DEV Community

Wesley Conde
Wesley Conde

Posted on

Scheduled and delayed messages:Routing with TTL and dead-letter exchange in RabbitMQ

#csharp #scheduling #events #rabbitmq

The challenge of scheduling events without plugins

Often, we need to delay message delivery in our messaging architectures: for example, sending a reminder email 2 hours after account creation, or retrying a temporarily failed transaction after a planned 10-minute delay.

While RabbitMQ offers an official delayed messages plugin (rabbitmq-delayed-message-exchange), it is not always available or permitted in managed cloud environments (like AWS Amazon MQ).

The onkai-unified-bus bypasses this limitation by designing a native topology based on message Time-To-Live (TTL) expiration directed toward a Dead-Letter Exchange (DLX).

The delayed message routing flow

To delay a message, we create an indirect routing flow:

  1. Publish the event to a temporary queue that has no active consumers.
  2. Configure a x-message-ttl limit on this queue equivalent to the desired delay.
  3. Configure a Dead-Letter Exchange (DLX) pointing to the final target exchange.
  4. When the TTL expires, RabbitMQ automatically evicts the message from the temporary queue and routes it to the exchange configured in the DLX. 
package main

import (
    "context"
    "fmt"
    "github.com/rabbitmq/amqp091-go"
)

// SetupDelayedTopology declares a consumer-less queue with TTL and DLX arguments
func SetupDelayedTopology(ch *amqp091_go.Channel, delayMs int) (string, error) {
    delayQueueName := fmt.Sprintf("delay-queue-%dms", delayMs)

    // Declares the queue with specific TTL and DLX configurations
    _, err := ch.QueueDeclare(
        delayQueueName,
        true,  // durable
        false, // auto-delete
        false, // exclusive
        false, // no-wait
        amqp091_go.Table{
            "x-message-ttl":             int32(delayMs),       // Retention period
            "x-dead-letter-exchange":    "main-event-exchange", // Destination post expiration
            "x-dead-letter-routing-key": "notifications.send",
        },
    )
    return delayQueueName, err
}
Enter fullscreen mode Exit fullscreen mode

Benefits of the native approach

Using this routing pattern:

  • Eliminates dependencies on third-party broker plugins.
  • Leverages the native persistence and reliability guarantees of the RabbitMQ engine.
  • Supports varied delay durations by declaring queues with specific TTLs on the fly.

Technical terms demystified

  • TTL (Time-To-Live): The maximum period of time a message can remain in a queue before being discarded or moved to a dead-letter exchange.
  • Dead-Letter Exchange (DLX): A normal RabbitMQ exchange where expired, rejected, or failed messages are routed automatically.
  • Delayed Topology: An arrangement of exchanges and queues configured to temporarily hold messages for a controlled time.

link: https://github.com/wesleyskap/onkai-unified-bus

Top comments (0)