Agent Skills: Prometheus Metrics for Python/Flask Apps

Prometheus Metrics for Python/Flask Apps

This skill wires Prometheus metrics into a Python/Flask application safely under multi-worker gunicorn and adds an optional Redis-backed pattern for cross-worker business metrics.

When to Use This Skill

Use this skill when:

• Adding a /metrics endpoint to a Python/Flask service
• The service runs (or might run) under gunicorn with more than one worker
• You need cross-worker business metrics (signups, jobs, queue depth, etc.) and Redis is available
• You are debugging inflated rate() values or phantom traffic in PromQL — counter values appear to "reset" because scrapes bounce between workers with private registries

The Multi-Worker Gunicorn Trap (Read This First)

prometheus_client keeps application metrics (Counter, Histogram, Gauge) in per-process memory. Under gunicorn with N workers, each worker has its own private registry. Prometheus scrapes hit a random worker each time, so:

• Counter values appear to "reset" as scrapes bounce between workers → PromQL rate() manufactures phantom traffic (often 5–10× the real rate for a 4-worker setup)
• Gauges that should be aggregate-able (e.g. queue depth) report whichever worker answered the scrape — not the cluster-wide value
• Histograms double-count or miss buckets the same way

What does NOT prove this bug: process_start_time_seconds, python_info, process_cpu_seconds_total, python_gc_*, and other metrics from ProcessCollector / PlatformCollector / GCCollector will always return N distinct values across scrapes — once per worker PID. prometheus_client does not aggregate those collectors across processes by design, regardless of whether multiproc is wired correctly. Diagnose using application metrics (Counter/Histogram/Gauge you declared), not process metrics.

Fix: every Step in this skill assumes multi-worker. The pieces are:

1. prometheus_flask_exporter.GunicornPrometheusMetrics (multiproc-aware)
2. PROMETHEUS_MULTIPROC_DIR env var set, dir cleaned on startup
3. gunicorn_conf.py with child_exit hook so dead workers release their shards
4. Custom Gauges declare multiprocess_mode='livesum' (or similar) — or they are silently dropped
5. Redis as the source-of-truth for cross-worker business counters (optional)

Skip any of these and metrics will look fine in dev (single process) and silently wrong in prod.

What This Skill Creates

1. requirements.txt entries — prometheus_client, prometheus_flask_exporter, optionally redis
2. Metrics initialization in the main app file, wired through create_app() (post-fork)
3. gunicorn_conf.py — child_exit hook for multiproc shard cleanup
4. Deploy snippets — Dockerfile/docker-compose/systemd showing PROMETHEUS_MULTIPROC_DIR setup + startup dir cleanup
5. Custom metric examples — file-multiproc-safe (with Gauge mode) and Redis-backed (cross-worker aggregate)
6. Starter Grafana dashboard JSON — references/starter-dashboard.json, importable as-is
7. Prometheus scrape config snippet — for whoever runs the Prometheus server
8. Verification checklist — counter monotonicity and rate() sanity post-deploy

Step 1: Gather Project Information

IMPORTANT: Before making changes, ask the user these questions:

1. "What is your application tag/name?" (e.g., "my-api", "ingest-worker")
   • This is the application label on all metrics — must match the application_tag you use in [[byteforge-loki-logging]] so logs and metrics correlate in Grafana
2. "What is your main application file?" (e.g., "app.py", "server.py")
   • Where create_app() lives
3. "Does this service run under gunicorn with more than one worker?" (yes/no — assume yes if unsure)
   • If yes: full multiproc setup (Steps 4 and 5)
   • If no: simpler single-process path, but still scaffold the multiproc setup behind a PROMETHEUS_MULTIPROC_DIR env-var gate so the service stays safe when scaled up later
4. "Is Redis available to this service?" (yes/no)
   • If yes: scaffold Step 6's Redis-backed business metrics pattern
   • If no: skip Step 6; document it as a follow-up if Redis is added later
5. "What's the metrics scrape path?" (default: /metrics)
   • Override only if /metrics collides with an existing route

Step 2: Add Dependencies to requirements.txt

Add:

