PostgreSQL Primary and Standby Server Configuration
Ensuring your PostgreSQL database is highly available and resilient to failures is crucial for business continuity. This involves setting up a primary server for read/write operations and one or more standby servers that can take over if the primary fails. This process is known as replication.
Understanding Replication Concepts
PostgreSQL offers several replication methods, but the most common for high availability is streaming replication. In this model, the primary server continuously sends Write-Ahead Log (WAL) records to the standby servers. The standby servers then apply these records to stay synchronized.
Streaming replication keeps standby servers in sync with the primary by sending WAL records.
The primary server writes all changes to its Write-Ahead Log (WAL). Streaming replication efficiently transmits these WAL records to standby servers in near real-time. Standby servers then replay these WAL records to reconstruct the database state, ensuring they are a mirror of the primary.
PostgreSQL's streaming replication is a robust mechanism for maintaining data consistency across multiple database instances. The primary server, upon committing a transaction, writes the changes to its WAL files. These WAL files are then streamed over the network to one or more standby servers. The standby servers receive these WAL segments and apply them to their own data files, effectively replaying the transactions that occurred on the primary. This continuous flow of WAL data ensures that the standby servers are always up-to-date, with minimal lag, making them ready to assume the primary role if needed.
Key Components for Configuration
Configuring primary and standby servers involves several key PostgreSQL parameters and settings. These ensure that replication is enabled, secure, and efficient.
| Parameter | Primary Server Setting | Standby Server Setting |
|---|---|---|
| <code>wal_level</code> | <code>replica</code> or <code>logical</code> | <code>replica</code> or <code>logical</code> |
| <code>max_wal_senders</code> | Number of standbys + 1 (for base backup) | Not applicable |
| <code>wal_keep_size</code> (or <code>wal_keep_segments</code> in older versions) | Sufficient to prevent WAL file deletion before standbys receive them | Not applicable |
| <code>hot_standby</code> | Not applicable | <code>on</code> |
| <code>primary_conninfo</code> | Not applicable | Connection string to the primary server |
| <code>restore_command</code> | Not applicable | Command to retrieve WAL files if streaming fails (e.g., using <code>pg_receivewal</code> or file copy) |
Steps for Setting Up Replication
The setup process typically involves preparing the primary, taking a base backup of the primary, configuring the standby, and starting the replication.
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1. Prepare the Primary Server
Ensure <code>wal_level</code> is set to <code>replica</code> or <code>logical</code> in <code>postgresql.conf</code>. Also, configure <code>pg_hba.conf</code> to allow replication connections from the standby server. Restart PostgreSQL after these changes.
2. Take a Base Backup
Use <code>pg_basebackup</code> to create an initial copy of the primary database cluster on the standby server. This command copies all necessary data files and WAL segments.
<code>pg_basebackup</code> is essential for creating the initial synchronized copy of your database for the standby server.
3. Configure the Standby Server
On the standby server, create a <code>standby.signal</code> file in the data directory (or use <code>recovery.signal</code> for PostgreSQL 12+) and configure <code>postgresql.conf</code> with <code>primary_conninfo</code> pointing to the primary. If not using streaming replication for WAL archiving, set up a <code>restore_command</code>.
4. Start Replication
Start the PostgreSQL service on the standby server. It will connect to the primary using the <code>primary_conninfo</code> and begin receiving WAL records. You can verify the replication status using SQL queries like <code>pg_stat_replication</code> on the primary and <code>pg_stat_wal_receiver</code> on the standby.
Monitoring and Failover
Regular monitoring of replication lag and connection status is vital. In the event of a primary failure, a manual or automated failover process is initiated to promote a standby server to become the new primary. This ensures minimal downtime.
The diagram illustrates the flow of Write-Ahead Log (WAL) records from the primary PostgreSQL server to the standby server. The primary writes WAL records to disk, and these records are then streamed over the network to the standby. The standby server acts as a WAL receiver, continuously applying these records to its own data files to maintain synchronization. This process ensures that the standby is a near real-time replica of the primary, ready to take over if the primary becomes unavailable.
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Learning Resources
The official PostgreSQL documentation provides a comprehensive overview of replication, including streaming replication and warm standbys.
A practical, step-by-step guide to setting up streaming replication between two PostgreSQL servers, covering configuration and verification.
This blog post explores the intricacies of PostgreSQL replication, discussing different types and best practices for high availability.
A detailed explanation of PostgreSQL's Write-Ahead Log (WAL) mechanism, which is fundamental to understanding replication.
Learn about replication slots, a feature that ensures WAL records are not removed before a standby server has consumed them.
Discusses strategies for achieving high availability in PostgreSQL, including replication and failover mechanisms.
A Stack Exchange discussion on how to monitor replication lag using the <code>pg_stat_replication</code> view.
Official documentation for the <code>pg_basebackup</code> utility, essential for creating base backups for replication.
A video tutorial demonstrating the practical steps involved in setting up a primary and standby PostgreSQL server for replication.
An overview of different replication methods available in PostgreSQL, comparing their use cases and benefits.