span volume → **sampling**          | Latency breakdown, cross-service dependency    |
| **Logs**    | "What exactly happened?" (event detail) | Volume + **indexing** strategy      | Forensics, audit, the specifics of one request |

Reach for metrics first (cheap, aggregate), traces to localize, logs for the detail. Link all three by `trace_id`/`correlation_id` so you can pivot: alert fires on a metric → jump to an exemplar trace → read that trace's logs.

## Structured Logging

Emit JSON, one object per event — never string-interpolated prose. Structured fields are queryable in any backend; `f"user {id} did {action}"` is not.

```python
import json, logging, sys
from datetime import datetime, timezone
from contextvars import ContextVar

correlation_id: ContextVar[str] = ContextVar("correlation_id", default="")
trace_id: ContextVar[str] = ContextVar("trace_id", default="")

class JsonFormatter(logging.Formatter):
    def format(self, r: logging.LogRecord) -> str:
        data = {
            "ts": datetime.now(timezone.utc).isoformat(),
            "level": r.levelname, "logger": r.name, "msg": r.getMessage(),
            "correlation_id": correlation_id.get(), "trace_id": trace_id.get(),
        }
        if r.exc_info:
            data["exception"] = self.formatException(r.exc_info)
        if hasattr(r, "fields"):
            data.update(r.fields)          # structured extras
        return json.dumps(data)

h = logging.StreamHandler(sys.stdout); h.setFormatter(JsonFormatter())
logging.getLogger().addHandler(h); logging.getLogger().setLevel(logging.INFO)

logging.getLogger(__name__).info("order processed",
    extra={"fields": {"order_id": order.id, "total": order.total}})
```

TypeScript: use a child-logger pattern so request context is bound once and inherited — `pino`/`winston` do this natively; prefer them over hand-rolling.

```typescript
// pino: bound context + fast JSON. child() inherits parent fields.
const log = pino();
const reqLog = log.child({ correlationId, traceId });
reqLog.info({ orderId, total }, "order processed");  // object first, message second
```

### Log levels

| Level | When | Example |
| ----- | ---- | ------- |
| TRACE | Fine-grained, off in prod | Loop iterations, var values |
| DEBUG | Diagnostics, off in prod | Function entry/exit, intermediate state |
| INFO  | Normal business events | Request done, job completed, user action |
| WARN  | Recoverable / degraded | Retry attempted, deprecated API, slow query |
| ERROR | Failure needing attention | Exception caught, operation failed |
| FATAL | Cannot continue | Startup config missing, data corruption |

### Logging discipline (the expensive mistakes)

- **Never log secrets/PII** — passwords, tokens, full card numbers, emails, request bodies. Redact at the formatter (allowlist fields), not by remembering at each call site. PII in logs is a compliance breach (GDPR/PCI) and log stores are rarely access-controlled like a DB.
- **Structured over interpolated** — attach IDs as fields, not baked into the message string, or you can't filter/aggregate by them.
- **Don't log in hot paths synchronously.** A blocking log write per iteration in a tight loop or per-row is a latency cliff. Use async/non-blocking appenders (`QueueHandler` in Python, pino's async transport) and log the summary, not each item.
- **Sample high-volume logs.** For chatty success paths, emit 1-in-N (keep 100% of WARN/ERROR). Reduces cost without losing the signal.
- **Consistent field names across services** — adopt OpenTelemetry semantic conventions for resource and signal attributes; keep any application correlation field stable across services. Cross-service queries depend on it.
- **Emit to stdout as JSON; let the platform collect it.** Don't manage log files/rotation inside the app in a containerized environment — the agent/sidecar (Promtail, Fluent Bit, Datadog agent) tails stdout.

## Distributed Tracing

**Use OpenTelemetry — the vendor-neutral standard (CNCF).** Don't hand-roll spans/tracers: the OTel SDK gives context propagation, batching, and OTLP export for free, and swaps backends (Jaeger, Tempo, Datadog, any OTLP endpoint) without code change. Maturity as of 2026: **tracing is stable/GA**; **metrics stable**; **logs stable spec** but SDK/collector logs support is newer than traces — verify your language SDK's status before relying on OTel logs vs a mature logging lib.

Architecture: **SDK in-process** (creates spans, propagates context) → **OTel Collector** (receive/process/export; the place to do batching, tail sampling, redaction, fan-out to backends). Run the Collector as a sidecar or gateway; keep exporters/sampling config there so apps stay backend-agnostic.

```python
from opentelemetry import trace
from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.sdk.resources import Resource
from opentelemetry.sdk.trace.export import BatchSpanProcessor
from opentelemetry.exporter.otlp.proto.grpc.trace_exporter import OTLPSpanExporter
from opentelemetry.instrumentation.fastapi import FastAPIInstrumentor

provider = TracerProvider(resource=Resource.create({"service.name": "order-service"}))
# OTLP/gRPC accepts an endpoint URL. `insecure=True` is appropriate only for a
# trusted in-cluster plaintext collector; configure TLS explicitly for remote use.
provider.add_span_processor(BatchSpanProcessor(OTLPSpanExporter(endpoint="http://otel-collector:4317", insecure=True)))
trace.set_tracer_provider(provider)
tracer = trace.get_tracer(__name__)

FastAPIInstrumentor.instrument_app(app)   # auto-spans for incoming requests + context extraction

@app.get("/orders/{order_id}")
async def get_order(order_id: str):
    with tracer.start_as_current_span("fetch_order") as span:
        span.set_attribute("order.id", order_id)      # low-cardinality-ok on spans (unlike metrics labels)
        return await repo.get(order_id)
```

