Replication and High Availability PostgreSQL

What You’ll Learn 📚

• Understanding the basics of replication and high availability
• Key terminology and definitions
• Simple to complex examples of PostgreSQL replication
• Common questions and troubleshooting tips

Introduction to Replication and High Availability

In the world of databases, replication refers to the process of copying data from one database server (the primary) to another (the replica). This ensures that your data is available even if one server fails. High availability is all about ensuring your database is up and running as much as possible. Together, they form a powerful duo to keep your data safe and accessible. 💪

Key Terminology 🗝️

• Primary Server: The main database server where all the writes occur.
• Replica Server: A copy of the primary server, used for read operations and backup.
• Failover: The process of switching to a replica server when the primary server fails.

Getting Started with a Simple Example

Let’s start with the simplest example of setting up replication in PostgreSQL. Don’t worry if this seems complex at first; we’ll break it down together! 😊

# Step 1: Install PostgreSQL on both primary and replica servers
sudo apt-get update
sudo apt-get install postgresql postgresql-contrib

# Step 2: Configure the primary server
# Edit the postgresql.conf file
sudo nano /etc/postgresql/12/main/postgresql.conf
# Add or modify the following lines
listen_addresses = '*'
wal_level = replica
max_wal_senders = 3

# Step 3: Configure the replica server
# Edit the recovery.conf file
sudo nano /var/lib/postgresql/12/main/recovery.conf
# Add the following lines
standby_mode = 'on'
primary_conninfo = 'host= port=5432 user=replicator password='

Progressively Complex Examples

# Step 1: Configure streaming replication on the primary server
# Edit the pg_hba.conf file
sudo nano /etc/postgresql/12/main/pg_hba.conf
# Add the following line
host replication replicator /32 md5

# Step 2: Restart PostgreSQL service
sudo systemctl restart postgresql

# Step 1: Install and configure a failover tool like repmgr
sudo apt-get install repmgr

# Step 2: Configure repmgr on both servers
# On the primary server
sudo nano /etc/repmgr.conf
node_id=1
node_name='primary'
conninfo='host= user=repmgr dbname=repmgr'

# On the replica server
sudo nano /etc/repmgr.conf
node_id=2
node_name='replica'
conninfo='host= user=repmgr dbname=repmgr'

Common Questions and Answers

1. What is the difference between synchronous and asynchronous replication?

   Synchronous replication requires the primary server to wait for confirmation from the replica before committing a transaction, ensuring data consistency. Asynchronous replication allows the primary to continue without waiting, which can improve performance but may lead to data lag.

2. How do I monitor replication status?

   You can use the pg_stat_replication view in PostgreSQL to check the status of your replication setup.

3. What happens if the primary server fails?

   If you have a failover mechanism in place, the replica server will take over as the new primary, ensuring high availability.

Troubleshooting Common Issues

Always back up your data before making changes to your database configuration!

• Replication not starting: Check your configuration files for typos and ensure network connectivity between servers.
• Data lag on replica: Investigate network latency and ensure your replica server has sufficient resources.

Remember, practice makes perfect! Try setting up replication in a test environment to get comfortable with the process. 🛠️

Practice Exercises

• Set up a basic replication between two local PostgreSQL instances on your machine.
• Experiment with both synchronous and asynchronous replication modes.
• Simulate a primary server failure and observe the failover process.

For more information, check out the PostgreSQL High Availability Documentation.