Docker container data goes blank when mapped with host data directory - docker

Question

I have created a Dockerfile like below. Just adding up an application in tomcat webapps.
FROM tomcat:9-alpine
ADD ./Spring3HibernateApp.war /usr/local/tomcat/webapps/
VOLUME /usr/local/tomcat/webapps
EXPOSE 8080
CMD ["catalina.sh","run"]
Build up a new image and named it test-app:0.1
docker build –t test-app:0.1 .
Spinning up new container like below, where I am using host data directory to mount a container data directory, so that I can make changes or list webapps content of container.
docker run -d --name=tomcat-01 -p 80:8080 --net=bridge -v /vol2/docker/sampleapp/tomcat-webapps:/usr/local/tomcat/webapps test-app:0.1
My problem: When I look at /vol2/docker/sampleapp/tomcat-webapps, I found it blank. However I am thinking it should list out contents of container from location /usr/local/tomcat/webapps. It’s actually cleaning up containers data as well, instead of persisting it and loading it in host data directory.
Am I missing anything?
If I simply remove –v from above command, it works fine and I am able to see contents inside default docker volume location, but not getting same result when I add –v.
Is my understand wrong?
I am referring “Mount a host directory as a data volume” through link https://docs.docker.com/engine/tutorials/dockervolumes/
Same command is working fine, when I am using mysql image and spinning up new container from it.
docker run -d --name=mysql-01 -p 3306:3306 -e MYSQL_ROOT_PASSWORD=password -e MYSQL_DATABASE=employeedb --net=bridge -v /vol2/docker/sampleapp/mysql-data:/var/lib/mysql mysql
This time, I am able to see containers data inside /vol2/docker/sampleapp/mysql-data

