# Send Command (/sdks/cpp/tutorials/command-send)



## Overview [#overview]

A **command** is KubeMQ's fire-and-confirm RPC pattern: you reach for it when you need to know that an action actually ran on the other end — "do-something" — but you don't need any data back, just a yes/no on execution. It's the middle ground between one-way pub/sub, where you get no confirmation at all, and a query, where the handler returns a result payload. Commands turn "I hope that worked" into a definite outcome your caller can branch on.

This sample builds that lesson: `client->SubscribeToCommands()` registers the handler, and `client->SendCommand()` blocks until the handler replies or the timeout set with `Command::Builder().SetTimeout(...)` expires. The handler builds its reply with `CommandReply::Builder().SetRequestId(cmd.id).SetResponseTo(cmd.response_to).SetExecuted(true)` — that correlation is what lets the broker route the response back to the exact caller waiting on it.

**Gotchas:** if no handler is subscribed (or it's still starting up), `SendCommand()` blocks for the full timeout before returning a failed status — there's no fast "nobody's listening" error. A handler that omits `SetRequestId`/`SetResponseTo` on the reply leaves the caller hanging until timeout. And a command's reply carries no business data — if you need the handler to return a value, use a query instead.

## Prerequisites [#prerequisites]

* KubeMQ server running on `localhost:50000`
* C++ SDK installed (vcpkg or CMake FetchContent)
* C++17 compiler (GCC 9+, Clang 9+, MSVC 2019+)

## Code [#code]

```cpp title="main.cc"
// Example: commands/send_command
//
// Demonstrates sending a command (RPC-style request) and receiving a response.
// A handler subscribes to the command channel, processes the command,
// and sends back an execution response.
//
// Channel: cpp-commands.send-command
// Client ID: cpp-commands-send-command-client
//
// Run with a KubeMQ server on localhost:50000
// (see https://docs.kubemq.io/deploy).

#include <kubemq/kubemq.h>

#include <atomic>
#include <chrono>
#include <iostream>
#include <thread>

int main() {
    std::cout << "[1] Connecting to localhost:50000" << std::endl;

    kubemq::ClientOptions options;
    options.set_address("localhost", 50000);
    options.set_client_id("cpp-commands-send-command-client");

    auto client_result = kubemq::Client::Create(options);
    if (!client_result.ok()) {
        std::cerr << "[ERROR] Failed to create client: " << client_result.status().message()
                  << std::endl;
        return 1;
    }
    auto& client = *client_result;

    std::string channel = "cpp-commands.send-command";
    std::atomic<bool> command_handled{false};

    // Subscribe to handle incoming commands on the channel.
    std::cout << "[2] Subscribing to commands on channel " << channel << std::endl;
    auto sub_result = client->SubscribeToCommands(
        channel, "",
        [&client, &command_handled](const kubemq::CommandReceive& cmd) {
            std::cout << "[4] Command received: channel=" << cmd.channel << " body=" << cmd.body
                      << std::endl;

            // Build and send a response indicating successful execution.
            auto now_epoch = std::chrono::duration_cast<std::chrono::seconds>(
                                 std::chrono::system_clock::now().time_since_epoch())
                                 .count();
            auto reply_result = kubemq::CommandReply::Builder()
                                    .SetRequestId(cmd.id)
                                    .SetResponseTo(cmd.response_to)
                                    .SetBody("executed")
                                    .SetExecuted(true)
                                    .SetExecutedAt(now_epoch)
                                    .Build();
            if (!reply_result.ok()) {
                std::cerr << "[ERROR] Build reply: " << reply_result.status().message()
                          << std::endl;
                return;
            }
            auto send_status = client->SendCommandResponse(*reply_result);
            if (!send_status.ok()) {
                std::cerr << "[ERROR] SendCommandResponse: " << send_status.message() << std::endl;
                return;
            }
            command_handled.store(true);
        },
        [](const kubemq::Status& err) {
            std::cerr << "[ERROR] Command subscription error: " << err.message() << std::endl;
        });
    if (!sub_result.ok()) {
        std::cerr << "[ERROR] SubscribeToCommands: " << sub_result.status().message() << std::endl;
        return 1;
    }
    auto& sub = *sub_result;

    // Allow subscription to fully establish before sending.
    std::this_thread::sleep_for(std::chrono::milliseconds(300));

    // Build and send a command with a 10-second timeout.
    std::cout << "[3] Sending command to channel " << channel << std::endl;
    auto cmd_result = kubemq::Command::Builder()
                          .SetChannel(channel)
                          .SetBody("do-something")
                          .SetTimeout(std::chrono::seconds(10))
                          .Build();
    if (!cmd_result.ok()) {
        std::cerr << "[ERROR] Build command: " << cmd_result.status().message() << std::endl;
        return 1;
    }
    auto resp_result = client->SendCommand(*cmd_result);
    if (!resp_result.ok()) {
        std::cerr << "[ERROR] SendCommand: " << resp_result.status().message() << std::endl;
        return 1;
    }
    std::cout << "[5] Command response: executed=" << std::boolalpha << resp_result->executed
              << std::endl;

    // Wait for the handler to finish processing.
    std::this_thread::sleep_for(std::chrono::seconds(1));

    // Cancel the subscription explicitly.
    // Note: The Subscription destructor also calls Cancel(), but explicit
    // cleanup is shown here for clarity and to match Go's defer pattern.
    sub->Cancel();
    std::cout << "[6] Subscription cancelled" << std::endl;

    auto close_status = client->Close();
    if (!close_status.ok()) {
        std::cerr << "[ERROR] Close failed: " << close_status.message() << std::endl;
        return 1;
    }
    std::cout << "[7] Client closed" << std::endl;

    return 0;
}
```

## How It Works [#how-it-works]

* Subscribes to handle commands with `SubscribeToCommands()`.
* The handler builds a `CommandReply` with `SetExecuted(true)` and sends it back.
* Sends a command using `Command::Builder()` with channel, body, and timeout.
* The `SendCommand()` call blocks until the handler responds or the timeout expires.

## Related [#related]

* [Pattern overview](/learn/rpc/getting-started)
* [C++ SDK Reference](/sdks/cpp/reference/rpc)
* [Handle Command](/sdks/cpp/how-to/rpc/command-handle)
* [Command Timeout](/sdks/cpp/how-to/rpc/command-timeout)