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new: doc: add doc on root level ``type`` key of ``metadata.yml``

rallly
Valentin Lab 12 months ago
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  1. 63
      README.org

63
README.org

@ -76,3 +76,66 @@ TBD
Most tools should check the =CHARM_STORE= bash environment variable
that should be the path to reach the root of this repository. If not
defined, most tools will look in =/srv/charm-store= by default.
* Specs
** charm type
Not all charm are intended to bring up services as having a container
always running and listening.
In ~metadata.yml~, the root level ~type~ can be one of:
- ~service~ (default)
If not specified, this is the default. A charm brings up a service.
It is meant to be *always running*. For instance, ~apache~, ~mysql~,
~postgres~ are services.
They usually open ports and are listening to provide their service,
or carry background listening of other ressources (like checking
time and sending scheduling command for the ~cron~ services), and or
use files to trigger or report on their activity.
It will have an entry in the final ~docker-compose.yml~, and thus, a
container will run and stay in memory and have a ~restart:
unless-stopped~ policy. They use CPU and memory ressources.
- ~run-once~
The entry is meant to describe *a command that run once*,
it will be called by a service and *will exit after execution*.
For instance, ~logrotate~, ~rsync-backup~, or ~letsencrypt~ are
of type ~run-once~.
They are meant to be run by service for specific events. They
usually will use relations to ensure they are called at specific
moment by service...
A command does not have an automatic ~restart~ policy as services
have.
They use CPU and memory ressources only when run and gives them back
once finished.
- ~stub~
The entry describes an entity that will *not be run at all*. It is
used to hold information in the ~compose.yml~ and often to *stand
for* a real service managed outside of ~compose.yml~ (on an other
host or on a different managing system, like a local installation or
LXC, virtualbox, ...).
For instance, ~stmp-stub~ can be used to stand for an external ~smtp~.
It is through their relation that they shine as they can provide
similar interface than actual services would have
provided. ~smtp-stub~ is a ~smtp-server~ provider and other charm
can connect to it.
They usually implement relation hooks, and are providers.
No entry will be created in the final ~docker-compose.yml~.
They use no CPU or memory ressources at all.
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