> For the complete documentation index, see [llms.txt](https://stephen-tsoi.gitbook.io/stephen-tsoi-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://stephen-tsoi.gitbook.io/stephen-tsoi-docs/messaging-troubleshooting/a-missing-event-among-billions-how-we-found-it-on-an-event-mesh/day-1-13-why-do-events-get-lost.md).

# Day 1/13 Why do events get lost?

You may be wondering why event messages would be lost, especially since event brokers support at-least-once delivery guarantee. To be exact, most event brokers support both at-most-once (direct messaging) and at-least-once (guaranteed messaging) delivery semantics. The following discussions apply only to guaranteed messaging. **A message has to be sent in “persistent” mode in guaranteed messaging.**

It is important to understand that the event broker alone does not provide end-to-end event message delivery guarantee. The responsibility for guaranteeing the delivery of an event message from a publishing application to the event broker lies with both the broker and the publishing application itself. Even for event message consumption, the consuming application has to work with the broker to guarantee that an event message has been duly processed before it is removed from the broker.

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