A self-built Modbus cache proxy runs for weeks without complaint in summer — until the first night the inverter shuts down, or the first firmware reboot of the SDongle. That's exactly when you find out whether you built a proxy or a time bomb. My first attempt was naive: poll, cache, serve. It worked perfectly during the day. At night, when the SUN2000 went to sleep, the poll loop hung in a read that never returned — and for hours the proxy silently served the last daytime values as if nothing was wrong.
That's the dangerous failure: not the crash (you notice that), but the proxy that keeps running and serves stale data while nobody is the wiser. This post is the reliability playbook that turned my proxy into something I trust. I build the proxy itself in the Modbus caching basics post — here it's purely about the robustness underneath.
The failure mode no beginner guide covers
Tutorials show the happy path: connect to the SDongle, read registers, done. What they leave out: the SDongle is slow, opinionated hardware. It drops the connection at night, it needs a minute after a firmware reboot before it answers again, and it can't handle reads fired back-to-back. A naive asyncio read without a timeout then blocks forever, and your cache freezes on its last value. HA dutifully keeps showing numbers — they're just no longer true.
