Stream Receive
Receive KubeMQ Queue messages with the C++ SDK using the downstream queue stream API for continuous consumption.
Overview
A downstream receiver is the persistent-connection way to pull queue messages: instead of opening and tearing down a request for every batch, you open one gRPC stream and reuse it across many poll cycles. That matters for any consumer that runs continuously — a worker loop, a background processor — where reconnecting per batch would add latency and churn on both the client and the broker.
The receiver is created once with NewQueueDownstreamReceiver(), then each call to Poll() fetches a batch with auto_ack = false so nothing is removed from the queue until you explicitly settle it. Every message carries a transaction_id; acknowledging it — individually, or as a batch with AckAll() — permanently removes it, while leaving it unacknowledged returns it for redelivery once the visibility timeout expires.
Gotchas: an unclosed receiver holds server-side state — always call Close() on both the receiver and the client when done; a crash between receiving and acknowledging redelivers the batch, so processing must be idempotent; and forgetting to set auto_ack = false silently drops the manual-settlement guarantee this pattern exists for.
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_stream/stream_receive
//
// Demonstrates receiving queue messages using NewQueueDownstreamReceiver + Poll.
// The receiver manages a persistent downstream stream with automatic reconnection.
//
// Channel: cpp-queues.stream-receive
// Client ID: cpp-queues-stream-receive-client
//
// Run with a KubeMQ server on localhost:50000
// (see https://docs.kubemq.io/deploy).
#include <kubemq/kubemq.h>
#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-queues-stream-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.stream-receive";
// Send a message to the queue so we have something to receive.
std::cout << "[2] Sending message to queue " << channel << std::endl;
auto msg_result =
kubemq::QueueMessage::Builder().SetChannel(channel).SetBody("message to receive").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;
}
std::cout << "[3] Message sent" << std::endl;
// Allow message to be committed to the queue.
std::this_thread::sleep_for(std::chrono::seconds(1));
// Open a downstream receiver.
std::cout << "[4] Opening downstream receiver" << std::endl;
auto receiver_result = client->NewQueueDownstreamReceiver();
if (!receiver_result.ok()) {
std::cerr << "[ERROR] NewQueueDownstreamReceiver: " << receiver_result.status().message()
<< std::endl;
return 1;
}
auto& receiver = *receiver_result;
// Poll for messages (manual ack).
kubemq::PollRequest poll_req;
poll_req.channel = channel;
poll_req.max_items = 10;
poll_req.wait_timeout_seconds = 5;
poll_req.auto_ack = false;
auto poll_result = receiver->Poll(poll_req);
if (!poll_result.ok()) {
std::cerr << "[ERROR] Poll: " << poll_result.status().message() << std::endl;
return 1;
}
if (poll_result->is_error()) {
std::cerr << "[ERROR] Poll response error: " << 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() << " tx=" << dm.transaction_id() << std::endl;
}
// Ack all received messages.
if (!poll_result->messages().empty()) {
auto ack_status = poll_result->AckAll();
if (!ack_status.ok()) {
std::cerr << "[ERROR] AckAll: " << ack_status.message() << std::endl;
return 1;
}
std::cout << "[6] Messages acknowledged" << std::endl;
}
// Close the receiver and client explicitly.
// Note: Destructors also handle cleanup, but explicit calls are shown
// for clarity and to match Go's defer pattern.
receiver->Close();
std::cout << "[7] Downstream receiver closed" << 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 << "[8] Client closed" << std::endl;
return 0;
}How It Works
- Creates a downstream receiver with
NewQueueDownstreamReceiver(). - Polls for messages using
receiver->Poll(poll_req)with configurable timeout and max items. - Messages can be individually settled with
Ack(),Nack(), orReQueue(). - The receiver maintains a persistent connection for efficient polling.
Related
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