Answer 1

When you mount a volume you overwrite the existing directory inside of the container. If you are looking to deploy .war or .jar files from outside of the container you would want to do the following:
FROM tomcat:9-alpine
VOLUME /usr/local/tomcat/webapps
EXPOSE 8080
CMD ["catalina.sh","run"]
Build it: docker build –t test-app:0.1 .
Then run your container like so: docker run -d --name=tomcat-01 -p 80:8080 -v /vol2/docker/sampleapp/tomcat-webapps:/usr/local/tomcat/webapps test-app:0.1 placing the Spring3HibernateApp.war in the /vol2/docker/sampleapp/tomcat-webapps directory.
Once you do this you can go docker logs -f tomcat-01 and watch as tomcat deploys the app like in my sample.war below.
Using CATALINA_BASE: /usr/local/tomcat
Using CATALINA_HOME: /usr/local/tomcat
Using CATALINA_TMPDIR: /usr/local/tomcat/temp
Using JRE_HOME: /usr/lib/jvm/java-1.8-openjdk/jre
Using CLASSPATH: /usr/local/tomcat/bin/bootstrap.jar:/usr/local/tomcat/bin/tomcat-juli.jar
25-Feb-2017 21:58:23.710 INFO [main] org.apache.catalina.startup.VersionLoggerListener.log Server version: Apache Tomcat/9.0.0.M17
25-Feb-2017 21:58:23.717 INFO [main] org.apache.catalina.startup.VersionLoggerListener.log Server built: Jan 10 2017 20:59:20 UTC
25-Feb-2017 21:58:23.717 INFO [main] org.apache.catalina.startup.VersionLoggerListener.log Server number: 9.0.0.0
25-Feb-2017 21:58:23.717 INFO [main] org.apache.catalina.startup.VersionLoggerListener.log OS Name: Linux
25-Feb-2017 21:58:23.717 INFO [main] org.apache.catalina.startup.VersionLoggerListener.log OS Version: 3.13.0-93-generic
25-Feb-2017 21:58:23.717 INFO [main] org.apache.catalina.startup.VersionLoggerListener.log Architecture: amd64
25-Feb-2017 21:58:23.717 INFO [main] org.apache.catalina.startup.VersionLoggerListener.log Java Home: /usr/lib/jvm/java-1.8-openjdk/jre
25-Feb-2017 21:58:23.718 INFO [main] org.apache.catalina.startup.VersionLoggerListener.log JVM Version: 1.8.0_111-internal-alpine-r0-b14
25-Feb-2017 21:58:23.718 INFO [main] org.apache.catalina.startup.VersionLoggerListener.log JVM Vendor: Oracle Corporation
25-Feb-2017 21:58:23.718 INFO [main] org.apache.catalina.startup.VersionLoggerListener.log CATALINA_BASE: /usr/local/tomcat
25-Feb-2017 21:58:23.718 INFO [main] org.apache.catalina.startup.VersionLoggerListener.log CATALINA_HOME: /usr/local/tomcat
25-Feb-2017 21:58:23.719 INFO [main] org.apache.catalina.startup.VersionLoggerListener.log Command line argument: -Djava.util.logging.config.file=/usr/local/tomcat/conf/logging.properties
25-Feb-2017 21:58:23.719 INFO [main] org.apache.catalina.startup.VersionLoggerListener.log Command line argument: -Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager
25-Feb-2017 21:58:23.719 INFO [main] org.apache.catalina.startup.VersionLoggerListener.log Command line argument: -Djdk.tls.ephemeralDHKeySize=2048
25-Feb-2017 21:58:23.720 INFO [main] org.apache.catalina.startup.VersionLoggerListener.log Command line argument: -Djava.protocol.handler.pkgs=org.apache.catalina.webresources
25-Feb-2017 21:58:23.720 INFO [main] org.apache.catalina.startup.VersionLoggerListener.log Command line argument: -Dcatalina.base=/usr/local/tomcat
25-Feb-2017 21:58:23.721 INFO [main] org.apache.catalina.startup.VersionLoggerListener.log Command line argument: -Dcatalina.home=/usr/local/tomcat
25-Feb-2017 21:58:23.721 INFO [main] org.apache.catalina.startup.VersionLoggerListener.log Command line argument: -Djava.io.tmpdir=/usr/local/tomcat/temp
25-Feb-2017 21:58:23.721 INFO [main] org.apache.catalina.core.AprLifecycleListener.lifecycleEvent Loaded APR based Apache Tomcat Native library 1.2.10 using APR version 1.5.2.
25-Feb-2017 21:58:23.721 INFO [main] org.apache.catalina.core.AprLifecycleListener.lifecycleEvent APR capabilities: IPv6 [true], sendfile [true], accept filters [false], random [true].
25-Feb-2017 21:58:23.722 INFO [main] org.apache.catalina.core.AprLifecycleListener.lifecycleEvent APR/OpenSSL configuration: useAprConnector [false], useOpenSSL [true]
25-Feb-2017 21:58:23.725 INFO [main] org.apache.catalina.core.AprLifecycleListener.initializeSSL OpenSSL successfully initialized (OpenSSL 1.0.2j 26 Sep 2016)
25-Feb-2017 21:58:23.838 INFO [main] org.apache.coyote.AbstractProtocol.init Initializing ProtocolHandler ["http-nio-8080"]
25-Feb-2017 21:58:23.861 INFO [main] org.apache.tomcat.util.net.NioSelectorPool.getSharedSelector Using a shared selector for servlet write/read
25-Feb-2017 21:58:23.868 INFO [main] org.apache.coyote.AbstractProtocol.init Initializing ProtocolHandler ["ajp-nio-8009"]
25-Feb-2017 21:58:23.870 INFO [main] org.apache.tomcat.util.net.NioSelectorPool.getSharedSelector Using a shared selector for servlet write/read
25-Feb-2017 21:58:23.874 INFO [main] org.apache.catalina.startup.Catalina.load Initialization processed in 653 ms
25-Feb-2017 21:58:23.908 INFO [main] org.apache.catalina.core.StandardService.startInternal Starting service Catalina
25-Feb-2017 21:58:23.908 INFO [main] org.apache.catalina.core.StandardEngine.startInternal Starting Servlet Engine: Apache Tomcat/9.0.0.M17
