# Topic stall URL: /rules/topic-stall Flags a logged uORB topic that goes silent for five times its usual interval. It observes that the flight log stopped getting samples, not which link in the chain stopped sending them. ## What it reads [#what-it-reads] The rule reads the uORB topics in the flight log, where each sample carries a `timestamp` in microseconds since boot. For each topic, the rule takes the median interval between samples as its usual rate. It then watches for a silence far longer than that. In the example dataset the topic that stalls is `obstacle_distance`, which comes from the companion computer. `perception_node` streams MAVLink `OBSTACLE_DISTANCE` at 10 Hz, and PX4's MAVLink receiver turns each message into an `obstacle_distance` sample. PX4 stamps that sample with its own clock when the message arrives, so a stall means messages stopped reaching PX4. ## When it fires [#when-it-fires] A finding is produced when a topic has no sample for 5 times its median period. For `obstacle_distance` at 10 Hz the median is 100 ms, so the rule fires after 500 ms of silence. The event sits on the last sample before the silence. | Value | Example from INC-0142 | | -------------------- | --------------------------- | | Last sample | 14:32:04.079 | | Median period before | 100 ms (10 Hz) | | Next sample | 14:32:11.604, 7.525 s later | | Stall threshold | 5 times the median period | The finding's title gives the silence: **Last obstacle\_distance sample before a 7.5 s stall**. On the timeline, the flight controller lane marks it **No obstacle\_distance for 7.5 s**. ## Wording and evidence level [#wording-and-evidence-level] Every finding reads **Observed: topic stopped updating**. The flight log shows the missing samples directly, so the level is observed. ## What the finding does not mean [#what-the-finding-does-not-mean] * **It does not say which link stopped.** The publisher, the companion's router and the serial link to the flight controller all sit in the chain. In INC-0142 the publisher was killed and the router stopped within 73 ms of the last sample, and either could explain the silence. * **A logger dropout looks the same.** When the logger loses data, samples go missing from the log for the length of the loss. Check for a [log\_dropout](/rules/log-dropout) finding at the same time. * **Its order against companion events can be unknown.** In INC-0142 the last sample and the kernel's kill line sit 39 ms apart, inside their 46 ms combined bound. In INC-0139 the companion clock had no anchor, so no order is claimed at all. * **A topic that was never logged cannot stall.** The run page's **Flight log topics** panel lists an expected topic that is missing as **Not logged**. ## Threshold and overrides [#threshold-and-overrides] The threshold scales with each topic's own rate. At 10 Hz it comes to 500 ms, and for a topic at 1 Hz it would come to 5 s. The scope is **All vehicles**. Change the threshold under **Rules** with **Edit**. The rule saves as `topic_stall 1.1`, and findings already produced keep version 1.0 and its threshold. See [Thresholds, scopes and overrides](/rules/configure). ## Example finding [#example-finding] In INC-0142, UAS-04 was on the return leg of Route Iron. The last `obstacle_distance` sample came at 14:32:04.079 and the next at 14:32:11.604, 7.525 s later. The **Findings** row labels it **obstacle\_distance stalled**. The kernel's kill line at 14:32:04.118 is 39 ms from the last sample. That is inside the 46 ms combined bound of the flight log and the journal, so the report lists the pair under Limitations. It claims no order between them. In INC-0139, UAS-07 went without `obstacle_distance` for 71 s, from 14:17:05.310 to 14:18:16.540, and held position. `perception_node` crashed on the companion, but the companion clock was never synchronized, so the crash cannot be placed before or after the stall. The report writes its finding as unknown and keeps the question open: did the stall start before the crash? ## Related [#related] * [log\_dropout](/rules/log-dropout) flags holes the logger itself left. * [process\_exit](/rules/process-exit) records the companion exits around both stalls. * [Alignment and event order](/concepts/alignment) explains when an order is reported as unknown. * [PX4 ULog](/collect/formats/px4-ulog) lists the topics Foxborne reads from a flight log.