Send & Receive
Send and receive messages on a KubeMQ Queue channel with the C++ SDK in a basic producer-consumer workflow.
Overview
Queue send/receive is the foundational operation for guaranteed-delivery, point-to-point messaging: you reach for it whenever work needs to survive past the moment it's created and be handled by exactly one consumer, not broadcast to every subscriber. Unlike pub/sub, a queued message sits durably on the broker until something pulls it, so the producer and consumer never need to be online at the same time — a slow or offline worker adds latency, it doesn't drop the message.
This tutorial builds the smallest possible version of that round trip: client->SendQueueMessage enqueues a message built with QueueMessage::Builder(), and client->PollQueue pulls it back using a PollRequest with a bounded wait_timeout_seconds. The auto_ack field controls settlement — set true and the broker acknowledges each message automatically the instant it's delivered.
Gotchas: auto-ack means "delivered," not "processed" — if your handler crashes after receiving but before finishing the work, the message is already gone with no chance to retry. Polling an empty queue isn't an error; PollQueue just returns an empty result once the wait timeout elapses, so always check is_error() rather than treating zero messages as a failure. And max_items caps how many messages a single poll can return, so don't assume one call drains the whole queue.
Prerequisites
- KubeMQ server running on
localhost:50000 - C++ SDK installed (vcpkg or CMake FetchContent)
- C++17 compiler (GCC 9+, Clang 9+, MSVC 2019+)
Code
// Example: queues/send_receive
//
// Demonstrates basic queue send and receive operations.
// A message is sent to a queue and then consumed (pulled) with auto-ack.
//
// Channel: cpp-queues.send-receive
// Client ID: cpp-queues-send-receive-client
//
// Run with a KubeMQ server on localhost:50000
// (see https://docs.kubemq.io/deploy).
#include <kubemq/kubemq.h>
#include <iostream>
int main() {
std::cout << "[1] Connecting to localhost:50000" << std::endl;
kubemq::ClientOptions options;
options.set_address("localhost", 50000);
options.set_client_id("cpp-queues-send-receive-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-queues.send-receive";
// Build and send a single queue message.
std::cout << "[2] Sending message to queue " << channel << std::endl;
auto msg_result = kubemq::QueueMessage::Builder()
.SetChannel(channel)
.SetBody("hello queue")
.SetMetadata("greeting")
.Build();
if (!msg_result.ok()) {
std::cerr << "[ERROR] Build message: " << msg_result.status().message() << std::endl;
return 1;
}
auto send_result = client->SendQueueMessage(*msg_result);
if (!send_result.ok()) {
std::cerr << "[ERROR] SendQueueMessage: " << send_result.status().message() << std::endl;
return 1;
}
if (send_result->is_error) {
std::cerr << "[ERROR] Send failed: " << send_result->error << std::endl;
return 1;
}
std::cout << "[3] Sent: id=" << send_result->message_id << std::endl;
// Receive (consume) messages from the queue via PollQueue with auto-ack.
std::cout << "[4] Polling queue " << channel << std::endl;
kubemq::PollRequest poll_req;
poll_req.channel = channel;
poll_req.max_items = 10;
poll_req.wait_timeout_seconds = 5;
poll_req.auto_ack = true;
auto poll_result = client->PollQueue(poll_req);
if (!poll_result.ok()) {
std::cerr << "[ERROR] PollQueue: " << poll_result.status().message() << std::endl;
return 1;
}
if (poll_result->is_error()) {
std::cerr << "[ERROR] Poll failed: " << poll_result->error() << std::endl;
return 1;
}
std::cout << "[5] Received: " << poll_result->messages().size() << " messages" << std::endl;
for (const auto& dm : poll_result->messages()) {
std::cout << " body=" << dm.message().body() << " metadata=" << dm.message().metadata()
<< std::endl;
}
// Close the client explicitly.
// Note: The Client destructor also calls Close(), but explicit
// cleanup is shown here for clarity and to match Go's defer pattern.
auto close_status = client->Close();
if (!close_status.ok()) {
std::cerr << "[ERROR] Close failed: " << close_status.message() << std::endl;
return 1;
}
std::cout << "[6] Client closed" << std::endl;
return 0;
}How It Works
- Sends a message to a queue using
QueueMessage::Builder()andSendQueueMessage(). - Receives messages using
PollQueue()with a configuredPollRequest(channel, max items, wait timeout, auto-ack). - Each received message includes body, metadata, and sequence information via
PollResponse::messages(). - With
auto_ack = true, messages are acknowledged automatically on receipt.
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