### Context propagation — W3C traceparent

A trace spans services only if context crosses the wire. The standard is the **W3C `traceparent` header**: `00-<32-hex trace-id>-<16-hex span-id>-<2-hex flags>` (flags bit 0 = sampled). OTel auto-instrumentation injects it on outgoing HTTP/gRPC and extracts it on incoming — so instrument BOTH the client and server side, or the trace breaks at the boundary and you get orphan traces. For non-HTTP hops (queues, Kafka), propagate `traceparent` as a message attribute manually. Legacy backends may use B3 (Zipkin) headers; configure the propagator to match, or run both.

### Sampling — head vs tail

You cannot afford 100% of spans at volume. Two strategies:

- **Head sampling**: decide at trace *start* (in the SDK), e.g. `ParentBased(TraceIdRatioBased(0.1))` keeps 10%. Cheap, no buffering. **Fatal limitation: the decision is made before the outcome is known**, so you cannot "keep all errors" — a 1% error might be dropped.
- **Tail sampling**: decide *after the whole trace finishes*, in the **Collector's `tailsamplingprocessor`** — keep 100% of error/slow traces + a sample of normal ones. Requires buffering every span of a trace until complete, so **all spans of one trace must reach the same Collector instance**: put a load-balancing exporter (routing by trace-id) in front of the tail-sampling collectors, or they'll each see partial traces and sample wrongly.

Rule of thumb: head-sample for cost control at the edge; add tail sampling in the Collector when you need "always keep errors/slow." Propagate the sampled flag so a downstream service doesn't independently drop spans of a kept trace.

### Correlation & exemplars

Put `trace_id` into every log line's structured fields (bind it in a middleware/context var) so a trace pivots to its logs. Link metrics → traces with **exemplars**: a sampled trace-id attached to a histogram bucket observation, letting Grafana jump from a latency spike on a graph to the exact slow trace. Prometheus needs `--enable-feature=exemplar-storage` and OpenMetrics exposition; the client library attaches the exemplar at `observe()` time.

## Metrics

**Use a real client (`prometheus_client`, OTel metrics) — don't hand-roll registries.** They handle concurrency, exposition format, and label management correctly.

```python
from prometheus_client import Counter, Gauge, Histogram, start_http_server

REQS = Counter("http_requests_total", "Requests", ["method", "route", "status"])
INFLIGHT = Gauge("http_inflight_requests", "In-flight requests")
LAT = Histogram("http_request_duration_seconds", "Latency", ["method", "route"],
                buckets=(.005, .01, .025, .05, .1, .25, .5, 1, 2.5, 5))

start_http_server(9090)                        # exposes /metrics for scrape

@INFLIGHT.track_inprogress()
def handle(req):
    with LAT.labels(req.method, req.route).time():   # times the block
        resp = process(req)
    REQS.labels(req.method, req.route, resp.status).inc()
```

- **Types**: Counter (monotonic totals — `_total`), Gauge (up/down current value — memory, queue depth), Histogram (bucketed distribution — latency; enables `histogram_quantile` percentiles server-side). Prefer Histogram over Summary for latency (Summary quantiles can't be aggregated across instances).
- **Naming (Prometheus)**: base unit suffix — `_seconds`, `_bytes`, `_total`. `http_request_duration_seconds`, not `_ms`.
- **⚠ Label cardinality is the #1 killer.** Each unique label-value combination is a **separate time series** stored in memory. Never use unbounded values as labels — user_id, request_id, email, full URL with IDs, raw error message. One high-card label can create millions of series and OOM Prometheus. Keep labels to bounded sets (method, route *template*, status class). Put the high-card identifier in a **trace or log**, not a metric label.

### Dashboards & SLOs — RED and USE

- **RED** (request-driven services): **R**ate (req/s), **E**rrors (failed req/s or ratio), **D**uration (latency distribution, alert on p95/p99). One RED panel set per service tells you if users are hurting.
- **USE** (resources: CPU, disk, pool, queue): **U**tilization (% busy), **S**aturation (queued/waiting work — the leading indicator), **E**rrors. Saturation usually predicts trouble before utilization saturates.
- Golden Signals (Google SRE) = latency, traffic, errors, saturation — RED + saturation. Alert on symptom signals (RED), use USE to diagnose the cause.

