Running two puppetdb in front of a single postgresql here and it's fine. The 
puppetservers use one puppetdb for everything and the second puppetdb is for 
monitoring and interactive use. The only hiccoughs are when trans-oceanic links 
blip.

The puppetdb instances are managed using puppet and at my SLA levels I get to 
shrug and say that agent runs will fail for a minute or so during the upgrade. 
Depending on your SLA you may have to upgrade at a specific time or swap 
servers behind a load balancer or something else quicker.

If I had a tighter SLA I would build new upgraded puppetdb hosts instead of 
upgrading in place. (Then repoint the puppetservers and turn down the old 
puppetdb hosts.) I haven't had schema issues during puppetdb upgrades.

For the puppet 4->5 upgrade I have a new 1xPostgreSQL9.6+2xPuppetDB5 set built, 
the upgraded puppetservers will fill them up soon enough.

On Thu, Nov 02, 2017 at 03:35:33PM +0100, Fabrice Bacchella wrote:
> Is there any risk running multiple puppetdb reading or perhaps writing to a 
> single postgres database ?
> 
> For my understanding, all puppetdb's data are stored in pg, so there is no 
> coherency problems, but too much caching on pdb's side might break that 
> assumption.
> 
> There is also the schema consistency. Upgrading one puppetdb without checking 
> the other might break it.
> 
> Is there any other failure case that I didn't think about ?
> 
> The goal is not to have a full HA solution, but be able to upgrade the 
> servers regularly without service disruption, as our puppetdb is used by 
> other tools than the puppet server.
> 
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