Event retention in Microsoft Azure EventHub

Question

I was checking on details about message retention in event hub. Suppose, I have set the retentionPolicy as 1 day and I had send some messages. Then, if I change the message retentionPolicy to 3 days, will the existing eventData also be retained for 3 days?

Answer 1

Absolutely Yes.

And one more important detail about retention policy - EventHubs does not apply the retention policy at message level. Its at file-system level. EventHubs is a high-throughput event ingestion pipeline. In-short it's a stream of events on cloud - to provide higher thruput & performance - we don't deal with any event-level operations (for example, an equivalent offering is ServiceBus Q/Topics - TimeToLive property on Message). Behind the covers - eventhubs actually stores data in pages (of lets say, for explanation sake 10 MB). Retention policy will be applied only on these pages. So - some of your messages which were sent 10 days before might also still be present, even if you have a retention policy of 1 day - if you have very-very-low data rates on the hub - making into that page.

Answer 2

Yes, If you use UpdateEventHubAsync to update the message retention period. However, the actual message cleanup time is not guaranteed. The azure infrastructure may decide to cleanup based on its dynamics.