# Prometheus metrics
prometheus_client>=0.19.0
prometheus_flask_exporter>=0.23.0

If the user answered "yes" to Redis (Step 1, Q4), also add:

redis>=5.0.0

Install:

pip install -r requirements.txt

Step 3: Wire Metrics into the Flask App

Add inside create_app() in {app_file}.py, after Flask(__name__) and before registering blueprints/routes:

import os
from flask import Flask
from prometheus_flask_exporter.multiprocess import GunicornPrometheusMetrics


def create_app() -> Flask:
    app = Flask(__name__)

    # ... existing setup (logging, etc.) ...

    # Prometheus metrics — multiproc-aware. Reads PROMETHEUS_MULTIPROC_DIR
    # automatically; if unset, falls back to single-process mode (only
    # safe for dev or workers=1).
    metrics = GunicornPrometheusMetrics(
        app,
        defaults_prefix='flask',
        group_by='endpoint',  # group HTTP latency by Flask endpoint name, not URL path
    )
    metrics.info(
        'app_info',
        'Application info',
        application='{application_tag}',
    )

    # ... register blueprints, configure Api, etc.
    return app


app = create_app()

CRITICAL: Replace:

• {app_file} → your main application filename
• {application_tag} → the tag from Step 1, Q1

Why inside create_app() and not at module level: gunicorn imports the module in each worker post-fork. Module-level initialization runs in the master process and breaks multiproc shard ownership. This matches the same fork-safety rule that [[byteforge-loki-logging]] documents for the Loki handler.

Why group_by='endpoint': URL-path grouping explodes cardinality (one time series per unique URL — /users/123, /users/456, ...). Endpoint grouping uses Flask's route name (get_user), keeping cardinality bounded.

What this gives you for free

• /metrics endpoint mounted automatically
• flask_http_request_total{method,status,endpoint} — request counter
• flask_http_request_duration_seconds{method,status,endpoint} — latency histogram (p50/p95/p99 via PromQL)
• flask_http_request_exceptions_total{method,endpoint} — unhandled exception counter
• Default process metrics: process_resident_memory_bytes, process_cpu_seconds_total, process_start_time_seconds, GC stats

Note: raw prometheus_client path (only if you can't use GunicornPrometheusMetrics)

Some services have reason not to adopt prometheus_flask_exporter (custom transport, non-Flask consumer of the same registry, etc.). The canonical bare-bones multiproc pattern in upstream prometheus_client docs looks like this:

from prometheus_client import (
    generate_latest, CollectorRegistry, CONTENT_TYPE_LATEST, multiprocess,
)
import os

def get_metrics():
    if os.environ.get('PROMETHEUS_MULTIPROC_DIR'):
        registry = CollectorRegistry()
        multiprocess.MultiProcessCollector(registry)
        return generate_latest(registry), CONTENT_TYPE_LATEST
    return generate_latest(), CONTENT_TYPE_LATEST

Gotcha: the fresh CollectorRegistry() does not include ProcessCollector, PlatformCollector, or GCCollector — those are attached to the global REGISTRY. The resulting /metrics will be missing process_*, python_info, and python_gc_* entirely. To restore them:

from prometheus_client import ProcessCollector, PlatformCollector, GC_COLLECTOR

registry = CollectorRegistry()
multiprocess.MultiProcessCollector(registry)
ProcessCollector(registry=registry)
PlatformCollector(registry=registry)
registry.register(GC_COLLECTOR)

These three collectors are per-worker (sampled at scrape time from whichever worker responds), not multiproc-aggregated. That is prometheus_client's documented behavior — not a bug, and not something to "fix." See the Multi-Worker Gunicorn Trap section above.

Step 4: Create gunicorn_conf.py

Create gunicorn_conf.py in the project root:

"""Gunicorn config — required for prometheus_client multiproc safety.

When a worker dies, mark_process_dead() removes its gauge_live*_<pid>.db
shards so 'livesum' / 'liveall' / 'livemax' / 'livemin' / 'livemostrecent'
gauges stop counting the dead worker.

Counter, Histogram, and non-live Gauge mode shards ('mostrecent', 'max',
'min', 'all', 'sum') are NOT touched by mark_process_dead — they are
preserved intentionally so aggregated values survive worker churn.
"""
from prometheus_client import multiprocess


def child_exit(server, worker):
    multiprocess.mark_process_dead(worker.pid)

That single hook is the only thing this file must do for metrics. Add other gunicorn settings (workers, bind, timeout) however the project normally configures them.

