MongoDB Replication Example Using a Docker

This MongoDB replication example uses a Docker engine to run replica set members.

This platform is mainly for developing and testing.

Getting Started

You will need an Docker engine where you can run containers. If you want to avoid running all replset members on one host, you can also use Docker Swarm.

Example Working Scenarios

This section describes how this example is designed to work.

All practices for MongoDB replication applies also to this example

Initial Deployment: 3-member Replica Set

To create replica set with three members you can use this script:

cat > variables <<EOF
MONGODB_DATABASE=db
MONGODB_USER=user
MONGODB_PASSWORD=password
MONGODB_ADMIN_PASSWORD=adminPassword
MONGODB_REPLICA_NAME=rs0
MONGODB_KEYFILE_VALUE=xxxxxxxxxxxx
MONGODB_SMALLFILES=true
MONGODB_SERVICE_NAME=mongodb"
EOF
source variables

IMAGE_NAME=centos/mongodb-32-centos7

network_name="mongodb-replset"
docker network create ${network_name}

docker run -d --cidfile $CIDFILE_DIR/replset0 --name=replset-0 --hostname=replset-0 --network ${network_name} --network-alias mongodb --env-file=variables $IMAGE_NAME run-mongod-replication
docker exec replset-0 bash -c "while ! [ -f /tmp/initialized ]; do sleep 1; done"
docker run -d --cidfile $CIDFILE_DIR/replset1 --name=replset-1 --hostname=replset-1 --network ${network_name} --network-alias mongodb --env-file=variables $IMAGE_NAME run-mongod-replication
docker exec replset-1 bash -c "while ! [ -f /tmp/initialized ]; do sleep 1; done"
docker run -d --cidfile $CIDFILE_DIR/replset2 --name=replset-2 --hostname=replset-2 --network ${network_name} --network-alias mongodb --env-file=variables $IMAGE_NAME run-mongod-replication
docker exec replset-2 bash -c "while ! [ -f /tmp/initialized ]; do sleep 1; done"

run-mongod-replication command have to be run in container (same script as for OpenShift StatefulSet replication example.

Parameters for docker run command:

--name and --hostname have to be set to the same value for each container to proper inter-container addressing
same --network-alias has to be added to all containers to be able to automatically connect containers together (alias has to be equal to $MONGODB_SERVICE_NAME). This allows dynamic adding of members to replicaset.
all environmental variables required for replication have to be set - see help of the image (TODO write it somewhere - deprecated)

To be able to select a container, which initialize the ReplicaSet, HOSTNAME of one container has to match this regular expression: .*-0. If this container does not use persistent storage (mounted directory into container) it can't be restarted.

And later from one of the containers you can easilly connect to MongoDB:

sh-4.2$ mongo $MONGODB_DATABASE -u $MONGODB_USER -p$MONGODB_PASSWORD --host $MONGODB_REPLICA_NAME/localhost
MongoDB shell version: 3.2.6
connecting to: sampledb
rs0:PRIMARY>

Note: You can also use host version of mongo shell, but you have to substitute values of environmental variables and IP address instead of localhost by yourself.

During the lifetime of your deployment, one or more of those members might crash or fail. It is possible to configure container to get restarted automatically (see docker run reference.

Note: for production usage, you should maintain as much separation between members as possible. It is recommended to run containers on different hosts.

Adding member

To add a new member into replicaset run:

docker run -d --cidfile $CIDFILE_DIR/replset3 --name=replset-3 --hostname=replset-3 --network ${network_name} --network-alias mongodb --env-file=variables $IMAGE_NAME run-mongod-replication
docker exec replset-3 bash -c "while ! [ -f /tmp/initialized ]; do sleep 1; done"

New container is created and it automatically connects to the replica set.

Removing member

To prevent possible data lost, automatic removing of members from replicaset is not supported.

To do it:

stop the container (for example docker stop replset-2)
connect to replica set and remove the member

Known Limitations

Adding or removing new member to replica set takes some time (elections, syncing,...), so after your command finished it may take some time until replica set is ready

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