Delayed Delivery

Messages don't need to be dispatched immediately. Delayed delivery is a feature that sends messages into the future to be delivered and processed at a later time.

Delayed delivery is used for:

Timeout messages sent by sagas
Delayed retries, to retry a message after successive delays when immediate retries don't result in successful processing
Explicitly sending a message with a delay, as described below

Only send operations can be deferred. Publish and reply operations cannot be deferred.

Delaying messages using HandleCurrentMessageLater

The HandleCurrentMessageLater() method was primarily used to defer messages when using Versions 2.x and below, before the Defer functionality was introduced in Versions 3 and above. While this API is still supported in Versions 6 and below, there are significant caveats in using this API.

Calling this method would create a copy of the message that has the same identifier, header, and body. This message would then be put at the end of the queue. The endpoint will eventually pick up this message once all the other messages in its queue have been processed. To make this work, the message pipeline will not abort which means any business transaction that's part of calling this method will also get committed.
If the endpoint's queue is empty, or the condition to put the message back into the queue is incorrect, the message goes back into the queue immediately causing the endpoint to process the same message without any delay. This behavior can cause an endless loop which will manifest itself as a high system resource utilization by the endpoint.

HandleCurrentMessageLater() cannot be used in conjunction with the Outbox.

This method will be deprecated in Version 7.0. It is recommended to use either Delayed Retries or one of the deferring mechanisms below, depending on the scenario.

Delaying message dispatching

Delaying a message is done using SendOptions and the DelayDeliveryWith method. This allows to defer the sending of a message to any endpoint. The behavior of delayed handling using DelayDeliveryWith can be seen in Delayed Delivery Sample.

Using a TimeSpan

Delays delivery of a message for a specified duration.

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var sendOptions = new SendOptions();

sendOptions.DelayDeliveryWith(TimeSpan.FromMinutes(30));

await handlerContext.Send(new MessageToBeSentLater(), sendOptions)
    .ConfigureAwait(false);
// OR
await endpoint.Send(new MessageToBeSentLater(), sendOptions)
    .ConfigureAwait(false);

Using a DateTime

Delays delivery of a message until a specified point in time.

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var options = new SendOptions();
options.DoNotDeliverBefore(new DateTime(2016, 12, 25));

await handlerContext.Send(new MessageToBeSentLater(), options)
    .ConfigureAwait(false);
// OR
await endpoint.Send(new MessageToBeSentLater(), options)
    .ConfigureAwait(false);

Caveats

Delayed delivery of messages is supported when one of the following requirements are met:

The transport supports delayed delivery natively.
The Timeout Manager feature is enabled and the endpoint is not a send-only endpoint.

When deferring a message, it is sent to the Timeout Manager requesting it to deliver the message at a later time or deferred by using the transports native capability to defer messages.

The Timeout Manager is enabled by default. However, it is automatically disabled for send-only endpoints and transports that support delayed delivery natively (i.e. Azure Service Bus).

When relying on Timeout Manager, the sending endpoint must be running when the timeout is reached in order for the message to be sent. If the endpoint is not running when the timeout is reached then the message will be sent when the endpoint is next started.

If specifying a time that is in the past then the message will still be slightly delayed. The message will not be sent until the Timeout Manager has processed the request.

How it works

NServiceBus provides delayed delivery feature for transports that don't have native support for delayed message delivery, i.e. for MSMQ and SQL Server transports. Transports that support delayed message delivery natively don't require persistence to store timeouts. To learn more about NServiceBus delayed message store refer to the Timeout Manager article.