Setting up logical replication

Before we dive into the Python code, let’s first set up logical replication in PostgreSQL.

1. Enable logical replication in the postgresql.conf file by uncommenting the wal_level parameter and setting it to logical. Restart the PostgreSQL server to apply the changes.

2. Create a replication slot using the pg_create_logical_replication_slot function. You can do this through a SQL client like psql or using the psycopg2 library in Python.

import psycopg2

conn = psycopg2.connect(
    host="localhost",
    port=5432,
    dbname="your_database",
    user="your_user",
    password="your_password"
)
cursor = conn.cursor()
cursor.execute("SELECT * FROM pg_create_logical_replication_slot('my_replication_slot', 'wal2json')")
cursor.close()
conn.close()

In the above code, replace your_database, your_user, and your_password with your actual database credentials. We have created a replication slot named my_replication_slot using the wal2json plugin.

1. Configure the replication in the pg_hba.conf file by allowing the replication user to connect from your Python application server. Append the following line to the file:

host replication your_user your_application_server_ip/32 trust

Make sure to replace your_user and your_application_server_ip with appropriate values.

1. Restart the PostgreSQL server for the changes to take effect.

Consuming logical replication events in Python

Now that we have set up logical replication, let’s see how we can consume the replicated events in Python using the Psycopg2 library.

import psycopg2
from psycopg2 import sql

conn = psycopg2.connect(
    host="localhost",
    port=5432,
    dbname="your_database",
    user="your_user",
    password="your_password"
)
conn.set_isolation_level(psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT)

cursor = conn.cursor()
cursor.execute("REPLICATION my_replication_slot")

while True:
    message = conn.poll()
    if message is None:
        continue
    if message["action"] == "begin":
        continue
    if message["action"] == "commit":
        break
    if message["action"] == "insert":
        table_name = message["table"]
        data = message["row"]
        # Process the replicated data, perform any required actions
        # ...
cursor.close()
conn.close()

In the above code, we establish a connection to the PostgreSQL database and set the isolation level to ISOLATION_LEVEL_AUTOCOMMIT. We then create a cursor and start the replication using the REPLICATION command with the name of the replication slot.

We enter into a loop where we continuously poll for replication events. Once we receive an event, we check the type of action performed (begin, commit, insert, etc.) and process the replicated data accordingly.

Please note that this is just a basic example to get you started. You may need to handle other replication events, errors, and implement error handling and retry mechanisms in your production code.

Conclusion

In this blog post, we have explored how to use logical replication in Python by leveraging the Psycopg2 library. We covered setting up logical replication in PostgreSQL and consuming replicated events in Python. Logical replication opens the door to building scalable and reliable systems and is a valuable feature in PostgreSQL.