25-Feb-2017 21:58:23.951 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployWAR Deploying web application archive /usr/local/tomcat/webapps/sample.war
25-Feb-2017 22:00:25.223 INFO [localhost-startStop-1] org.apache.catalina.util.SessionIdGeneratorBase.createSecureRandom Creation of SecureRandom instance for session ID generation using [SHA1PRNG] took [120,610] milliseconds.
25-Feb-2017 22:00:25.253 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployWAR Deployment of web application archive /usr/local/tomcat/webapps/sample.war has finished in 121,302 ms
25-Feb-2017 22:00:25.258 INFO [main] org.apache.coyote.AbstractProtocol.start Starting ProtocolHandler [http-nio-8080]
25-Feb-2017 22:00:25.270 INFO [main] org.apache.coyote.AbstractProtocol.start Starting ProtocolHandler [ajp-nio-8009]
25-Feb-2017 22:00:25.278 INFO [main] org.apache.catalina.startup.Catalina.start Server startup in 121403 ms
Lastly, the MySQL volume mount works like it does because the base image had the volume exposed before as it was build VOLUME /var/lib/mysql if you wanted the same thing to occur in the catalina app you would have to copy their Dockerfile and add a VOLUME /usr/local/tomcat/webapps/ to it like below.
FROM openjdk:8-jre-alpine
ENV CATALINA_HOME /usr/local/tomcat
ENV PATH $CATALINA_HOME/bin:$PATH
RUN mkdir -p "$CATALINA_HOME"
WORKDIR $CATALINA_HOME
# let "Tomcat Native" live somewhere isolated
ENV TOMCAT_NATIVE_LIBDIR $CATALINA_HOME/native-jni-lib
ENV LD_LIBRARY_PATH ${LD_LIBRARY_PATH:+$LD_LIBRARY_PATH:}$TOMCAT_NATIVE_LIBDIR
RUN apk add --no-cache gnupg
# see https://www.apache.org/dist/tomcat/tomcat-$TOMCAT_MAJOR/KEYS
# see also "update.sh" (https://github.com/docker-library/tomcat/blob/master/update.sh)
ENV GPG_KEYS 05AB33110949707C93A279E3D3EFE6B686867BA6 07E48665A34DCAFAE522E5E6266191C37C037D42 47309207D818FFD8DCD3F83F1931D684307A10A5 541FBE7D8F78B25E055DDEE13C370389288584E7 61B832AC2F1C5A90F0F9B00A1C506407564C17A3 79F7026C690BAA50B92CD8B66A3AD3F4F22C4FED 9BA44C2621385CB966EBA586F72C284D731FABEE A27677289986DB50844682F8ACB77FC2E86E29AC A9C5DF4D22E99998D9875A5110C01C5A2F6059E7 DCFD35E0BF8CA7344752DE8B6FB21E8933C60243 F3A04C595DB5B6A5F1ECA43E3B7BBB100D811BBE F7DA48BB64BCB84ECBA7EE6935CD23C10D498E23
RUN set -ex; \
for key in $GPG_KEYS; do \
gpg --keyserver ha.pool.sks-keyservers.net --recv-keys "$key"; \
done
ENV TOMCAT_MAJOR 9
ENV TOMCAT_VERSION 9.0.0.M17
# https://issues.apache.org/jira/browse/INFRA-8753?focusedCommentId=14735394#comment-14735394
ENV TOMCAT_TGZ_URL https://www.apache.org/dyn/closer.cgi?action=download&filename=tomcat/tomcat-$TOMCAT_MAJOR/v$TOMCAT_VERSION/bin/apache-tomcat-$TOMCAT_VERSION.tar.gz
# not all the mirrors actually carry the .asc files :'(
ENV TOMCAT_ASC_URL https://www.apache.org/dist/tomcat/tomcat-$TOMCAT_MAJOR/v$TOMCAT_VERSION/bin/apache-tomcat-$TOMCAT_VERSION.tar.gz.asc
RUN set -x \
\
&& apk add --no-cache --virtual .fetch-deps \
ca-certificates \
tar \
openssl \
&& wget -O tomcat.tar.gz "$TOMCAT_TGZ_URL" \
&& wget -O tomcat.tar.gz.asc "$TOMCAT_ASC_URL" \
&& gpg --batch --verify tomcat.tar.gz.asc tomcat.tar.gz \
&& tar -xvf tomcat.tar.gz --strip-components=1 \
&& rm bin/*.bat \
&& rm tomcat.tar.gz* \
\
&& nativeBuildDir="$(mktemp -d)" \
&& tar -xvf bin/tomcat-native.tar.gz -C "$nativeBuildDir" --strip-components=1 \
&& apk add --no-cache --virtual .native-build-deps \
apr-dev \
gcc \
libc-dev \
make \
"openjdk${JAVA_VERSION%%[-~bu]*}"="$JAVA_ALPINE_VERSION" \
openssl-dev \
&& ( \
export CATALINA_HOME="$PWD" \
&& cd "$nativeBuildDir/native" \
&& ./configure \
--libdir="$TOMCAT_NATIVE_LIBDIR" \
--prefix="$CATALINA_HOME" \
--with-apr="$(which apr-1-config)" \
--with-java-home="$(docker-java-home)" \
--with-ssl=yes \
&& make -j$(getconf _NPROCESSORS_ONLN) \
&& make install \
) \
&& runDeps="$( \
scanelf --needed --nobanner --recursive "$TOMCAT_NATIVE_LIBDIR" \
| awk '{ gsub(/,/, "\nso:", $2); print "so:" $2 }' \
| sort -u \
| xargs -r apk info --installed \
| sort -u \
)" \
&& apk add --virtual .tomcat-native-rundeps $runDeps \
&& apk del .fetch-deps .native-build-deps \
&& rm -rf "$nativeBuildDir" \
&& rm bin/tomcat-native.tar.gz
# verify Tomcat Native is working properly
RUN set -e \
&& nativeLines="$(catalina.sh configtest 2>&1)" \
&& nativeLines="$(echo "$nativeLines" | grep 'Apache Tomcat Native')" \
&& nativeLines="$(echo "$nativeLines" | sort -u)" \
&& if ! echo "$nativeLines" | grep 'INFO: Loaded APR based Apache Tomcat Native library' >&2; then \
echo >&2 "$nativeLines"; \
exit 1; \
fi
ADD ./Spring3HibernateApp.war /
VOLUME ${CATALINA_HOME}
EXPOSE 8080
CMD ["catalina.sh", "run"]

Answer 2

When you mount a host directory into the container, anything inside the container at that path is no longer available. It is still there in the underlying image or volume, but it will be superseded by your volume mount.
That is simply how host volume mounts work: Whatever is in the directory on your host is slapped into place inside the container, and takes precedence over whatever was previously at that path (if anything was).

Answer 3

I am afraid your VOLUME /usr/local/tomcat/webapps clears this directory.
You need to do it differently.