Question: What is a PostgreSQL cluster?

Answer

A PostgreSQL cluster refers not to a group of servers, but rather to a single instance of PostgreSQL consisting of one database server process (postgres) and multiple databases managed by it. This concept can be somewhat confusing because "cluster" often implies a set of nodes working together, but in PostgreSQL, it mainly relates to the architecture on a single server.

Understanding PostgreSQL Clusters

In PostgreSQL, a "cluster" is a collection of databases that are managed by a single PostgreSQL server process. These databases share the same PostgreSQL installation (binary files) but operate independently in terms of data storage, transaction logs, and other operational aspects.

Each cluster has its own:

• Data directory: This is where all the data files for the databases in the cluster are stored.
• Port number: PostgreSQL clusters typically listen on a specific port for connections; different clusters on the same host must use different ports.
• Configuration files: Configuration files (postgresql.conf, pg_hba.conf, etc.) are specific to each cluster.

Setting Up a PostgreSQL Cluster

To create a new PostgreSQL cluster, you typically use the initdb command, which initializes a new data storage area and prepares it to become a new cluster. Here's an example command:

initdb -D /path/to/new/cluster/data

After initializing a cluster, you can start the PostgreSQL server for that cluster using the pg_ctl command:

pg_ctl -D /path/to/new/cluster/data -l logfile start

Multi-node PostgreSQL Clusters

Although the basic definition of a cluster in PostgreSQL doesn't involve multiple nodes, you can set up a multi-node configuration for high availability or read scaling. Tools like Pgpool, Patroni, and technologies such as streaming replication are used to create a cluster of multiple PostgreSQL servers.

Example: Setting Up Streaming Replication

Streaming replication involves a primary node and one or more standby nodes. Here's a very simplified setup:

1. Primary Node Configuration: Edit postgresql.conf:

   wal_level = replica
   max_wal_senders = 3

   And pg_hba.conf:

   host replication replicator 192.168.1.0/24 md5

2. Standby Node Setup: Use pg_basebackup to clone the primary:

   pg_basebackup -h primary_host -D /var/lib/postgresql/data -U replicator -P -v

   Then configure the recovery by creating a standby.signal file and setting appropriate parameters in postgresql.conf.

This setup allows changes on the primary node to be sent to the standby nodes, ensuring that they are up-to-date and can take over in case the primary fails.

Conclusion

Ultimately, understanding how PostgreSQL defines and manages clusters is crucial for effective database management and scaling solutions. Whether you're managing a single node or setting up a robust multi-node environment, PostgreSQL offers the tools and configurations necessary to ensure data integrity and availability.