I would like to install Icinga 2. My requirements are quite limited (w.r.t. the number of targets). Can I run Icinga 2 without Redis, for reasons of simplicity? The installation manual is not clear about this [1].
yes you can, but without redis there is no icingadb so have to use the “old” ido feature with mysql/mariadb or postgres, and the monitoring module.
here is the old documentation
https://icinga.com/docs/icinga-2/2.12/doc/02-installation/#configuring-db-ido-mysql
Thanks for the quick response. I was a bit confused on the different documentation locations for the various Icinga products (Web, DB, Web DB, …).
Since there seem to be no plans to remove the deprecated IDO feature in the near future, I will give it a try.
There may not be a date yet when support for the IDO backend is dropped, but basing off a fresh installation of it may not be the best idea nonetheless.
I’d strongly advice against it, especially since you don’t have a big environment, there isn’t much speaking against Icinga DB. I also think that such small installations don’t get much maintenance so you’ll certainly be surprised once the support for the IDO backend is indeed dropped and things just don’t work anymore.
But if you don’t keep this system running longer than, say two years, there is indeed no need to worry about the removal of the IDO backend. I’d still strongly advice against it, you’ll miss some features some day otherwise 
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found it in the latest doc, in the deprecated section:
https://icinga.com/docs/icinga-2/latest/doc/14-features/#ido-database-db-ido