Discovery from code

You get the same answer vrobots topic list gives, as data your program can branch on.

cargo run -p vrobots-examples --bin ex11_topic_discovery
./target/cpp-build/ex11_topic_discovery
python examples/python/ex11_topic_discovery.py

Discovery needs no robot and no connect. It answers "is the simulator publishing, and under which ids" before you have a handle to ask with, which is the order those two questions actually occur in.

The calls

Two listing calls and one capability question. From crates/vrobots-sdk/src/discovery.rs:

#![allow(unused)]
fn main() {
pub fn list_topics(timeout: Duration) -> VrResult<Vec<TopicInfo>>
pub fn list_topics_with(timeout: Duration, options: &ConnectOptions) -> VrResult<Vec<TopicInfo>>
pub fn discovery_covers_all_transports() -> bool
}

inline std::vector<TopicInfo> list_topics(double timeout_s = 1.5,
                                          const vrsdk_connect_options_t* options = nullptr)

def list_topics(
    timeout: float = 1.5, router: Optional[str] = None
) -> list[TopicInfo]: ...

Rust splits the plain and the options-taking call in two; C++ and Python fold both into one function with defaulted trailing arguments, and Python narrows the options to the one field that matters here, router. Neither wrapper exposes discovery_covers_all_transports: it exists only as vrsdk_discovery_covers_all_transports in crates/vrobots-sdk-capi/include/vrobots_sdk.h.

The listing calls block for timeout and print nothing; what they return is the vector described below.

The result is sorted by (sys_id, key), so it is stable run to run. list_topics_with takes a ConnectOptions for the one field that matters here, router_endpoint, which reaches a simulator on another host. discovery_covers_all_transports returns true in this build and exists so a future build without shared memory can say that camera streams are missing rather than let an absent stream read as a broken camera.

An empty vector is a legitimate result, not an error. VrError::Session is what you get when zenoh will not open or the iceoryx2 registry cannot be read.

TopicInfo

Observed versus registered

observed is the field the rest of the struct depends on, and it exists because the two transports answer "what topics are there" by completely different means.

flowchart TB
  Q["list_topics(window)"] --> Z["zenoh: subscribe to vrobots/** for the window"]
  Q --> I["iceoryx2: read the service registry"]
  Z --> ZO["observed = true<br/>it published during the window<br/>hz, samples, bytes are measurements"]
  I --> IR["observed = false<br/>the service is defined<br/>counters are 0, nothing was measured"]
  IR --> L{"live?"}
  L -->|true| A["a process holds it open"]
  L -->|false| D["stale record, owning process is gone"]

Zenoh has no registry. The only honest way to enumerate it is to subscribe to vrobots/** for a window and report what arrived, so a topic appears only if it published during your window, and its counters are real measurements. The consequence is a false negative on slow topics: vrobots/*/z/frames publishes at 1 Hz, so a 0.5 s window loses it entirely. State runs at 25 Hz, so about a second is enough for it.

iceoryx2 does have a registry. An entry means the service is defined, which is not the same as the service producing frames: it may be streaming, or it may be a dead leftover from a process that exited. live is what separates those two. Nothing was watched either way, so samples, bytes and hz are all zero and mean "not measured" rather than "zero traffic".

Gotcha. A router endpoint makes the result asymmetric. Zenoh topics come back from wherever the simulator is, while the iceoryx2 half only ever sees this host, so a remote simulator lists states and services and no camera streams. That is what shared memory means, not a discovery failure.

Reading the flag

From examples/rust/src/bin/ex11_topic_discovery.rs, the print loop is a three-way branch on observed and live rather than a two-way one:

#![allow(unused)]
fn main() {
    println!("\n{:<4} {:>7} {:>9}  topic", "wire", "Hz", "bytes");
    for t in &topics {
        // `observed` decides whether the numbers mean anything at all.
        let (hz, bytes) = if t.observed {
            (format!("{:.1}", t.hz), t.bytes.to_string())
        } else if t.live {
            ("-".to_string(), "-".to_string())
        } else {
            ("stale".to_string(), "-".to_string())
        };
        println!("[{}] {hz:>7} {bytes:>9}  {}", t.transport.tag(), t.key);
    }
}

        std::printf("\n%-4s %7s %9s  topic\n", "wire", "Hz", "bytes");
        for (const vrsdk::TopicInfo& t : topics) {
            // `observed` decides whether the numbers mean anything at all.
            if (t.observed) {
                std::printf("[%s] %7.1f %9llu  %s\n", t.transport, t.hz,
                            static_cast<unsigned long long>(t.bytes), t.key.c_str());
            } else {
                std::printf("[%s] %7s %9s  %s\n", t.transport, t.live ? "-" : "stale", "-",
                            t.key.c_str());
            }
        }

    print(f"\n{'wire':<4} {'Hz':>7} {'bytes':>9}  topic")
    for t in topics:
        # `observed` decides whether the numbers mean anything at all.
        if t.observed:
            hz, nbytes = f"{t.hz:.1f}", str(t.bytes)
        elif t.live:
            hz, nbytes = "-", "-"
        else:
            hz, nbytes = "stale", "-"
        print(f"[{t.transport}] {hz:>7} {nbytes:>9}  {t.key}")

Rust reaches the wire tag through a method, t.transport.tag(), because Transport is an enum; C++ and Python both hand you t.transport already as the string "z" or "i".

Printing 0.0 for a registry entry would be reporting a measurement nobody took. - says the number does not exist, and stale says the entry does not either.

Grouping by robot

The reason to do this in code rather than at the command line is that the result is data. Grouping by sys_id is how a program answers "which robots exist, and does the one I want have a camera".

#![allow(unused)]
fn main() {
    let mut by_robot: BTreeMap<Option<u32>, Vec<&str>> = BTreeMap::new();
    for t in &topics {
        by_robot.entry(t.sys_id).or_default().push(&t.key);
    }
}

        std::map<std::optional<std::uint32_t>, std::vector<std::string>> by_robot;
        for (const vrsdk::TopicInfo& t : topics) {
            by_robot[t.sys_id].push_back(t.key);
        }

    by_robot: dict[int | None, list[str]] = defaultdict(list)
    for t in topics:
        by_robot[t.sys_id].append(t.key)

The absent id is Option<u32> in Rust, std::optional<std::uint32_t> in C++ and None in Python, and all three sort or group on it directly. Rust and C++ get the ordering for free from BTreeMap and std::map; Python's dict does not order, so the example sorts the keys itself before printing.

manager and scene sit where an id would in the key, so they can never collide with a robot, and they parse as None.

The whole example prints the table and then the grouping:

listening for 1.5s ...

wire      Hz     bytes  topic
[i]       -         -  vrobots/1/i/cam/front_left/720p_rgba8
[i]       -         -  vrobots/1/i/cam/front_right/720p_rgba8
[z]     1.3      1760  vrobots/1/z/frames
[z]    25.3     45600  vrobots/1/z/state

by robot:
  sys_id 1: 4 topic(s)
      vrobots/1/i/cam/front_left/720p_rgba8
      vrobots/1/i/cam/front_right/720p_rgba8
      vrobots/1/z/frames
      vrobots/1/z/state

Next: Versions and pins

See also: The vrobots command, Two transports, one simulator, The topic namespace