Question: What are the differences between PostgreSQL clustering and MySQL clustering?

Answer

Clustering in database systems refers to a method where multiple servers or instances work together to serve as a single system, enhancing the availability and fault tolerance of the database. PostgreSQL and MySQL both support clustering, but they do so in different ways with distinct characteristics.

PostgreSQL Clustering

PostgreSQL does not have built-in clustering support in the same way as MySQL. Instead, it relies on third-party tools and extensions to achieve clustering. Some popular options include:

• Patroni: A template for PostgreSQL High Availability. It creates a failover system where several nodes can handle data replication and automatic failover.
• PostgreSQL Automatic Failover (PAF): It leverages Corosync and Pacemaker for creating high availability setups.
• Pgpool-II: Acts as a middleware that sits between PostgreSQL servers and clients. It provides connection pooling, load balancing, and automated failover.

The main approach in PostgreSQL is to use primary-replica replication. One node acts as the primary node which handles writes and multiple replicas can handle read queries. Replication can be synchronous or asynchronous.

Example of setting up a simple replication with PostgreSQL:

# On the primary server
# Edit postgresql.conf
wal_level = replica
max_wal_senders = 3
archive_mode = on

# Edit pg_hba.conf to allow replica to connect
host    replication     all             replica_host_ip/32         trust

# Restart PostgreSQL service

MySQL Clustering

MySQL offers several built-in technologies for clustering:

• MySQL Cluster (NDB Cluster): This is a technology that lets you spread your data across multiple servers with no single point of failure. It is best suited for read-heavy loads but also supports write operations.
• InnoDB Cluster: Uses Group Replication, which provides a way to create a cluster of servers that can each accept reads and writes, ensuring data consistency through advanced conflict detection and resolution mechanisms.

MySQL's approach is generally more tightly integrated compared to PostgreSQL's reliance on external tools. MySQL Cluster automatically manages the balance between nodes, which simplifies some aspects of deployment and management.

Example of initiating a MySQL InnoDB Cluster:

# On each server participating in the cluster
SET SQL_LOG_BIN=0;
CREATE USER 'repl'@'%' IDENTIFIED BY 'password' REQUIRE SSL;
GRANT REPLICATION SLAVE ON *.* TO 'repl'@'%';
SET SQL_LOG_BIN=1;

# Creating the cluster
CREATE CLUSTER myCluster;

Conclusion

Choosing between PostgreSQL and MySQL clustering depends largely on specific project requirements and existing infrastructure. PostgreSQL, with its flexible third-party tooling approach, might be preferred in environments where custom configurations are necessary. MySQL, with its built-in solutions, offers simplicity and tight integration for users looking for an out-of-the-box clustering solution.