Docker: Waiting for jmx port to be open before starting the exporter

docker/Dockerfile

@@ -1,8 +1,13 @@
-FROM openjdk:10.0.1-10-jre-slim-sid
+FROM openjdk:10.0.2-13-jre-slim-sid
 
 ARG EXPORTER_VERSION=2.0.0
 ARG EXPORTER_SHA512=7baa4e13f0a3c4228ba9d6cb848027f8837de0a0bc2a6b4fc3d8265e00a53fe926a6eac75a32a84de5e0771b355c1a8715dd46886c134710c6f26f477010b9d3
 
+RUN apt-get update && apt-get install -y --no-install-recommends \
+		netcat \
+  && apt-get clean \
+  && rm -rf /var/lib/apt/lists/*
+
 RUN mkdir -p /etc/cassandra_exporter /opt/cassandra_exporter
 ADD https://github.com/Yelp/dumb-init/releases/download/v1.2.1/dumb-init_1.2.1_amd64 /sbin/dumb-init
 ADD https://github.com/criteo/cassandra_exporter/releases/download/${EXPORTER_VERSION}/cassandra_exporter-${EXPORTER_VERSION}-all.jar /opt/cassandra_exporter/cassandra_exporter.jar

docker/run.sh

@@ -20,4 +20,12 @@ while IFS='=' read -r name value ; do
   fi
 done < <(env)
 
+host=$(grep -m1 'host:' /etc/cassandra_exporter/config.yml | cut -d ':' -f2)
+port=$(grep -m1 'host:' /etc/cassandra_exporter/config.yml | cut -d ':' -f3)
+
+while ! nc -z $host $port; do
+  echo "Waiting for Cassandra JMX to start on $host:$port"
+  sleep 1
+done
+
 /sbin/dumb-init /usr/bin/java ${JVM_OPTS} -jar /opt/cassandra_exporter/cassandra_exporter.jar /etc/cassandra_exporter/config.yml