Why this matters: if you use any live* gauge mode (e.g. multiprocess_mode='livesum' for in-flight request counts), missing this hook means a crashed worker keeps contributing to the sum forever. For Counters, Histograms, and non-live Gauge modes, the hook is a no-op — those shards are preserved either way. So "missing _dead markers" or "non-shrinking shard count after a worker dies" is not evidence of a broken multiproc setup — that's the documented behavior for non-live* modes.

Step 5: Deploy-Side Setup (Dockerfile / docker-compose / systemd)

Three things must happen at container/service startup, in this order:

1. PROMETHEUS_MULTIPROC_DIR is exported
2. That directory is created fresh (stale shards from a previous container exit will inflate counters)
3. gunicorn is launched with -c gunicorn_conf.py

Option A: Docker Compose with an entrypoint script

entrypoint.sh:

#!/bin/sh
set -e

: "${PROMETHEUS_MULTIPROC_DIR:=/tmp/prometheus_multiproc}"
export PROMETHEUS_MULTIPROC_DIR

rm -rf "$PROMETHEUS_MULTIPROC_DIR"
mkdir -p "$PROMETHEUS_MULTIPROC_DIR"

exec gunicorn -c gunicorn_conf.py -b 0.0.0.0:{port} {app_module}:app "$@"

CRITICAL: Replace:

• {port} → your bind port (e.g. 7843)
• {app_module} → the import path of your app (e.g. my_api for my_api.py)

Dockerfile:

COPY entrypoint.sh /app/entrypoint.sh
RUN chmod +x /app/entrypoint.sh
ENTRYPOINT ["/app/entrypoint.sh"]

docker-compose.yaml — mount tmpfs over the multiproc dir to avoid hitting the container's writable layer for every metric increment:

services:
  {app_name}:
    tmpfs:
      - /tmp/prometheus_multiproc
    environment:
      - PROMETHEUS_MULTIPROC_DIR=/tmp/prometheus_multiproc

Option B: systemd unit

[Service]
Environment=PROMETHEUS_MULTIPROC_DIR=/run/prometheus_multiproc_{service_name}
RuntimeDirectory=prometheus_multiproc_{service_name}
RuntimeDirectoryMode=0755
ExecStartPre=/bin/sh -c 'rm -rf $PROMETHEUS_MULTIPROC_DIR/*'
ExecStart=/path/to/venv/bin/gunicorn -c gunicorn_conf.py -b 127.0.0.1:{port} {app_module}:app

RuntimeDirectory is on tmpfs by default under systemd and is cleaned up when the service stops.

Why tmpfs

Metrics shards are written on every increment. On a busy service, that is thousands of file writes per second. tmpfs keeps them in RAM, avoiding disk I/O and SSD wear.

Step 6: Custom Application Metrics

Two patterns. Use Pattern A for per-request metrics that are naturally per-process (timings, error counts, in-flight requests). Use Pattern B for business metrics that need a single global value across workers.

Pattern A: File-multiproc metrics (the default)

For Counters and Histograms, the multiproc collector sums across workers automatically. No extra work:

from prometheus_client import Counter, Histogram

orders_placed_total = Counter(
    'orders_placed_total',
    'Orders placed, by product category',
    ['category'],
)

order_processing_seconds = Histogram(
    'order_processing_seconds',
    'Time spent processing an order',
    ['category'],
    buckets=(0.01, 0.05, 0.1, 0.5, 1.0, 2.5, 5.0, 10.0),
)

# In your route:
with order_processing_seconds.labels(category='digital').time():
    place_order(...)
orders_placed_total.labels(category='digital').inc()

Gauges are different — you MUST declare multiprocess_mode. Without it, the multiproc collector silently drops the metric:

from prometheus_client import Gauge

# 'livesum' = sum across all live workers (good for in-flight requests, active jobs)
# 'liveall' = expose one series per live worker (good for debugging)
# 'max' / 'min' = aggregate by max/min across workers
in_flight_orders = Gauge(
    'in_flight_orders',
    'Orders currently being processed',
    multiprocess_mode='livesum',
)

in_flight_orders.inc()
try:
    place_order(...)
finally:
    in_flight_orders.dec()

Pattern B: Redis-backed business metrics

For metrics where you want one true global value across workers (e.g. "total signups today", "queued background jobs", "active subscribers"), keep the source of truth in Redis and expose a custom collector that reads Redis at scrape time.

