new: [mongo] add ``README.org`` with some help

mongo/README.org

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+#+TITLE: Mongo Charm
+
+* Quick information
+
+** Upgrade
+
+
+Since 4.0.6 based on alpine 3.9, alpine is not packaging mongo
+anymore. So we use the official version (that is huge). We still
+must change entrypoint to enforce proper directory paths.
+
+A ``Dockerfile`` is provided for this mean.
+
+You can fetch latest official tags:
+#+begin_quote
+docker-tags-fetch mongo -l 10 -f '^[0-9]+\.[0-9]+\.[0-9]+$'
+#+end_quote
+
+change it in docker file, then
+
+#+begin_quote
+docker build . -t docker.0k.io/mongo:"$TAG"-myc
+#+end_quote
+
+
+** General Access
+
+You can access mondo db, if running, through
+
+
+#+begin_src sh
+SERVICENAME=mongo
+compose exec "$SERVICENAME" /usr/bin/mongo --quiet --eval "some js code"
+#+end_src
+
+if not running, you might want to run it.
+
+
+The following could be added to actions
+
+*** list databases
+
+
+#+begin_src sh
+SERVICENAME=mongo
+compose exec -T mongo /usr/bin/mongo --quiet \
+    --eval "JSON.stringify(db.adminCommand({listDatabases: 1}))" | \
+    jq -r '.databases[] | .name'
+#+end_src
+
+
+*** Dump database
+
+This one will use the probably already builded image from your project on your
+host to run a competing mongo with a host directory to store the dump.
+
+#+begin_src sh
+PROJECTNAME=myc
+SERVICENAME=mongo
+docker run --rm --name mongo_client -ti \
+     -v /tmp/mongo:/tmp/mongo --network "${PROJECTNAME}_default" \
+     -u root \
+     --entrypoint /bin/sh "${PROJECTNAME}_$SERVICENAME"
+#+end_src
+
+Then inside docker:
+
+#+begin_src sh
+apk add mongodb-tools
+#+end_src
+
+Then for each databases:
+
+#+begin_src sh
+DBNAME="xxx"
+cd /tmp/mongo
+mongodump -h mongo -d "$DBNAME" -o mdump/
+#+end_src
+
+
+*** Drop database
+
+
+#+begin_src sh
+DBNAME=xxx
+PROJECTNAME=myc
+SERVICENAME=mongo
+docker exec -i "${PROJECTNAME}_${SERVICE}_1" /usr/bin/mongo --quiet < \
+    <(echo "
+use $DBNAME;
+db.dropDatabase();
+")
+#+end_src
+
+Note: if some connection are active to the database, it might not be
+deleted, you should check by relisting the database.
+
+
+
+*** Restore database
+
+
+#+begin_src sh
+SOURCE_DIR=/tmp/gogocarto
+docker run --rm -ti \
+    -v "$SOURCE_DIR":/tmp/backups/gogocarto \
+    -w /tmp/backups \
+    --entrypoint mongorestore \
+    --network myc_default \
+    myc_mongo \
+    --host rs01/mongo /tmp/backups/
+#+end_src
+
+There should not have any errors.
+
+Please note that '/tmp/backups' in the last example should contain
+one directory per database.
+
+Note also that you should drop database (see previous section) prior
+to restore them.