Event bus message delivery guarantee...

[javadevmtl]

Hi if we want to do a fire and forget, if eb.send() does not throw an error does it mean the message got delivered or does the receiving verticle have to reply with an "ack" message to be 100%?

[Sean Burns]

From http://vertx.io/docs/vertx-core/java/#_the_theory 

Best-effort delivery

Vert.x does it’s best to deliver messages and won’t consciously throw them away. This is called best-effort delivery.

However, in case of failure of all or parts of the event bus, there is a possibility messages will be lost.

If your application cares about lost messages, you should code your handlers to be idempotent, and your senders to retry after recovery.

So depending on the importance of your message, if you want 100% knowledge of delivery, you should probably ack the message. Then retry if no ack recieved. 

But remember the receiving verticle must handle "duplicates" (Ie be idempotent), because in theory it could receive the message and the ack could be lost (which would cause a retry).

[javadevmtl]

That i read... but does it mean eb.send does not throw an error then the message got sent? Im not talking about if the receiving verticle got it or not and processed it.

[javadevmtl]

I.e if eb.send without a reply handler. If it didn't throw an error. Im assuming it got sent. Im not assuming the receiving side processed it or not.

[Asher Tarnopolski]

i think that when you work in cluster, errors like no consumer handler or delivery timeout will be returned as a callback to reply handler. not sure about sendLocal though. 

[riscad...@gmail.com]

If you there's no registered handler I believe you'll get an error back stating no handler was registered. If there is a handler but it does not respond to the sender for some reason. It will eventually fail with whatever timeout was configured in the send DeliveryOptions... or the default eventbus 30s timeout.

[Jochen Mader]

You only receive an error if you registered a callback handler.

In all other cases the event is simply discarded.

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[javadevmtl]

Thanks. That's what i wanted to know...