## Log Aggregation — and the indexing cost trap

**The single biggest cost/architecture decision: index everything vs index labels only.**

- **ELK / Elasticsearch / OpenSearch**: indexes *every field* → fast arbitrary full-text and field queries, but storage and compute scale with total log volume (expensive at scale; index bloat, hot-node pressure).
- **Grafana Loki**: indexes *only labels* (stream metadata), stores log content compressed and **unindexed** → cheap ingest/storage; content queries are brute-force scans over the time-and-label-narrowed set (`{app="api"} |= "timeout"`). Great when you filter by labels then grep; slow if you need ad-hoc full-text over everything.
- **Loki cardinality trap (same killer as metrics):** every unique label-value set is a separate *stream*. Putting `trace_id`/`user_id`/`pod_ip` as a Loki **label** explodes stream count and destroys performance. Keep labels low-cardinality (app, env, level, namespace); filter high-card values as *content* in the query, not as labels.
- **Datadog / hosted**: priced per ingested/indexed GB — use ingestion filters/exclusion rules (e.g. drop health-check logs) and index only what you'll query.

```yaml
# Promtail → Loki: LOW-cardinality labels only; do NOT add trace_id/user_id as labels
scrape_configs:
  - job_name: app
    static_configs: [{ targets: [localhost], labels: { job: app, __path__: /var/log/app/*.log } }]
    pipeline_stages:
      - json: { expressions: { level: level, service: service } }
      - labels: { level:, service: }        # bounded sets only
```

```yaml
# Datadog agent: exclude noise at ingest to control cost
logs_config:
  processing_rules:
    - type: exclude_at_match
      name: drop_healthchecks
      pattern: "GET /health"
```

## Alerting

```yaml
groups:
  - name: service
    rules:
      - alert: HighErrorRate                # symptom, not cause
        expr: sum(rate(http_requests_total{status=~"5.."}[5m])) / sum(rate(http_requests_total[5m])) > 0.05
        for: 5m
        labels: { severity: critical }
        annotations:
          summary: "Error rate {{ $value | humanizePercentage }}"
          runbook_url: "https://wiki/runbooks/high-error-rate"     # every alert links a runbook
      - alert: HighLatencyP95
        expr: histogram_quantile(0.95, sum(rate(http_request_duration_seconds_bucket[5m])) by (le)) > 1
        for: 10m
        labels: { severity: warning }
      - alert: ServiceDown
        expr: up == 0
        for: 1m
        labels: { severity: critical }
```

| Severity | Response | Examples |
| -------- | -------- | -------- |
| Critical | Page now | Service down, error-rate SLO breach, data loss |
| Warning  | Business hrs | Rising latency, approaching limits, retry spikes |
| Info     | Log only | Deploy started, config changed |

Principles:

- **Alert on symptoms (user impact), not causes.** Page on error rate/latency (RED); CPU high with healthy latency is not an incident. Symptoms say *what's broken*; use USE/traces to find *why*.
- **Every alert is actionable and has a runbook.** Non-actionable alerts cause fatigue → ignored pages. Delete or downgrade alerts nobody acts on.
- **Thresholds from SLOs, not vibes.** Prefer **SLO burn-rate alerts** (fast burn = page, slow burn = ticket) over static thresholds — they alert proportional to error-budget consumption and cut false pages during low traffic.
- **`for:` avoids flapping** — require the condition to hold before firing.

## Request middleware (correlation + span + metrics in one place)

```python
class ObservabilityMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        corr = request.headers.get("X-Correlation-ID", str(uuid.uuid4()))
        correlation_id.set(corr)
        span = trace.get_current_span()               # created by FastAPIInstrumentor
        span.set_attribute("correlation_id", corr)
        trace_id.set(format(span.get_span_context().trace_id, "032x"))  # into logs
        start = time.perf_counter()
        try:
            resp = await call_next(request)
            LAT.labels(request.method, request.url.path).observe(time.perf_counter() - start)
            REQS.labels(request.method, request.url.path, str(resp.status_code)).inc()
            resp.headers["X-Correlation-ID"] = corr
            return resp
        except Exception as e:
            span.record_exception(e); span.set_attribute("error", True)
            raise
```

Note: use `url.path` *template* (route pattern), not the raw path with IDs, as the metric label — else cardinality explodes.

## Verification Checklist

- [ ] Logs are JSON to stdout with consistent field names; `correlation_id` + `trace_id` on every line
- [ ] No secrets/PII in logs; redaction is enforced at the formatter, not per-call-site
- [ ] Hot-path logging is async/sampled; success paths sampled, 100% of WARN/ERROR kept
- [ ] Tracing uses the OTel SDK; both client and server sides instrumented so `traceparent` propagates (no orphan traces)
- [ ] Sampling chosen deliberately: head for cost, tail (in Collector, with trace-id load balancing) if "always keep errors"
- [ ] Metric labels are bounded — no user_id/request_id/raw-path/error-string as labels
- [ ] Latency uses Histogram with sensible buckets; percentiles computed via `histogram_quantile`
- [ ] Dashboards follow RED (services) / USE (resources); metric names carry base-unit suffixes
- [ ] Loki labels are low-cardinality (no trace_id/user_id as labels); backend indexing cost understood (Loki labels-only vs ELK index-all)
- [ ] Alerts fire on symptoms, have `for:`, link a runbook, and derive thresholds from SLOs (prefer burn-rate)
- [ ] Logs, metrics, and traces are cross-linkable by shared IDs; exemplars wired if metric→trace pivot is needed