On Fri, Jul 15, 2016 at 4:31 PM, Derek Mahar <derek.ma...@gmail.com> wrote:
> This is an interesting development. > > How is sd_notify related to a service of Type=notify? > When a service is declared Type=Notify: it is expected that the daemon sends a notification message via sd_notify(3) or an equivalent call when it has finished starting up ( https://www.freedesktop.org/software/systemd/man/systemd.service.html#Type=) When you start a container, you use systemd-nspwan and it sends the "ready message" as soon as the container started not when it is ready. I implemented a new option in systemd-nspawn that allows to delegate this responsibility to the container itself and I am working to make it work in rkt, however this feature will probably available after coreos will use system v231. > > While the container is a highly useful extension or derivative of the > process concept, it surprises me that containers don't better expose or > promote useful inter-process communication mechanisms such as semaphores, > signals, pipes, or message queues to facilitate container coordination on a > host. > > Derek > > On Friday, 15 July 2016 05:51:30 UTC-4, Alban Crequy wrote: >> >> Hi, >> >> There is work going on to implement systemd's mechanism for notifying >> when an application is ready (sd_notify): >> https://github.com/coreos/rkt/issues/1464 >> >> That's useful when the rkt pods are started as systemd services. When >> this work in rkt is done, a rkt pod (e.g. PostgreSQL) would be able to >> notify systemd on the host when it is ready to receive requests (fully >> initialized), and systemd on the host could then only start other >> dependencies (e.g. another rkt pod connecting to PostgreSQL). >> >> For this to work, applications must implement the sd_notify mechanism. >> But I see that PostgreSQL has support for sd_notify: >> >> https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=7d17e683fcc28a1b371c7dd02935728cd2cbf9bf >> >> >> Since sd_notify is not there yet in rkt, it is more difficult but in >> some cases it is possible to use socket activation when the dependency >> can be expressed in terms of a client connecting to the socket of a >> server. See "Socket-activated service": >> https://coreos.com/rkt/docs/latest/using-rkt-with-systemd.html >> >> Cheers, >> Alban >> >> On Fri, Jul 15, 2016 at 2:15 AM, Derek Mahar <derek...@gmail.com> wrote: >> > What is the recommended approach to synchronizing containers in >> CoreOS? For >> > example, how might CoreOS ensure the following container execution >> order at >> > startup? >> > >> > 1. Start PostgreSQL Server >> > 2. Create schema (using separate one-shot Liquibase container) >> > 3. Launch application >> > >> > The application cannot run without its database schema and Liquibase >> cannot >> > create the database schema without PostgreSQL Server being ready to >> receive >> > SQL statements. Which mechanisms in CoreOS can we use to guarantee >> this >> > ordering? >> > >> > Thank you, >> > >> > Derek >> > -- Alessandro Puccetti
