Question: What is a quorum in a PostgreSQL cluster?

Answer

In a PostgreSQL cluster, especially when using replication configurations such as streaming replication or synchronous replication, the concept of 'quorum' becomes significant for ensuring data integrity and high availability.

Definition of Quorum

A quorum is the minimum number of voting members that must agree on a network decision before it is considered valid. This concept is crucial in distributed systems, including database clusters, to avoid issues like split-brain scenarios where two parts of a cluster believe they are active and accepting writes without knowledge of each other's state.

Quorum in PostgreSQL

PostgreSQL does not directly implement quorum-based consensus within its core architecture unlike some other distributed databases (e.g., Cassandra or etcd). However, the idea of quorum is relevant in the context of failover management tools used with PostgreSQL, such as Patroni or Pgpool-II, which support automatic failover and require a quorum to promote a new primary server.

These tools typically use an external service (like Consul, Zookeeper, or etcd) to manage the state of the cluster and help ensure that only one node can be promoted to primary at a time based on the quorum of available nodes.

Practical Example with Patroni

Let's consider an example with Patroni, which uses a distributed configuration store (like etcd, ZooKeeper, or Consul) for managing the PostgreSQL cluster state.

scope: postgres
namespace: /db/
name: pgnode1

restapi:
  listen: 0.0.0.0:8008
  connect_address: 192.168.1.100:8008

etcd:
  host: 192.168.1.200:2379

bootstrap:
  dcs:
    ttl: 30
    loop_wait: 10
    retry_timeout : 10
    maximum_lag_on_failover: 1048576
    synchronous_mode: true
    synchronous_mode_strict: false
    postgresql:
      use_pg_rewind: true
      use_slots: true
      parameters:
        max_connections: 100

In this configuration, Patroni will communicate with etcd to handle leader elections and ensure that there is consensus about which node is the leader. The synchronous_mode option can help ensure that a certain number of replicas have confirmed the reception of write operations before returning success to the client, akin to maintaining a quorum for transaction commits.

Conclusion

While PostgreSQL itself does not handle quorum directly within the database engine, the concept is applied through external tools that enhance PostgreSQL's capabilities in clustered environments. These setups help prevent data inconsistencies and ensure high availability of the database service.