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Solutions to errors you may hit while running the iii engine and workers.

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KVM not accessible

Workers boot inside a microVM that needs read/write access to /dev/kvm. On Linux hosts where your user is not in the kvm group (common on WSL2), starting a worker fails with KVM not accessible. Add your user to the kvm group:
Then restart your session so the new group membership applies. On WSL2, run wsl --shutdown from a Windows terminal and reopen your distribution.
For why workers run in microVMs, see Engine.

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An observer’s target never runs

List the observers of the observed function:
Find yours by its target and read its state:
  • pending or error: reason says why, for example the source or the target is not registered, the source’s worker has not accepted the observer yet, or it refused the observer’s args (INVALID_ARGS, or a code from the worker such as INVALID_CRON_EXPRESSION). See Observer states.
  • active with fires and dropped at 0: no call has reached it. Check these:
    • Only calls that start after the observer turned active, end with an outcome its capture fires on (success by default), and pass its filters run the target. A call already running when the observer turned active does not, so call the function again.
    • A function whose worker matches observers to calls (with onObservationMatch or onObservationDeliver) gives a call only to the observers its args select. Compare the observer’s args with that function’s documentation, for example the method and path of an http::on-request observer or the expression of a cron::on-tick observer. See Observable functions.
  • dropped or activation_failures above 0: calls reached it and were dropped or failed, counted by reason (OVERFLOW, LOOP_GUARD, …).

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Logger lines do not show up

Logger sends its lines to the engine. They do not appear in your worker’s output. Look in the engine’s output (under iii compose --up, .iii/compose/<namespace>/engine.log), or run:
If they are not there either, the worker’s OpenTelemetry export may be off (OTEL_ENABLED=false). Logger then falls back to the language’s own logging, which the Rust SDK prints only with a tracing subscriber. See Where the lines go.