This is preferred over Pattern A for business metrics because:

• The value is correct even if a worker just restarted (no shard re-aggregation lag)
• It survives the whole gunicorn pool being recycled
• Other services can read/increment the same counter

Add a module metrics_redis.py:

import os
import redis
from prometheus_client.core import CounterMetricFamily, GaugeMetricFamily
from prometheus_client.registry import Collector

REDIS_URL = os.environ.get('REDIS_URL', 'redis://localhost:6379/0')
METRIC_KEY_PREFIX = 'metrics:{application_tag}:'

_redis_client = redis.Redis.from_url(REDIS_URL, decode_responses=True)


def incr(name: str, amount: int = 1) -> None:
    """Increment a monotonic business counter. Safe across workers and processes."""
    _redis_client.incrby(f'{METRIC_KEY_PREFIX}{name}', amount)


def set_gauge(name: str, value: float) -> None:
    """Set a gauge value. Safe across workers."""
    _redis_client.set(f'{METRIC_KEY_PREFIX}gauge:{name}', value)


class RedisBusinessMetricsCollector(Collector):
    """Custom collector that reads business metrics from Redis at scrape time."""

    def __init__(self, application_tag: str):
        self.application_tag = application_tag

    def collect(self):
        # Counters — keys matching metrics:<tag>:<name> (non-gauge)
        counter_keys = _redis_client.keys(f'{METRIC_KEY_PREFIX}*')
        for key in counter_keys:
            if ':gauge:' in key:
                continue
            name = key.split(':', 2)[-1]
            value = int(_redis_client.get(key) or 0)
            m = CounterMetricFamily(
                name,
                f'Business counter: {name}',
                labels=['application'],
            )
            m.add_metric([self.application_tag], value)
            yield m

        # Gauges — keys matching metrics:<tag>:gauge:<name>
        gauge_keys = _redis_client.keys(f'{METRIC_KEY_PREFIX}gauge:*')
        for key in gauge_keys:
            name = key.rsplit(':', 1)[-1]
            value = float(_redis_client.get(key) or 0)
            m = GaugeMetricFamily(
                name,
                f'Business gauge: {name}',
                labels=['application'],
            )
            m.add_metric([self.application_tag], value)
            yield m

Register the collector once, inside create_app() (after GunicornPrometheusMetrics):

from prometheus_client import REGISTRY
from metrics_redis import RedisBusinessMetricsCollector

REGISTRY.register(RedisBusinessMetricsCollector(application_tag='{application_tag}'))

Use it from anywhere:

from metrics_redis import incr, set_gauge

# In a signup route:
incr('signups_total')

# In a background job:
set_gauge('jobs_queue_depth', queue.size())

CRITICAL: Replace {application_tag} in METRIC_KEY_PREFIX with the tag from Step 1, Q1.

Naming the keys

Redis keys collide silently when reused. Always namespace by application_tag. The prefix metrics:{application_tag}: keeps each service's counters isolated and makes Redis introspection easy (redis-cli KEYS 'metrics:my-api:*').

Cardinality warning

Custom collectors that read Redis on every scrape can explode if you create one Redis key per high-cardinality dimension (user ID, request ID). Keep labels low-cardinality (status, category, region) — the same rule that applies to Prometheus metrics in general.

Step 7: Post-Deploy Verification

Run these checks after deploying. Skipping this step is how the multi-worker bug ships to production unnoticed.

Check 1: An application Counter is monotonically non-decreasing

Pick a Counter that you have declared in your application (or flask_http_request_total if you used GunicornPrometheusMetrics from Step 3). Do not substitute process_*, python_gc_*, or python_info — those are per-worker by design (see Multi-Worker Gunicorn Trap section) and would false-FAIL this check even with multiproc wired correctly.

for i in 1 2 3 4 5; do
  curl -s https://<your-host>/metrics | grep '^<your_counter_name>{' | head -3
  echo "---"
  sleep 2
done

Pass: values stay the same or increase. Fail: values bounce (down, then up, then down) — that means scrapes are landing on different workers with private registries. Re-check Steps 3, 4, 5.

