Docker: Connecting to external database

Question

I have hbase instance running which my app connects, We are planning to move the app into docker container but keep hbase running outside of docker. I could make the my app running in docker container connect to hbase by using add-host option while running docker container as below

docker run -dit --add-host hbasehost:xxx.xxx.xxx.xxx mydockerimage

However what we need is auto-scaling function of swarm, as we have multiple services to run, what is the correct way of achieving this if i want to run my app as docker service instead of individual container, i couldn't find any references to "--add-host" in "docker service"

Answer 1

Update: As of docker 1.13 you now have a similar flag to add entries to /etc/host.

To add a host at service creation, you can use the --host flag:

docker service create --name myapp --host "hbasehost:xxx.xxx.xxx.xxx" --replicas 5 myimage

To update the service and add an additional host after its creation, you use the --host-add flag:

docker service update --host-add "hbase:x.x.x.x" myapp

You can also remove a host using --host-rm.

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Original answer

--add-host only appends an host:IP association in /etc/hosts.

You can switch from using --add-host to using environment variables with --env even though this will require slight changes to your app to use the environment variable instead of the hostname to connect to hbase.

# Applies environment variables for all tasks in a service.

docker service create --name myapp --replicas=5 --env HBASEHOST=xxx.xxx.xxx.xxx myimage

Then you can scale the service using:

docker service scale myapp=20

Additional tasks should be able to use the environment variable to connect to hbase.

Source: I'm an ex-Docker Swarmkit maintainer

Answer 2

If you pointed your docker hosts to an internal dns server or a dns server that you can create records on, You can create A record for your hbasehost on that DNS server. Containers will use Internal Docker DNS then fall back to hosts DNS Server Settings. But setting a record in /etc/hosts get the job done (weired).

My docker swarm node's resolv.conf:

nameserver 10.110.1.25
nameserver 10.110.1.26

Now verify from inside one of the containers:

root@05d05f934c68:/# ping dc1.mydomain.net
PING dc1.mydomain.net (10.110.1.25): 56 data bytes
64 bytes from 172.20.21.5: icmp_seq=0 ttl=121 time=0.929 ms