# Send Query (/sdks/cpp/tutorials/query-send)



## Overview [#overview]

This tutorial builds the RPC half of KubeMQ's request/reply patterns: a **query**, where the caller blocks for a handler's data payload instead of just a completion status. Reach for it whenever a caller needs an answer — fetching a record, running a lookup, or asking another service to compute a value on demand. You'll run a handler and a sender in the same process to see the full round trip.

The sender builds a request with `kubemq::Query::Builder()` and calls `client->SendQuery(...)`, which blocks until a reply arrives. `client->SubscribeToQueries(...)` registers a handler callback; the handler builds a reply with `kubemq::QueryReply::Builder().SetRequestId(q.id).SetResponseTo(q.response_to)` — copied from the incoming query — plus `SetBody(...)`, sent with `client->SendQueryResponse(...)`. KubeMQ routes that reply back to the caller waiting on it.

**Gotchas:** the timeout passed to `SetTimeout(...)` must cover however long the handler takes to run — a slow handler leaves `resp_result->executed` false even though the handler eventually succeeds. No handler subscribed yet also times out rather than erroring immediately. The body is a plain byte payload you encode yourself, and every status-returning call should be checked with `.ok()` before dereferencing the result.

## 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: queries/send_query
//
// Demonstrates sending a query and receiving a response with data.
// Unlike commands, queries return a body payload in the response.
//
// Channel: cpp-queries.send-query
// Client ID: cpp-queries-send-query-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-queries-send-query-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-queries.send-query";
    std::atomic<bool> query_handled{false};

    // Subscribe to handle queries and return data.
    std::cout << "[2] Subscribing to queries on channel " << channel << std::endl;
    auto sub_result = client->SubscribeToQueries(
        channel, "",
        [&client, &query_handled](const kubemq::QueryReceive& q) {
            std::cout << "[4] Query received: channel=" << q.channel << " body=" << q.body
                      << std::endl;

            // Build and send a response with a JSON body.
            auto now_epoch = std::chrono::duration_cast<std::chrono::seconds>(
                                 std::chrono::system_clock::now().time_since_epoch())
                                 .count();
            auto reply_result = kubemq::QueryReply::Builder()
                                    .SetRequestId(q.id)
                                    .SetResponseTo(q.response_to)
                                    .SetBody(R"({"result":"data","status":"ok"})")
                                    .SetMetadata("ok")
                                    .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->SendQueryResponse(*reply_result);
            if (!send_status.ok()) {
                std::cerr << "[ERROR] SendQueryResponse: " << send_status.message() << std::endl;
                return;
            }
            query_handled.store(true);
        },
        [](const kubemq::Status& err) {
            std::cerr << "[ERROR] Query subscription error: " << err.message() << std::endl;
        });
    if (!sub_result.ok()) {
        std::cerr << "[ERROR] SubscribeToQueries: " << 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 query with a 10-second timeout.
    std::cout << "[3] Sending query to channel " << channel << std::endl;
    auto query_build = kubemq::Query::Builder()
                           .SetChannel(channel)
                           .SetBody("fetch-data")
                           .SetTimeout(std::chrono::seconds(10))
                           .Build();
    if (!query_build.ok()) {
        std::cerr << "[ERROR] Build query: " << query_build.status().message() << std::endl;
        return 1;
    }
    auto resp_result = client->SendQuery(*query_build);
    if (!resp_result.ok()) {
        std::cerr << "[ERROR] SendQuery: " << resp_result.status().message() << std::endl;
        return 1;
    }
    std::cout << "[5] Query response: executed=" << std::boolalpha << resp_result->executed
              << " body=" << resp_result->body << 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 queries and return data in the response body.
* The handler builds a `QueryReply` with a JSON body and sends it back.
* Sends a query using `Query::Builder()` with channel, body, and timeout.
* Unlike commands, the query response includes a body payload with data.

## Related [#related]

* [Pattern overview](/learn/rpc/getting-started)
* [C++ SDK Reference](/sdks/cpp/reference/rpc)
* [Handle Query](/sdks/cpp/how-to/rpc/query-handle)
* [Cached Query](/sdks/cpp/how-to/rpc/query-cached)