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
