Re: [gentoo-user] Cross system dependencies

[thegeezer]

On 06/28/2014 07:06 PM, J. Roeleveld wrote:
> On Saturday, June 28, 2014 01:39:41 PM Neil Bothwick wrote:
>> On Sat, 28 Jun 2014 11:36:11 +0200, J. Roeleveld wrote:
>>> I need a way to add dependencies to services which are provided by
>>> different servers. For instance, my mail server uses DNS to locate my
>>> LDAP server which contains the mail aliases. All these are running on
>>> different machines. Currently, I manually ensure these are all started
>>> in the correct sequence, I would like to automate this to the point
>>> where I can start all 3 servers at the same time and have the different
>>> services wait for the dependency services to be available even though
>>> they are on different systems.
>>>
>>> All the dependency systems in the init-systems I could find are all
>>> based on dependencies on the same server. Does anyone know of something
>>> that can already provide this type of dependencies? Or do I need to
>>> write something myself?
>> With systemd you can add ExecStartPre=/some/script to the service's unit
>> file where /some/script waits for the remote services to become available,
>> and possibly return an error if the service does not become available
>> within a set time.
> That method works for any init-system and writing a script to check and if
> necessary fail is my temporary fall-back plan. I was actually hoping for a
> method that can be used to monitor availability and, if necessary, stop
> services when the dependencies disappear.
>
> --
> Joost
>

the difficulty is in identifying failed services.
local network issue / load issue could mean your services start bouncing.
the best way is to have redundancy so it doesn't matter as much

having said all of that::

systemd will start servers and buffer network activity - how this works
for non local services would be interesting to see.

with openrc :
you could on the DNS server have a service which is just a batch script
that uses watches for pid / program path in "ps" which outputs ACK or
NAK to a file in an NFS share  say /nfs/monitoring/dns

then on the mail server you could have a service that polls
/nfs/monitoring/dns for NAK or ACK
you can then choose to have this service directly start your dependent
services, or if you adjust /etc/init.d/postfix to have depends =
"mymonitorDNS" which is an empty shell of a service. your watchdog
service could stop / start the empty shell of a script mymonitorDNS, and
then postfix depends on mymonitorDNS
this would save you from "i've just stopped the mail server for
maintenance and my watchdogservice has just restarted it due to a
NAK>ACK event"

or...
you could have a central master machine which has it's own services,
watchdog and monitor... i.e. /etc/init.d/thepostfixserver start  /
depends on thednsserver which just runs
# ssh postfixserver '/etc/init.d/postfix start' 

or...
puppet and it's kin