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who_need_help/docs/performance.md

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Performance measurement

No production capacity, minimum resource requirement, SLO, alert threshold, pool size, or autoscaling threshold is known yet. The repository therefore contains a reproducible measurement profile, not a capacity claim or blocking resource preflight.

The profile uses a separate Compose project, generated independent secrets, and a separate PostgreSQL volume. Its Traefik instance is constrained to that exact Compose project; its router, service, middleware, and Host rule are unique, so the load profile cannot join the staging router. It exercises:

  • dynamic public home and Safety pages;
  • database-backed readiness queries;
  • Phoenix WebSocket upgrades and protocol heartbeats;
  • the configured number of web and worker replicas;
  • cross-node Phoenix PubSub after each run.

It does not authenticate users, create requests, send chat messages, update locations, render a real browser, or reproduce an unknown future production traffic mix. It also does not sample the k6 load-generator container itself. Its results only describe the recorded host, container versions, replica counts, database state, and experiment inputs.

Create the isolated profile

./scripts/ensure-local-load-env.sh
./scripts/load-stack-up.sh

The generated .env.load is ignored, restricted to mode 600, and contains independent PostgreSQL and application secrets. Edit its LOAD_* inputs to define a specific experiment. Values in .env.load.example are reproducible measurement points, not recommendations.

Run and compare replica counts

./scripts/load-run.sh two-web
./scripts/load-stack-up.sh 3
# Set LOAD_WEB_REPLICAS=3 in .env.load so the recorded expected topology
# matches the running topology, then:
./scripts/load-run.sh three-web

Each run writes ignored evidence below output/performance/<label>/:

  • the exact inputs and Docker host observation;
  • a machine-readable k6 summary and complete console report;
  • periodic Docker CPU/memory/network/block-I/O/PID samples for every load stack container;
  • before/after application table counts and their diff;
  • final Compose state, readiness response, application logs, and cross-node PubSub result.

The k6 image is pinned to version 2.1.0 by digest and runs locally with anonymous usage reporting disabled. No Grafana Cloud account, API token, OpenAI API, or usage-based service is involved.

Observed local comparison

Observed on 2026-07-18 with Docker Engine 29.6.2 on the recorded 32-CPU, 100289265664-byte Docker host. Neither application containers nor the load generator had CPU or memory limits. Both runs used 40 HTTP VUs, 40 WebSocket VUs, a 30-second duration, a 0.1-second HTTP think time, and five-second socket sessions. Each HTTP iteration requested the home page, Safety page, and database-backed readiness endpoint.

Observation 2 web replicas 3 web replicas
HTTP checks 35,160 / 35,160 passed 35,163 / 35,163 passed
HTTP failed rate 0% 0%
Observed HTTP rate 1,169.12 requests/s 1,169.42 requests/s
HTTP duration p95 2.87 ms 3.36 ms
WebSocket opened / heartbeat replies 240 / 240 240 / 240
WebSocket errors 0 0
WebSocket connect p95 46.65 ms 24.12 ms
Average CPU per web container 66.57%, 66.63% 50.25%, 49.77%, 49.18%
Maximum CPU per web container 79.80%, 79.21% 58.16%, 59.23%, 59.08%
Maximum observed memory per web container 268.3, 238.3 MiB 221.0, 223.2, 219.4 MiB

Each resource series contained 11 sequential docker stats --no-stream samples. Both before/after database diffs were empty, both cross-node PubSub probes passed, readiness remained healthy, and the captured proxy/web/worker logs contained no application error or warning. The public staging root and readiness endpoint also returned HTTP 200 while the isolated project was running.

An earlier harness smoke exposed that fixed Traefik router/service names are global to the Docker provider: starting a second Compose project temporarily made staging readiness return HTTP 404. The load project was stopped immediately and staging returned HTTP 200. The final Compose configuration parameterizes the project constraint, router/service name, Docker network, and router rule. Merged-config inspection found no shared router/service label key, the two running proxies showed different exact project constraints, and both origins returned HTTP 200 concurrently before the recorded final runs.

The nearly identical request rate is expected from the selected paced workload: 40 HTTP VUs each issue three requests and then wait 0.1 seconds, giving an approximate workload ceiling of 1,200 requests/s before response time is added. The result therefore does not show the saturation point of two replicas and does not justify an HPA threshold. The lower per-container CPU and lower WebSocket connect p95 with three replicas are observations for this run only; the slightly higher HTTP p95 also shows why one short comparison is not a production sizing study.

Ignored evidence:

  • output/performance/two-web-final-20260718/
  • output/performance/three-web-final-20260718/

Stop the isolated containers without deleting their database volume:

./scripts/load-stack-stop.sh

The stop script reads the actual LOAD_PROJECT, refuses the staging project name, and does not delete the volume. Volume deletion is intentionally not automated.