Send & Receive

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: SendQueueMessageAsync enqueues a message on a channel, and a QueueDownstreamReceiver's PollAsync pulls it back within a bounded WaitTimeoutSeconds. With AutoAck = false, settlement is manual — each message must be explicitly confirmed with AckAsync(), rejected with NackAsync(), or returned to the queue with ReQueueAsync() once your handler decides its outcome.

Gotchas: if your handler crashes before calling AckAsync(), the message stays locked to that receiver only until the broker's visibility timeout expires, after which it reappears for redelivery — write handlers that tolerate seeing the same message twice. Polling an empty queue isn't an error; PollAsync just blocks up to WaitTimeoutSeconds and returns a batch with no messages. And the downstream receiver is meant to be created once and reused across many PollAsync calls, not recreated per poll.

Prerequisites

• KubeMQ server running on localhost:50000
• C# SDK installed (dotnet add package KubeMQ.SDK.CSharp)

Code

// KubeMQ .NET SDK — Queues: Basic Send and Receive
//
// This example demonstrates sending a queue message and receiving it with acknowledgment.
// Queue messages are pull-based and processed by exactly one consumer.
// Uses QueueDownstreamReceiver.PollAsync for transactional message settlement.
//
// Prerequisites:
//   - KubeMQ server running on localhost:50000
//   - dotnet run

using KubeMQ.Sdk.Client;
using KubeMQ.Sdk.Queues;
using System.Text;

// TODO: Replace with your KubeMQ server address
await using var client = new KubeMQClient(new KubeMQClientOptions
{
    ClientId = "csharp-queues-send-receive-client",
});
await client.ConnectAsync();

Console.WriteLine("Connected to KubeMQ server");

// Send a queue message
var sendResult = await client.SendQueueMessageAsync(new QueueMessage
{
    Channel = "csharp-queues.send-receive",
    Body = Encoding.UTF8.GetBytes("Process order #1234"),
    Tags = new Dictionary<string, string> { ["priority"] = "high" }
});

Console.WriteLine($"Sent message: {sendResult.MessageId}");

// Receive via downstream receiver (supports manual settlement)
await using var receiver = await client.CreateQueueDownstreamReceiverAsync();

var batch = await receiver.PollAsync(new QueuePollRequest
{
    Channel = "csharp-queues.send-receive",
    MaxMessages = 1,
    WaitTimeoutSeconds = 10,
    AutoAck = false,
});

if (batch.HasMessages)
{
    foreach (var msg in batch.Messages)
    {
        Console.WriteLine($"Received: {Encoding.UTF8.GetString(msg.Body.Span)}");
        await msg.AckAsync();
        Console.WriteLine("Message acknowledged");
    }
}
else
{
    Console.WriteLine("No messages received");
}

Console.WriteLine("Done.");

// Expected output:
// Connected to KubeMQ server
// Sent message: <message-id>
// Received: Process order #1234
// Message acknowledged
// Done.

How It Works

• SendQueueMessageAsync delivers the message to the broker, which persists it and assigns a unique MessageId. The returned sendResult confirms delivery.
• CreateQueueDownstreamReceiverAsync opens a long-lived gRPC stream to the broker. One receiver instance can be reused across many PollAsync calls — create it once and dispose it with await using.
• PollAsync with AutoAck = false locks the batch for this receiver. Each message must be explicitly settled with AckAsync(), NackAsync(), or ReQueueAsync().
• msg.AckAsync() removes the message from the queue permanently. Unanswered messages return to the queue after the broker's visibility timeout.

Related

• Pattern overview
• C# SDK Reference
• Ack All
• Ack & Reject