Check 2: An application metric stays consistent across scrapes (not bouncing per worker)

Pick any application Counter, Histogram bucket, or Gauge declared with multiprocess_mode set. Across 10 consecutive scrapes, the value should be identical (or trending monotonically for Counters — never bouncing between unrelated values).

METRICS_URL="https://<your-host>/metrics"
for i in $(seq 1 10); do
  curl -fsS "$METRICS_URL" | grep '^<your_app_metric_name> '
  sleep 1
done | sort -u

Pass: one unique value (Gauge) or a small monotonically increasing set (Counter). Fail: many unrelated values — each row from a different worker's private registry, which means MultiProcessCollector is not in the response path.

Do NOT use process_start_time_seconds, process_cpu_seconds_total, python_info, python_gc_*, or any metric from ProcessCollector / PlatformCollector / GCCollector for this check. Those are never aggregated across workers — they return one value per worker PID regardless of whether multiproc is wired. Using them as a multiproc diagnostic produces false negatives.

Check 3: rate() returns a sensible value in Grafana

sum(rate(flask_http_request_total{application="{application_tag}"}[5m]))

Compare to your nginx/gateway access-log rate. They should agree within ~10%. If rate() is 3–10× higher, the counter is "resetting" because scrapes bounce between workers with private registries.

Check 4: Custom Gauges show up

curl -s https://<your-host>/metrics | grep -E '^(in_flight_orders|jobs_queue_depth)'

If a Gauge is missing entirely, it was declared without multiprocess_mode and the multiproc collector dropped it. Add multiprocess_mode='livesum' (or similar) and restart.

Step 8: Starter Grafana Dashboard

A starter dashboard is included at references/starter-dashboard.json. To install:

1. Grafana → Dashboards → Import → Upload JSON
2. Select the Prometheus datasource
3. The dashboard is parameterized on application — pick {application_tag} from the dropdown

It includes:

• Request rate (req/s) by status class (2xx/4xx/5xx)
• Latency p50 / p95 / p99
• In-flight requests
• Error rate (5xx / total)
• Process memory + CPU
• Open file descriptors
• Worker restart events (gaps in process_start_time_seconds)

For the Redis-backed business metrics, add panels manually — the metric names are project-specific.

Step 9: Prometheus Scrape Config

Hand this snippet to whoever runs the Prometheus server. The exact location depends on whether they use static configs, file-based SD, or Kubernetes ServiceMonitors.

Static config

scrape_configs:
  - job_name: '{application_tag}'
    metrics_path: /metrics
    scrape_interval: 15s
    static_configs:
      - targets: ['<host>:<port>']
        labels:
          application: '{application_tag}'

Notes for the Prometheus operator

• Set application as a target label so it appears on every metric — this matches the application_tag convention used by [[byteforge-loki-logging]] so logs and metrics correlate
• 15s is a good default scrape interval; faster than 5s rarely pays off and inflates storage
• If the service is behind nginx/gatekeeper, the scrape config should hit the internal address (avoid the public URL and any auth_request layer) — alternatively, whitelist the Prometheus source IP in nginx for /metrics

The Four Traps

Things that look fine in dev and break in prod. All four are real and have shipped to production before this skill existed.

| # | Trap | Symptom | Fix | |---|------|---------|-----| | 1 | No multiproc wiring under multi-worker gunicorn | rate() 5–10× too high; an application Counter "resets" between scrapes; a Gauge bounces between unrelated values | Steps 3, 4, 5 — all three are needed; any one missing leaves the bug. Do not use process_start_time_seconds plurality as the diagnostic — it is always per-worker regardless of multiproc wiring | | 2 | Gauges declared without multiprocess_mode | Gauge disappears from /metrics after enabling multiproc | Add multiprocess_mode='livesum' (or 'liveall', 'max', 'min') | | 3 | PROMETHEUS_MULTIPROC_DIR not cleaned on startup | Counter values appear pre-loaded from a prior container's shards; rate() initially negative then settles | rm -rf $PROMETHEUS_MULTIPROC_DIR/* in entrypoint BEFORE launching gunicorn | | 4 | High-cardinality labels (user ID, request ID, full URL path) | Prometheus storage/memory blows up; queries slow to crawl | Use group_by='endpoint' not URL path; never label by user/request ID; for Redis collector, never key on high-cardinality dimensions |

