Question: How do you set up high availability in PostgreSQL?

Answer

High Availability (HA) in PostgreSQL is crucial for ensuring that your database system remains available and reliable, even in the event of hardware failures or maintenance operations. Setting up a high availability system involves several components and strategies, including replication, failover mechanisms, and load balancing. Here are the essential steps to set up high availability in PostgreSQL:

1. Choose a Replication Strategy

PostgreSQL supports several replication methods, with the most popular being streaming replication. Streaming replication allows real-time mirroring of data from a primary server to one or more standby servers.

-- On the primary server, modify the postgresql.conf:
wal_level = replica
max_wal_senders = 3
archive_mode = on
archive_command = 'cp %p /path_to_archive/%f'

-- Modify pg_hba.conf to allow replication connections:
host replication all standby_ip/32 trust

2. Set Up Standby Servers

On the standby servers, you need to configure them to follow the primary server:

-- On the standby server, create a recovery.conf file:
standby_mode = 'on'
primary_conninfo = 'host=primary_ip port=5432 user=replication_user password=replication_pass'
trigger_file = '/path_to_trigger/trigger_file'

Use pg_basebackup to clone the primary server:

pg_basebackup -h primary_ip -D /var/lib/postgresql/data -U replication_user -v -P --xlog-method=stream

3. Implement Failover Mechanisms

Failover is critical for HA setups. Tools like Patroni, which uses etcd, Consul, or ZooKeeper as a distributed configuration store, can automate failover processes.

# Example Patroni configuration snippet
scope: postgres
namespace: /db/
name: node1

restapi:
    listen: 0.0.0.0:8008
    connect_address: node1_ip:8008

etcd:
    host: etcd_ip:2379

bootstrap:
    dcs:
        ttl: 30
        loop_wait: 10
        retry_timeout: 10
        maximum_lag_on_failover: 1048576
        postgresql:
            use_pg_rewind: true
            use_slots: true
            parameters:
                wal_level: replica

4. Load Balancing

For read scalability, use a load balancer to distribute read queries across multiple standby servers. Pgpool-II and HAProxy are commonly used for this purpose.

5. Regular Testing and Monitoring

Regularly test your HA setup through planned failovers. Monitor your replication lag and system performance to ensure everything operates smoothly.

In summary, setting up high availability in PostgreSQL requires careful planning and regular maintenance. By leveraging streaming replication, automated failover tools like Patroni, and effective load balancing, you can create a robust environment resilient to failures.