fix: VectorSinkFailing alert never fires for sinks with transport-only or 5xx-only failures - #269
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…y or 5xx-only failures
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The VectorSinkFailing expression adds two independent aggregations: transport failures from vector_http_client_errors_total and error responses from vector_http_client_responses_total{status=~"5..|429"}. Vector registers counters lazily, so a sink that fails only at the transport level (connection refused, DNS, timeout) has no responses series for the 5xx matcher, and a sink that only receives 5xx/429 has no errors series. In PromQL, adding a vector to an empty vector produces an empty result, so in both cases the sink drops out of the expression and the alert never fires, even at a 100% failure rate. A dead endpoint that refuses connections is exactly the transport-only case.
The fix pads each addend with
or 0 * sum by (pod, component_id) (rate(vector_http_client_requests_sent_total[...])), so both sides of the addition always carry the full set of (pod, component_id) present in the denominator. A failing sink now yields its true failure ratio, a healthy sink yields 0, and an idle sink evaluates to 0/0 = NaN and is filtered by the threshold.