Failure Decoder

| Symptom | Cause | Fix | |---------|-------|-----| | rate(flask_http_request_total[5m]) returns 5–10× the real traffic | Multi-worker gunicorn without multiproc — each worker keeps a private registry, scrapes bounce between them, PromQL reads the bouncing as counter resets and manufactures phantom traffic | Apply Steps 3, 4, 5 in full | | An application Counter's value bounces (e.g. 1247 → 893 → 1402 → 651) across consecutive scrapes | Same as above — multiproc collector isn't in the /metrics response path; scrapes are landing on different workers with private registries | Confirm PROMETHEUS_MULTIPROC_DIR is set in the running container (docker exec ... env \| grep PROMETHEUS), confirm GunicornPrometheusMetrics(app) is called in create_app(), confirm gunicorn_conf.py is passed with -c | | Gauge metric missing from /metrics entirely | Gauge was declared without multiprocess_mode — the multiproc collector drops aggregation-ambiguous gauges silently | Add multiprocess_mode='livesum' (or 'liveall'/'max'/'min'/'mostrecent' as appropriate) | | process_start_time_seconds, python_info, or process_cpu_seconds_total returns N distinct values | Not a bug. ProcessCollector, PlatformCollector, and GCCollector are per-worker by design and are never aggregated by MultiProcessCollector. Diagnose using application metrics (Counter/Histogram/Gauge) instead | None — this is documented prometheus_client behavior, not a multiproc problem | | /metrics is missing process_* and python_info entirely (under raw prometheus_client, not GunicornPrometheusMetrics) | ProcessCollector and PlatformCollector were not registered on the fresh CollectorRegistry() used by MultiProcessCollector — they only live on the global REGISTRY | Register them explicitly: see "Note: raw prometheus_client path" in Step 3 | | /metrics returns 500 with FileNotFoundError in logs pointing at PROMETHEUS_MULTIPROC_DIR | Env var is set, but the directory does not exist or is not writable by the gunicorn user | Add the mkdir -p $PROMETHEUS_MULTIPROC_DIR step to entrypoint/ExecStartPre; verify file permissions match the runtime user | | Counter values appear inflated at container startup and slowly decrease | Stale shards from a previous container's exit weren't cleaned, multiproc collector summed them with the new worker shards | Add rm -rf $PROMETHEUS_MULTIPROC_DIR/* to entrypoint BEFORE launching gunicorn | | redis.exceptions.ConnectionError in /metrics response | RedisBusinessMetricsCollector is registered but Redis is unreachable; the custom collector raises during scrape and the whole /metrics response fails | Wrap the collect() method body in a try/except redis.RedisError that logs and yields nothing — a Redis outage should degrade business metrics, not kill the scrape | | Custom Redis-backed metric reports 0 even though incr() is being called | application_tag mismatch between METRIC_KEY_PREFIX in writes and reads, or wrong Redis DB number in REDIS_URL | redis-cli KEYS 'metrics:*' to inspect; align prefixes; verify REDIS_URL is identical across the app and the collector | | Cardinality explosion — Prometheus storage 100×, queries time out | High-cardinality label in a metric (user ID, request ID, full URL path with IDs) | Find the offending metric in /metrics (look for thousands of unique label combinations on one metric); switch to group_by='endpoint'; never label by entity ID |

Report Back

When done, report:

1. Files created/modified (with paths)
2. The values for application_tag, app file, gunicorn multi-worker (yes/no), Redis (yes/no)
3. Output of Step 7 Check 1 (application Counter monotonicity across 5 scrapes) — values should only go up or stay the same
4. Output of Step 7 Check 2 (application metric stability across 10 scrapes piped through sort -u) — one unique value for a Gauge, or a small monotonic set for a Counter
5. Whether the starter dashboard was imported and is showing data

Do not use process_start_time_seconds, process_cpu_seconds_total, or other ProcessCollector metrics in the verification — they are per-worker by design and produce false negatives. See the Multi-Worker Gunicorn Trap section.

If any check fails, walk through the Failure Decoder — every row maps a real symptom back to the Step that fixes it.