Define schema for mapping service resources into DNS

[stevvooe]

While there has been discussion in moby/moby#24973 and
#192, the adoption of a clear schema for mapping service resources into the DNS space is unclear.

The following presents a schema for mapping cluster-level FQDNs from various components:

Resource	Component	Structure	Examples
Cluster	<cluster>	<cluster>	local, cluster0
Namespace	<namespace>	<namespace>.<cluster>	production.cluster0, development.local, system
Node	<node>	<node>.<cluster>	node0.local
Job	<job>	<job>.<namespace>.<cluster>	job0.production.cluster0
Slot	<slot>	<slot id>.<job>.<namespace>.<cluster>	1.job0.production.cluster0
Task	<task>	<task id>.<slot id>.<job>.<namespace>.<cluster>	abcdef.1.job0.production.cluster0

@mavenugo @mrjana @aluzzardi

[stevvooe]

@aluzzardi This is related to what we discussed today.

@mavenugo Is there any progress on making this happen?

[aluzzardi]

@stevvooe How does this map with #1193 since we've adopted <e.ServiceAnnotations.Name>.<Slot>.<TaskID> as a model?

[stevvooe]

@aluzzardi That is just the naming convention, which goes left to right. DNS goes right to left. Completely compatible.

The only open item is the consistency of active slots. We need a way for tasks to discover all of the hostnames of other tasks, via DNS or other, regardless of DNS-RR or VIP mode. This will allow us to support host-identity based services, like zk, etcd, nats, etc.

[mostolog]

Perhaps I'm confused, but shouldn't DNS SRV records will perfectly match for this use case?

Regarding zookeeper config, as stated by documentation, the config file file should look like:

...
server.1=zoo1:2888:3888
server.2=zoo2:2888:3888
server.3=zoo3:2888:3888
...

A Zookeeper image with a defined entrypoint, could execute:

nslookup -querytype=srv _zookepeer._tcp.swarmmode.com

# _service._proto.name.  TTL   class SRV priority weight port target.
_zookepeer._tcp.swarmmode.com.   100 IN    SRV 10       10     2888 zookeper.1.swarmmode.com
_zookepeer._tcp.swarmmode.com.   100 IN    SRV 10       10     2888 zookeper.2.swarmmode.com
_zookepeer._tcp.swarmmode.com.   100 IN    SRV 10       10     2888 zookeper.3.swarmmode.com

And echo records for each server to config file.

Each time a service-task is executed, it should publish SRV entries at swarm internal DNS, and when stopped, removed from it. Of course, I dare to say this without having any clue about Docker internal DNS (or whatever it uses)

Regards.

[stevvooe]

@mostolog That is somewhat the goal here but we first need a plan for mapping these into the SRV records in a consistent manner. At this point, it is fairly ad-hoc, which is very disappointing.

[mostolog]

I have been reading #192 and trying to reply, but at the end I just think I probably lack the needed knowledge to discuss this.

I'll only say that, IMHO, it makes much more sense to have abcdef-1.job-0.cluster-0 rather than abcdef.1.job0.production.cluster0.

[mostolog]

@stevvooe Somehow related to this:

docker run -h myhost...

adds an entry on /etc/host with: ip container-name
Is there any way to specify a domain or set this entry to FQDN? ie: ip myhost.domain.com on /etc/host

Seems doing:

docker run -h myhost.domain.com...

it's not very polite, cause hostname should be only myhost

Is this a missing feature I could request on https://github.com/docker/docker ? Am I missing something?
Thanks

[stevvooe]

#192 is a little ambitious. I wrote that proposal very early on. We mostly keep it open to consider some of the namespacing concepts, but let's not consider that a hard fast model.

Like I've said before, this isn't about a single fix. There needs to be a concerted effort to manage the DNS name mapping.

[vovimayhem]

Will this allow the routing mesh to route requests to different services running on the same port? Is that even being planned for the routing mesh dns stuff???

[stevvooe]

@vovimayhem No, this is more about mapping services into DNS. To multiplex services on the same port, you'll need to introduce an L7 load balancer to manage that in your infrastructure.

[vovimayhem]

Thank you for clarifying that! I've been searching for clear info (a confirmation) about this for almost a month now!

[mostolog]

@stevvooe I'm back! 😝
Have you considered enabling DNS registering in out-of-docker DNS server?

It will also be interesting being able to register under a specific domain tree
Background-related to this: could a docker node create multiple swarm clusters (each having a domain scope)?

[stevvooe]

@mostolog Let's keep the discussion focused on the proposal at hand. The presented ideas are interesting but they are orthogonal to the goal of creating a clear DNS-based service discovery, which is the topic at hand.