On Saturday, June 28, 2014 09:23:17 PM thegeezer wrote: > 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
I know that. A proper system for this would have a configurable amount of retries with a wait-time in between. > 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. It would, but I am not going to migrate my servers to something like systemd without a clear and proven advantage. For me, that currently does not exist. It also would not work as not all the software I run will happily wait while the rest of the stack starts. I would end up in a bigger mess thanks to timeout issues during startup. > 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 Yes, but in order to access the NFS share, I need DNS to be running. Chicken- egg problem. > 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" That is the problem I have with these watchdog services. During boot, I want it to wait. But it needs to understand not to start a service when I stopped it during runtime. Otherwise it could prevent a clean shutdown as well... > 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 Last time I looked at puppet, it seemed too complex for what I need. I will recheck it again. Thanks, Joost
