AzureRecipes

Introduction

Application often needs some kind of an event scheduler, which triggers a process in a particular point in time. This schedule must be configurable and could include multiple occurrences in a particular interval. Luckily, the Azure Service Bus provides the functionality of scheduled-messages, even in its most basic plan. Based in this, such an event scheduler can be easily implemented with following advantages:

• Highly cost-efficient and scalable
• Robustness thanks to included retry-handling and dead-letter queue (all configurable)
• Order-guarantee and for most situations sufficient time-precision
• Monitoring functionality and possibilities for manual interactions (e.g. re-dispatched dead-lettered messages)

Getting Started

The snippet implements:

• An initial definition of an event (trigger) which normally would take place after this information is persisted in a datastore. This is shown with the HTTP triggered function PostEventConfig.
• The function HandleTriggerQueueMessage bases on a Service Bus trigger and is called when the configured due date is reached. It included:
  • Check if the event-/trigger-definition is still current (example: for Cosmos DB stored entities this can be done by comparing the ETag information). If the event has become obsolete, the process is aborted. Concept: If the event-definition has been updated, a new message has been placed in the Service Bus.
  • Check if the event has a definition of recurrence. If so, the next iteration is scheduled and dispatched to the Service Bus.
  • (Triggering of the process which is connected to the event)

public static async Task ScheduleTrigger(ServiceBusMessage message, DateTimeOffset dueTimestamp, ILogger log, CancellationToken cancellationToken = default)
{
    try
    {
        await using var sender = ServiceBusClient.Value.CreateSender(Configurations.ServiceBusQueueName);
        var seqNum = await sender.ScheduleMessageAsync(message, dueTimestamp, cancellationToken);
        log.LogInformation($"Trigger scheduled for MessageId {message.MessageId} -> SequenceNumber = {seqNum}");
    }
    catch (Exception e)
    {
        log.LogError(e, "Dispatch message to ServiceBus failed");
    }
}

Deployment (Azure Resources)

References

• MSDN Message sequencing and timestamps

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