wait is a really small Docker utility that blocks until another container is accepting TCP connections.

The default operation looks for a single exported port. Use it like this:

$ docker run -d --name mycontainer some-image-or-other
$ docker run --link mycontainer:mycontainer n3llyb0y/wait
waiting for TCP connections
172.17.0.105:5432 up!

note: it doesn't matter what the link alias is.

If the container you are linking to has multiple port exports then you will need to let wait know which ones you are interested in. To do so, set the PORTS environment variable at run:

$ docker run -d --name mycontainer some-image-or-other
$ docker run -e PORTS=80,443 --link mycontainer:mycontainer n3llyb0y/wait
waiting for TCP connections
172.17.0.105:80 up!
172.17.0.105:443 up!

wait will also interrogate linked services that share the same port (as within a cluster, for example)

$ docker run -d -p 9200 --name cluster1 some-image-or-other
$ docker run -d -p 9200 --name cluster2 some-image-or-other
$ docker run -e PORTS=9200 --link cluster1:w1 --link cluster2:w2 n3llyb0y/wait
waiting for TCP connections
172.17.0.105:9200 up!
........172.17.0.106:9200 up!

By default each connection attempt will bail after 30 seconds. You can specify an override using the TIMEOUT env variable:

$ docker run -d -p 9200 -p 32000 --name myservice some-image-or-other
$ docker run -e PORTS="9200 32000" -e TIMEOUT=10 --link myservice:w1 n3llyb0y/wait
waiting for TCP connections
172.17.0.105:9200 up!
..............................172.17.0.105:32000 WARN: unable to connect

experimental usage in docker-compose.yml

I've used this utility in docker-compose yaml files in order to orchestrate a wait period when running automated tests. I don't know how it will behave in other situations!

example docker-compose.yml:

db:
  image: postgres
  ports:
    - "5432"
es:
  image: dockerfile/elasticsearch
  ports:
    - "9200"
wait:
  image: n3llyb0y/wait
  environment:
    PORTS: "5432 9200"
  links:
    - es
    - db

Here the wait container explicitly links to the db and es containers and passes on the ports required to wait for.

Rather than using docker-compose up, instead use

docker-compose run wait

This launches the linked services and backgrounds them, then returns control after the wait container finishes. It looks like this might need to be tweaked for future versions of docker-compose but as of version 1.1.0 it does just what I need.

TODO Better echo output Some tests would be good

credits

The single port usage idea and dockerfile was pulled from aanand/wait: https://github.com/aanand/docker-wait