Environment variables (HTTP server) ​

All server-side settings can be configured by env var — convenient for Docker, Kubernetes, CI, and systemd. Variables relevant only to the CLI (CHROMIUM_PATH when Chromium is auto-installed, for example) are duplicated here because the server also reads them; see CLI → Environment variables for their CLI-specific nuances.

Setting them ​

Shell ​

export PORT=8080
export CONCURRENCY=4
export API_KEY=s3cret
chartjs2img serve

Docker ​

docker run -p 8080:8080 \
  -e PORT=8080 \
  -e API_KEY=s3cret \
  -e CONCURRENCY=4 \
  chartjs2img

docker-compose ​

services:
  chartjs2img:
    build: .
    ports:
      - "3000:3000"
    environment:
      API_KEY: s3cret
      CONCURRENCY: 8
      CACHE_MAX_ENTRIES: 2000
      CACHE_TTL_SECONDS: 7200
      MAX_RENDER_TIME_SECONDS: 45

systemd ​

[Service]
Environment=PORT=3000
Environment=API_KEY=s3cret
Environment=CONCURRENCY=8
ExecStart=/usr/local/bin/chartjs2img serve

Tuning guidelines ​

CONCURRENCY ​

Match it to available CPU cores. Each active render uses one browser tab and 50-150 MB of memory. Too high and you'll see OOMs or page timeouts; too low and clients wait in the queue.

Typical values:

• 2 cores → CONCURRENCY=2
• 8 cores → CONCURRENCY=6 (leave headroom for I/O + OS)
• 32 cores → CONCURRENCY=16 (memory becomes the bottleneck, not CPU)

Watch GET /health → renderer.concurrency.pending to decide whether to raise the ceiling or scale out horizontally.

CACHE_MAX_ENTRIES / CACHE_TTL_SECONDS ​

Identical requests are cheap; wide-variety requests don't benefit. Profile with X-Cache-Hit: true/false:

• Dashboard with ~50 charts shown many times a day → large cache, long TTL.
• Ad-hoc one-offs → small cache, short TTL.
• Stateless horizontal scale-out → set CACHE_MAX_ENTRIES=0 and cache at the CDN instead (see Cache).

MAX_RENDER_TIME_SECONDS ​

Raise only when you have legitimately slow renders (huge datasets, slow CDN on a cold start). Lower to fail-fast on stuck charts.

PAGE_TIMEOUT_SECONDS ​

The safety-net timer only fires when the render finally block never runs (genuinely orphaned tab). You should rarely need to override the derived default; if you see Safety net fired after Xms warnings in the server logs without the request actually hanging, it indicates a bug in the renderer — please file an issue.

See also ​

• CLI → Environment variables — overlapping variables read by one-shot render calls.
• Authentication — API_KEY usage.
• Cache — how CACHE_MAX_ENTRIES and CACHE_TTL_SECONDS interact.