Catch 22 with `external_links` mutual dependency

[zeevallin]

As it turns out, when you got two or more projects that want to communicate with each other, docker-compose support is lacking. Starting up project_a requires me to have project_b running and vice versa.

# project_a/docker-compose.yml
app:
  external_links: projectb_app:b

# project_b/docker-compose.yml
app:
  external_links: projecta_app:a

For now I can circumvent this by running an ambassador proxy but it makes me wonder if this is something that should be solved by tooling in docker-compose. I'd love to hear your ideas.

Currently this increases developer sadness quota by 42%

[dnephin]

I am pretty sure this is a limitation of docker links, they can't be circular.

[zeevallin]

@dnephin Yeah, that's what I've gathered reading through documentation. It brings up a good question though. What is the preferred way of dealing with circular dependency?

[dnephin]

I think avoiding them is always nice, but if that's not an option, I've been suggesting you can use a dns service as part of your compose setup (since links are really just an easy way of doing dns).

There seem to be a lot of options for this:

• https://github.com/crosbymichael/skydock
• https://github.com/progrium/registrator
• https://github.com/bnfinet/docker-dns
• http://jasonwilder.com/blog/2014/07/15/docker-service-discovery/

I haven't had a chance to use any of these myself yet, but I would like to get around to it.

[aviau]

Currently this increases developer sadness quota by 42%

I can confirm that I feel the sadness.

Any updates on this?

[zeevallin]

@aviau As far as I've understood, this type of thing is fundamentally not compatible with how docker works.

[dnephin]

I believe the new docker networking will allow circular "links" to exist. You can follow #1676 for the compose side of things.

[zeevallin]

@dnephin Cheers.