> ## Documentation Index > Fetch the complete documentation index at: https://docs.cerulion.com/llms.txt > Use this file to discover all available pages before exploring further. # Inspect topics > Discover and watch live topics, open the TUI, and inspect recorded trace bags. The Cerulion lion at a DJ-style control panel with LIVE camera icons — broadcasting topics While a graph runs, every wired port becomes a topic you can observe. This guide covers listing and watching live topics, the interactive dashboard, and reading trace bag files after a run. The `topic` commands use iceoryx2 service discovery, so they work from any directory, no workspace required. Run them in a second terminal while a graph is running. ## List and inspect live topics `cerulion topic list` enumerates active topics, sorted by name. ```bash theme={null} cerulion topic list ``` ```text theme={null} TOPIC perception/camera/image perception/detector/detections ``` If nothing is publishing, it prints `No active topics.` `cerulion topic info ` prints details for a single topic: ```bash theme={null} cerulion topic info perception/camera/image ``` ## Watch messages with echo `cerulion topic echo ` subscribes and prints each message until you press Ctrl+C. ```bash theme={null} cerulion topic echo perception/camera/image ``` Echo pretty-prints `std_msgs/String` and `sensor_msgs/Image` messages (identified by their schema hash). For any other schema, it prints a hex preview of the payload. ## Measure publish rate `cerulion topic hz ` measures how often a topic publishes, until Ctrl+C. ```bash theme={null} cerulion topic hz perception/camera/image ``` For the periodic camera created with `--policy period_ms=33`, `topic hz` should report roughly 30 Hz. ## Open the interactive dashboard `cerulion tui` launches an interactive terminal dashboard. Logging is suppressed while the TUI is open. ```bash theme={null} cerulion tui ``` ## Inspect recorded traces `cerulion trace inspect ` reads `trace_*.jsonl` bag files from a directory (in lexicographic order) and prints one line per record: ```text theme={null} seq= t=ns schema=0x ``` | Flag | Value | Effect | | ------------------ | ------- | --------------------------------------- | | `-t` / `--filter` | `TOPIC` | Show only records for this exact topic. | | `-n` / `--limit` | `N` | Print at most N records. | | `-r` / `--reverse` | None | Reverse order (most recent first). | ```bash theme={null} cerulion trace inspect ./traces -t perception/camera/image -n 20 -r ``` Tracing is built into the runtime: every graph run automatically produces `trace_*.jsonl` bag files via the publish trace, so there's nothing extra to start or configure. `cerulion trace inspect` reads those bags afterward. ## Clean up stale services iceoryx2 keeps on-disk bookkeeping for each node. When a graph's topology changes between runs, stale entries from dead nodes can cause discovery errors. `cerulion clean` removes bookkeeping for **dead** nodes only; live sibling processes are left untouched. It prints cleanup counts and per-cause remediation. ```bash theme={null} cerulion clean ``` Run `cerulion clean` if you rewire a graph and then see stale-service errors on the next `graph run`. It will not disturb a graph that is currently running. ## Next steps Produce the topics you inspect here. Understand the message shapes that echo prints.