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PostgreSQL Performance & Query Optimization · Lesson

Replication Strategies (Streaming, Logical)

Understand and configure various replication methods like streaming and logical replication for high availability and read scaling.

Replication Strategies (Streaming, Logical) is a free PostgreSQL Performance & Query Optimization lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the PostgreSQL Performance & Query Optimization learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Why Data Replication?

Imagine your database goes down! Replication is key to preventing data loss and ensuring your application stays online. It creates copies of your database.

It's vital for:

  • High Availability (HA): If the main server fails, a copy can take over.
  • Read Scaling: Distribute read queries across multiple servers, reducing load on the primary.

The Primary-Replica Setup

The most common replication setup is a Primary-Replica (or Master-Slave) model. One database server acts as the primary, handling all writes.

  • Primary: Processes all write operations (INSERT, UPDATE, DELETE).
  • Replica(s): Receive copies of data changes from the primary and can serve read-only queries.

This ensures data consistency and offloads read traffic.

Streaming Replication: The Basics

Streaming Replication is PostgreSQL's built-in, physical replication method. It works by continuously shipping the Write-Ahead Log (WAL) records from the primary to one or more replicas.

WAL records are low-level descriptions of every change made to the database. Replicas apply these changes to stay in sync.

WAL Shipping in Action

Here's how streaming replication works:

  • The primary server writes all database changes to its WAL files.
  • A dedicated process (WAL sender) on the primary sends these WAL records to replica servers.
  • A process (WAL receiver) on each replica receives and applies these records.

This keeps the replica's data identical to the primary's, block by block.

Setting Up Streaming Replication (Primary)

To enable streaming replication on the primary, you need to configure postgresql.conf:

  • wal_level = replica (or higher)
  • max_wal_senders = 10 (number of replicas/connections)
  • listen_addresses = '*' (allow connections from replicas)

You'll also need to set up pg_hba.conf to allow replica connections.

ALTER SYSTEM SET wal_level = 'replica';
ALTER SYSTEM SET max_wal_senders = 10;
ALTER SYSTEM SET listen_addresses = '*';

-- After these, restart PostgreSQL

Initializing a Replica with pg_basebackup

Before a replica can stream WAL, it needs a full copy of the primary's data. pg_basebackup is the standard tool for this.

It creates a consistent snapshot of the primary's data directory, which then becomes the replica's initial data.

pg_basebackup -h primary_host -p 5432 -U replication_user -D /var/lib/postgresql/data_replica -F p -Xs stream -P

Configuring the Replica

After pg_basebackup, the replica needs a few files:

  • A standby.signal file in its data directory (for PostgreSQL 12+).
  • A postgresql.conf with hot_standby = on (if you want to run queries).
  • A primary_conninfo setting in postgresql.conf to tell it where to connect to the primary.

This allows it to start up and connect as a replica.

ALTER SYSTEM SET hot_standby = 'on';
ALTER SYSTEM SET primary_conninfo = 'host=primary_host port=5432 user=replication_user password=your_password';

-- Create an empty standby.signal file:
touch /var/lib/postgresql/data_replica/standby.signal

-- Then start the replica PostgreSQL instance

Logical Replication: A Different Approach

Logical Replication is a newer, more flexible replication method introduced in PostgreSQL 10. Unlike streaming replication, it replicates data based on its logical representation (row changes, not physical blocks).

This allows for selective replication of tables or even databases, and cross-version/cross-platform replication.

Publications and Subscriptions

Logical replication uses a publish-subscribe model:

  • A Publication is created on the primary, defining which tables or changes to publish.
  • A Subscription is created on the replica, defining which publications to subscribe to.

The primary then sends logical decoding output (changes) to the subscribers, which apply them.

Setting Up Logical Replication (Example)

Here's a simplified example of setting up logical replication:

On Primary:

CREATE PUBLICATION my_pub FOR TABLE my_table;

On Replica:

CREATE SUBSCRIPTION my_sub CONNECTION 'host=primary_host dbname=mydb user=repl_user password=xyz' PUBLICATION my_pub;

Remember to configure wal_level = logical and max_replication_slots on the primary.

ALTER SYSTEM SET wal_level = 'logical';
ALTER SYSTEM SET max_replication_slots = 5;

-- On Primary:
CREATE PUBLICATION my_pub FOR TABLE my_table;

-- On Replica:
CREATE SUBSCRIPTION my_sub CONNECTION 'host=primary_host dbname=mydb user=repl_user password=xyz' PUBLICATION my_pub;

Replication Strategy Check

You need to set up replication for a PostgreSQL database. You want an exact, block-for-block copy of your entire database on a standby server for high availability. Which replication method is best suited for this goal?

Recap: Streaming vs. Logical

We've explored two key PostgreSQL replication strategies:

  • Streaming Replication: A physical, continuous copy of the entire database using WAL files. Great for high availability and read scaling with identical replicas.
  • Logical Replication: A flexible, publish-subscribe model replicating logical changes (rows). Ideal for selective replication, cross-version upgrades, or integrating with other systems.

Choosing the right strategy depends on your specific needs for consistency, flexibility, and performance.

Frequently asked questions

Is the “Replication Strategies (Streaming, Logical)” lesson free?

Yes — the full text of “Replication Strategies (Streaming, Logical)” is free to read here on the web, and the PostgreSQL Performance & Query Optimization course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the PostgreSQL Performance & Query Optimization course, upgrade to CoddyKit PRO.

What will I learn in “Replication Strategies (Streaming, Logical)”?

Understand and configure various replication methods like streaming and logical replication for high availability and read scaling. You practise PostgreSQL Performance & Query Optimization with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start PostgreSQL Performance & Query Optimization?

No prior experience is required. PostgreSQL Performance & Query Optimization on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Replication Strategies (Streaming, Logical)” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this PostgreSQL Performance & Query Optimization lesson?

Yes. Every PostgreSQL Performance & Query Optimization lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Connection Pooling with PgBouncer
  2. Replication Strategies (Streaming, Logical)
  3. Sharding and Distributed PostgreSQL
  4. Read Scaling with Hot Standby and Load Balancing
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