Docker-compose links vs external_links

Question

I believe it is simple question but I still do not get it from Docker-compose documentations. What is the difference between links and external_links?

I like external_links as I want to have core docker-compose and I want to extend it without overriding the core links.

What exactly I have, I am trying to setup logstash which depends on the elasticsearch. Elasticsearch is in the core docker-compose and the logstash is in the depending one. So I had to define the elastic search in the depended docker-compose as a reference as logstash need it as a link. BUT Elasticsearch has already its own links which I do not want to repeat them in the dependent one.

Can I do that with external_link instead of link?

I know that links will make sure that the link is up first before linking, does the external_link will do the same?

Any help is appreciated. Thanks.

Answer 1

Use links when you want to link together containers within the same docker-compose.yml. All you need to do is set the link to the service name. Like this:

---
elasticsearch:
  image: elasticsearch:latest
  command: elasticsearch -Des.network.host=0.0.0.0
  ports:
    - "9200:9200"

logstash:
  image: logstash:latest
  command: logstash -f logstash.conf
  ports:
    - "5000:5000"
  links:
    - elasticsearch

If you want to link a container inside of the docker-compose.yml to another container that was not included in the same docker-compose.yml or started in a different manner then you can use external_links and you would set the link to the container's name. Like this:

---
logstash:
  image: logstash:latest
  command: logstash -f logstash.conf
  ports:
    - "5000:5000"
  external_links:
    - my_elasticsearch_container

I would suggest the first way unless your use case for some reason requires that they cannot be in the same docker-compose.yml