sdr-recorder/openspec/changes/archive/2026-03-21-recording-cleanup-cron/design.md

Context

The SDR recorder running in Docker on sdr-pi writes MP3 files to /home/m/recordings/ (mounted from the host). Despite squelch tuning, many files are noise — today's data shows 91 files totaling 476 KB, with the majority under 2 KB. These clutter the Nginx web browser and Samba share, making it hard to find real transmissions.

The recorder container runs as root (required for USB/SDR hardware access) and writes files owned by root, so cleanup must also run as root.

Goals / Non-Goals

Goals:

• Automatically remove recordings that are too small to contain real audio
• Make the size threshold easily adjustable via environment variable
• Keep everything version-controlled in docker-compose
• Run frequently enough that the web/Samba views stay clean

Non-Goals:

• Duration-based or content-based analysis (too complex, needs extra tools)
• Age-based retention policy (handled manually for now)

Decisions

Dedicated cleanup container in docker-compose

Add a lightweight Alpine container (sdr-cleanup) to docker-compose that shares the /home/m/recordings volume. This follows Docker's separation-of-concerns model — one container per responsibility. The container runs a simple find -delete loop with sleep instead of requiring a cron daemon. Since it runs as root (Alpine default), it can delete the root-owned files written by the recorder.

Alternatives considered:

• Host crontab: Simplest, but files are root-owned so needs sudo crontab, and isn't version-controlled in docker-compose
• Cron inside recorder container: Not our image, would need custom build
• Systemd timer on host: More ceremony than cron for the same result

File size threshold as the filter heuristic

Use file size (default 3 KB) rather than MP3 duration. This avoids needing ffprobe or similar tools. From today's data, files under 3 KB are consistently noise — real transmissions start around 5 KB. Configurable via SIZE_THRESHOLD environment variable in docker-compose.

5-minute sleep loop

Run every 5 minutes via sleep 300 in a loop. Lightweight and keeps the browsing experience clean.

Only delete files older than 1 minute

Add a minimum age filter (-mmin +1) to avoid deleting files still being written by rtl_airband.

Risks / Trade-offs

• Threshold too aggressive → Real short transmissions deleted. Mitigation: start conservative at 3 KB, tune via environment variable.
• Race condition with active writes → The -mmin +1 guard ensures we never delete a file rtl_airband is still writing.
• Container restart → If the cleanup container crashes, restart: unless-stopped brings it back. No state to lose.