Scheduling messages using RabbitMQ delayed exchange

[rvanoord]

Hi,

The MassTransit documentation indicates that it is possible to use RabbitMQ delayed exchanges for scheduling messages.

See: https://masstransit-project.com/advanced/scheduling/rabbitmq-delayed.html

In the example, the only configuration required is: 

var busControl = Bus.Factory.CreateUsingRabbitMq(cfg =>
{
    cfg.UseDelayedExchangeMessageScheduler();

However, when I try this configuration, no scheduled (delayed) messages are received at the destination queue.

As a work-around, I created a message consumer which handles ScheduleMessage<T> (where T is the actual message type) which subsequently delays messages using context.Defer(...). This seems to work, however we are getting a lot of traffic in the message queue which I can't explain.

       

        public virtual Task Consume(ConsumeContext<ScheduleMessage<T>> context)

        {

            var delay = context.Message.ScheduledTime - DateTime.UtcNow;

            if (delay > TimeSpan.Zero)

            {

                return context.Defer(delay);

            }

            if (context.ExpirationTime.HasValue && context.ExpirationTime.Value < DateTime.UtcNow)

            {

                return Task.CompletedTask;

            }

            return context.Send(context.DestinationAddress, context.Message.Payload);

        }

My question is whether this is the correct / expected way of using the delayed exchange message scheduler? I.e. is it really necessary to create a consumer for ScheduleMessage<T>, or is the call to UseDelayedExchangeMessageScheduler() all that should be required.

[Chris Patterson]

How are you sending the initial scheduled message? If you're sending it within a consumer, the configuration you specified is all that is required.

Otherwise, if you're sending outside a consumer, you'll need to create an IMessageScheduler (https://github.com/MassTransit/MassTransit/blob/develop/src/MassTransit/Scheduling/MessageScheduler.cs#L10) from the bus, using the RabbitMQ Scheduler Provider (https://github.com/MassTransit/MassTransit/blob/develop/src/MassTransit.RabbitMqTransport/Scheduling/DelayedExchangeScheduleMessageProvider.cs#L12)

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[Reng van Oord]

Thanks for your reply!

I noticed yesterday that the rabbitMQ delayed message exchange plugin only supports a delay of 2^32-1 milliseconds (roughly 49 days). If this limit is exceeded, messages are sent with no delay. A work-around is to delay messages a maximum of 49 days and subsequently reschedule them to extend the delay. Is this limitation already catered for in the default DelayedExchangeScheduleMessageProvider?

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[Chris Patterson]

No, honestly, I can't imagine scheduling a message that far in advance. If you exceed the limit, you're on your own.

An issue/PR to throw an exception if the duration is outside the limit would be accepted.

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[Eric Piraux]

Hi,

I do agree with Chris, a messaging system is not the place to store data for a long period. As soon as arrived the message should be processed.

If you need to fire an event after a long period of time put it somewhere in a database (with a publishdatetime) and poll it regularly.

Eric

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