If we don't specify any service properties when labelling containers,
the generated service will be named according to the container. However,
we change the container name on every deployment (as it is versioned),
which means that the auto-generated service name will be different in
each container.
That is a problem for two reasons:
- Multiple containers share a common router while a deployment is
happening. At this point, the router configuration will be different
between the containers; Traefik flags this as an error, and stops
routing to the containers until it's resolved.
- We allow custom labels to be set in an app's config. In order to
define custom configuration on the service, we'll need to know what
it will be called.
Changed to force the service name by setting one of its properties.
Replaces our current host-based HTTP healthchecks with Docker
healthchecks, and adds a new `healthcheck.cmd` config option that can be
used to define a custom health check command. Also removes Traefik's
healthchecks, since they are no longer necessary.
When deploying a container that has a healthcheck defined, we wait for
it to report a healthy status before stopping the old container that it
replaces. Containers that don't have a healthcheck defined continue to
wait for `MRSK.config.readiness_delay`.
There are some pros and cons to using Docker healthchecks rather than
checking from the host. The main advantages are:
- Supports non-HTTP checks, and app-specific check scripts provided by a
container.
- When booting a container, allows MRSK to wait for a container to be
healthy before shutting down the old container it replaces. This
should be safer than relying on a timeout.
- Containers with healthchecks won't be active in Traefik until they
reach a healthy state, which prevents any traffic from being routed to
them before they are ready.
The main _disadvantage_ is that containers are now required to provide
some way to check their health. Our default check assumes that `curl` is
available in the container which, while common, won't always be the
case.
Allow the hosts for accessories to be specified by host or role, or on
all app hosts by setting `daemon: true`.
```
# Single host
mysql:
host: 1.1.1.1
# Multiple hosts
redis:
hosts:
- 1.1.1.1
- 1.1.1.2
# By role
monitoring:
roles:
- web
- jobs
```
