dovecot - Jan 2021 - set up for Dovecot redundancy -- store on edge server only if link to backofc server is down, otherwise deliver to final dest?

I run Postfix + Dovecot.

Currently, I've got a cloud-instance of Postfix that handles all the
anti-spam/auth/routing.
On successful 'pass', it resends email -- over a VPN link -- to a
*local* Postfix instance, which then LMTP-delivers to a Dovecot instance on the
same box.

Works great.

I'd like to set up some IMAP redundancy, so that if the VPN link is down for
any reason, mail is stored/accessible, and -- eventually -- correctly
(re)delivered to the local Dovecot instance when the link's up again.

The goal state I'm considering looks like:

@cloud

	Postfix
	LMTP delivery from Postfix to "Dovecot1"

@local

	LMTP (re)delivery from "Dovecot1", @cloud, to "Dovecot2"

IDEALLY, no IMAP mail should ever be persistently stored on the @cloud Dovecot
instance -- unless the VPN link is down.
Then, when the VPN link is up again, the @cloud-stored IMAP mail should me
'moved' to the @local instance.

I.e., I do _not_ want a replicated store.

My question is how transparent/automated can this be done?

I'd guess that somewhere in Dovecot's config I need logic that provides
the conditional delivery to just @cloud vs 'all the way' to @local ...

Any suggestions as to how to put this VPN-conditional redundancy in place?

Perhaps I misunderstood the constraints, but (for a low nunber od accounts) what
would be wrong with simply fetching the mail on the local server from dovecot1
and delivering it to dovecot2 with IMAP idle to prevent delays?

If the VPN link is down mail will stay on dovecot1. You could even configure a
postponed deletion (hours or days) so you would be able to access recent mail if
dovecot2 is down.

Am 4. Januar 2021 14:52:20 MEZ schrieb PGNet Dev <pgnet.dev at
gmail.com>:
>I run Postfix + Dovecot.
>
>Currently, I've got a cloud-instance of Postfix that handles all the
>anti-spam/auth/routing.
>On successful 'pass', it resends email -- over a VPN link -- to a
>*local* Postfix instance, which then LMTP-delivers to a Dovecot
>instance on the same box.
>
>Works great.
>
>I'd like to set up some IMAP redundancy, so that if the VPN link is
>down for any reason, mail is stored/accessible, and -- eventually --
>correctly (re)delivered to the local Dovecot instance when the link's
>up again.
>
>The goal state I'm considering looks like:
>
>@cloud
>
>	Postfix
>	LMTP delivery from Postfix to "Dovecot1"
>
>@local
>
>	LMTP (re)delivery from "Dovecot1", @cloud, to
"Dovecot2"
>
>IDEALLY, no IMAP mail should ever be persistently stored on the @cloud
>Dovecot instance -- unless the VPN link is down.
>Then, when the VPN link is up again, the @cloud-stored IMAP mail should
>me 'moved' to the @local instance.
>
>I.e., I do _not_ want a replicated store.
>
>My question is how transparent/automated can this be done?
>
>I'd guess that somewhere in Dovecot's config I need logic that
provides
>the conditional delivery to just @cloud vs 'all the way' to @local
...
>
>Any suggestions as to how to put this VPN-conditional redundancy in
>place?
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