How do you pass an environment variable to Solr running inside Docker when the environment variable only exists inside the container? - postgresql

Question

I need to do a dataimport from a PostgreSQL container running inside docker to a Solr server also running inside of Docker.
In my docker run command I specify the --link option which creates the environment variable $POSTGRESQL_PORT_5432_TCP_ADDR inside the solr docker container, and I need to pass this into Solr to use in my solrconfig.xml file.
I've heard that this is possible by passing JVM environment variables to the Solr startup command, but docker run starts Solr automatically. The only workaround I've found is doing something like:
docker run --name solr -d -p 8983:8983 --link postgresql --volumes-from solr_cores makuk66/docker-solr /bin/true
Starting the container with bin/true so it does nothing, and then
docker exec -it solr /bin/bash
to get into the container, finally running the solr startup command myself with the flag
-Dsolr.database.ip=$POSTGRESQL_PORT_5432_TCP_ADDR
However this is an involved manual process, and I'm wondering if there's a better way.

Answer 1

Looking on the page Taking Solr to Production you see
The bin/solr script simply passes options starting with -D on to the JVM during startup. For running in production, we recommend setting these properties in the SOLR_OPTS variable defined in the include file. Keeping with our soft-commit example, in /var/solr/solr.in.sh, you would do:
SOLR_OPTS="$SOLR_OPTS -Dsolr.autoSoftCommit.maxTime=10000"
So all you need to do is edit the SOLR_OPTS environment variable in solr.bin.sh.
It's a bit different for Docker because you don't directly have access to solr.bin.sh, but it after some trial and error, it was as easy as adding this to my Dockerfile.
RUN echo 'SOLR_OPTS="$SOLR_OPTS -Dsolr.database.ip=$POSTGRESQL_PORT_5432_TCP_ADDR"' >> /opt/solr/bin/solr.in.sh
Then you can use it in the solrconfig.xml file as
${solr.database.ip}
An important thing to note is that you can call the JVM environment variable whatever you want as long as you make sure not to overwrite anything important. I could have called it
-Dsolr.potato
if I wanted to.

Answer 2

For some reason the solr.in.cmd file looks exactly the same as solr.in.sh which confused me on how to set variables there. In windows containers, the command to accomplish the same - from a dockerfile, would be:
RUN Add-Content C:\solr\bin\solr.in.cmd 'set SOLR_OPTS=%SOLR_OPTS% -Dsolr.database.ip=%POSTGRESQL_PORT_5432_TCP_ADDR%'