This is the fifth article of the Getting started with Docker series. In this article, I am going to show how to install a PostgreSQL cluster on three Docker containers.
The cluster will be configured in master/slave mode with one master and two slaves. PostgreSQL supports two cluster type: hot and warm standby. The former allows the slave to receive connections in read-only, the latter doesn’t allow the slaves to receive connections. In this tutorial, we will configure the cluster as hot standby.
Modify the start_containers.sh script
The first step to create a PostgreSQL cluster is to modify the start_containers.sh script to create two environment variables that we pass to the docker create command.
...
MASTER_NAME=${NODE1_NAME}
...
docker create -it --net ${PRIVATE_NETWORK_NAME} --ip ${NODE1_PRIVATE_IP} --hostname ${NODE1_NAME} --name ${NODE1_NAME} --env NODE_NAME=${NODE1_NAME} --env MASTER_NAME=${MASTER_NAME} -p ${NODE1_PORT}:5432 postgresql /bin/bash
docker create -it --net ${PRIVATE_NETWORK_NAME} --ip ${NODE2_PRIVATE_IP} --hostname ${NODE2_NAME} --name ${NODE2_NAME} --env NODE_NAME=${NODE2_NAME} --env MASTER_NAME=${MASTER_NAME} -p ${NODE2_PORT}:5432 postgresql /bin/bash
docker create -it --net ${PRIVATE_NETWORK_NAME} --ip ${NODE3_PRIVATE_IP} --hostname ${NODE3_NAME} --name ${NODE3_NAME} --env NODE_NAME=${NODE3_NAME} --env MASTER_NAME=${MASTER_NAME} -p ${NODE3_PORT}:5432 postgresql /bin/bash
...
These variables will be used by the startup scripts to start the master and the two slaves.
Modify the startup scripts
The two environment variables defined in the previous section will be used by the startup scripts to understand if the container contains the master of one of the two slaves.
Here the code of the src/postgresql/entrypoint.sh script. We added a sleep command for slave nodes.
#!/bin/bash
...
while [ -f /var/run/recovery.lock ]; do
sleep 1;
done;
...
Here the code of the src/postgresql/bin/entrypoint.sh script.
#!/bin/bash
DATA_DIRECTORY="/home/postgres/data/postgres"
LOGS_DIRECTORY="/home/postgres/data/logs"
echo ">>> TEST IF DATA DIRECTORY IS EMPTY"
if [ -z "$(ls -A $DATA_DIRECTORY)" ]; then
echo ">>> PREPARE NODE $NODE_NAME FOR FIRST STARTUP"
if [ "$NODE_NAME" = "$MASTER_NAME" ]
then
echo ">>> CREATE DATA DIRECTORY ON THE MASTER NODE"
/usr/lib/postgresql/9.5/bin/initdb -D $DATA_DIRECTORY
echo ">>> COPY CONFIGURATION FILES"
cp /usr/local/bin/cluster/postgresql/config/postgresql.conf $DATA_DIRECTORY
cp /usr/local/bin/cluster/postgresql/config/pg_hba.conf $DATA_DIRECTORY
else
echo ">>> $NODE_NAME IS WAITING MASTER IS UP AND RUNNING"
while ! nc -z $MASTER_NAME 5432; do sleep 1; done;
sleep 10
echo ">>> REPLICATE DATA DIRECTORY ON THE SLAVE $NODE_NAME"
/usr/lib/postgresql/9.5/bin/pg_basebackup -h 10.0.2.31 -p 5432 -U postgres -D $DATA_DIRECTORY -X stream -P
echo ">>> COPY recovery.conf ON SLAVE $NODE_NAME"
cp /usr/local/bin/cluster/postgresql/config/recovery.conf $DATA_DIRECTORY
sed -i "s/NODE_NAME/$NODE_NAME/g" $DATA_DIRECTORY/recovery.conf
sed -i "s/MASTER_NAME/$MASTER_NAME/g" $DATA_DIRECTORY/recovery.conf
fi
fi
echo ">>> START POSTGRESQL ON NODE $NODE_NAME"
/usr/lib/postgresql/9.5/bin/postgres -D $DATA_DIRECTORY > $LOGS_DIRECTORY/postgres.log 2>&1
The code checks whether the data directory is empty or not. In the latter case, the code assumes the data directory already exists and then we only startup PostgreSQL. In the former case, the code is different for the master and slaves.
For the master, the code is similar to the previous article. The data directory is initialized and the configuration files postgresql.conf and pg_hba.conf are copied in it.
For the slaves, the code will wait for the master is up and running and then they replicate the data directory from it with the pg_basebackup command. Finally, we copy the recovery.conf file and let it refers to the master node.
Configure the PostgreSQL cluster
To configure the PostgreSQL cluster as hot standby we need to uncomment the following lines in the postgresql.conf file.
hba_file = '/home/postgres/data/postgres/pg_hba.conf' wal_level = hot_standby listen_addresses = '*' max_wal_senders = 5 hot_standby = on
In the pg_hba.conf we will specify the host from which the replication is allowed.
host replication postgres node1 trust host replication postgres node2 trust host replication postgres node3 trust
How to verify if the cluster is working?
An easy way to verify if the cluster is working is to create a database on the master node and verify its replication on slaves. Run the following commands on the master node.
psql -h localhost -p 5432 -U postgres CREATE DATABASE mydb; \list \quit
Verify on slave nodes if the database has been replicated using the following commands. To verify the database is replicated on node3 use the same commands and replace 5433 port with 5434.
psql -h localhost -p 5433 -U postgres \list \q
What’s next?
In this article, we learned how to use the three containers created in the previous article to install a PostgreSQL cluster. You can download the source code here in the postgresql-cluster folder. In the next article, we will discuss another important Docker concept: volumes.
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