How to specify an iterator in the volume path when using docker-compose to scale up service?

Question

Background: I'm using docker-compose in order to place a tomcat service into a docker swarm cluster but I'm presently struggling with how I would approach the logging directory given that I want to scale the service up yet retain the uniqueness of the logging directory.

Consider the (obviously) made up docker-compose which simply starts tomcat and mounts a logging filesystem in which to capture the logs.

version: '2' services: tomcat: image: "tomcat:latest" hostname: tomcat-example command: /start.sh volumes: - "/data/container/tomcat/logs:/opt/tomcat/logs,z"

Versions

• docker 1.11
• docker-compose 1.7.1
• API version 1.21

Problem: I'm looking to understand how I would approach inserting a variable into the 'volume' log path so that the log directory is unique for each instance of the scaled service

say,

volumes:
    - "/data/container/tomcat/${container_name}/logs:/opt/tomcat/logs,z"

I see that based on project name (or directory I'm in) the container name is actually known, so could I use this ?

eg, setting the project name to 'tomcat' and running docker-compose scale tomcat=2 I would see the following containers.

• hostname/tomcat_1
• hostname/tomcat_2

So is there any way I could leverage this as a variable in the logging volume, Other suggestions or approaches welcome. I realise that I could just specify a relative path and let the container_id take care of this, but now if I attach splunk or logstash to the logging devices I'd need to know which ones are indeed logging devices as opposed to the base containers f/s. However Ideally I'm looking use a specific absolute path here.

Thanks in advance dockers! R.

Answer 1

You should really NOT log to the filesystem, and use a specialized log management tool like graylog/logstash/splunk/... instead. Either configure your logging framework in Tomcat with a specific appender, or log to sysout and configure a logging driver in Docker to redirect your logs to the external destination.

This said, if you really want to go the filesystem way, simply use a regular unnamed volume, and then call docker inspect on your container to find the volume's path on the filesystem :

[...snip...]
"Mounts": [
    {
        "Type": "volume",
        "Name": "b8c...SomeHash...48d6e",
        "Source": "/var/lib/docker/volumes/b8c...SomeHash...48d6e/_data",
        "Destination": "/opt/tomcat/logs",
[...snip...]

If you want to have nice-looking names in a specific location, use a script to create symlinks.

Yet, I'm still doubtfull on this solution, especially in a multi-host swarm context. Logging to an external, specialized service is the way to go in your use case.