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Failure and Dead Letter Queues

When a Request is passed to RequestHandler.Handle() it runs in your application code. If your application code fails, you have a number of options:
Any unhandled exception that leaves the Request Handling Pipeline (in other words is not intercepted by middleware) will Terminate Processing of the Request.

Retry (and Circuit Break) the Request on the Internal Bus

You can use Brighter's support for Polly policies to retry the operation on the Internal Bus. See Retry and Circuit Breaker
A circuit breaker is triggered when all Retry attempts fail, and will prevent further requests from succeeding.
Both the triggering of the circuit breaker, and requests passed to the Request Handler Pipeline while the circuit breaker is open will Terminate processing of that Request

Retry (with Delay) the Request on the External Bus

If you the failure is potentially retriable, but you want to retry on the External Bus (by making the message available to be consume from the External Bus again) then you can throw a DeferMessageAction exception. Upon receipt of a DeferMessageAction the pump will Reject the message and Requeue it. with a delay. The delay is configured by the External Bus Subscription.RequeueDelayInMilliseconds property.
You can configure a limit on the number of requeue attempts by setting the Subscription.RequeueCount. A value of -1 will allow infinite retries.

Terminate processing of that Request

An unhandled exception leaving the pipeline results in us terminating processing of the Request.
  • On an Internal Bus that exception will bubble out to the caller.
  • On an External Bus we will nack (or reject) the message. On a queue this will delete the message for all consumers, on a stream we will increment the offset past that message for a consumer group.
We do this because you have responsibility to handle exceptions thrown in your code, not the framework and we assume that non-recovered errors are not potentially retriable.
On and Exernal Bus if your middleware supports a Dead Letter Queue (DLQ), and it is configured in your subscription, when we reject a message it will be copied to the DLQ.
On an External Bus, to prevent discarding too many messages, you can set an Subscription.UnacceptableMessageLimit. If the number of messages terminated due to unhandled exceptions equals or exceeds this limit, the message pump processing the External Bus will terminate.

Run a Fallback

If you want to take action before exiting a handler, due to a failure you can use a Fallback policy. See Fallback Policy for more details

Use Custom Middleware

If none of the above options meet your needs, you can define custom approaches to exception handling by building your own middleware, see Pipeline.