Logical Replication in PostgreSQL: Easy steps to set up

Introduction

One of the major goals of storing data in a database is to ensure data availability. You would want to access your data at any time that you want. Having a single copy of the database can be dangerous. In case of an unprecedented event, you won’t be able to access it. That’s why it’s a good idea for you to have multiple copies of your database. 

PostgreSQL database management system makes this possible. It supports the concept of replication that you can use to create multiple copies of your database. 

Table of Contents

• Prerequisites
• What is Logical Replication?
• The Architecture of PostgreSQL Logical Replication
• PostgreSQL Logical Replication Process
• Basic Syntax
• A Real-life Example
• Limitations in PostgreSQL Logical Replication
• Conclusion

Prerequisites

Make sure that your computer is installed with the following:

• PostgreSQL version 10.0 and above. 

What is Logical Replication?

When deploying applications in a production environment, it is always good for you to have multiple copies of the database. After making the copies, you must ensure that they are all in sync. This process of keeping database copies in sync is known as replication. 

There are two types of replication in PostgreSQL: 

1. Physical replication
2. Logical replication

Physical replication was introduced in PostgreSQL 9.0. In this mechanism, every change in the master is streamed via WAL (write-ahead logging) then applied in the destination/standby server. However, physical replication is associated with a number of limitations. These include:

• It cannot replicate a part of the database (selective replication). 
• It cannot write to the standby server. 
• Replication cannot be done between 2 different major versions. 
• Replication cannot be done between different platforms, for example, Windows and Linux. 

To take care of the above limitations, PostgreSQL 10.0 introduced the concept of logical replication. PostgreSQL logical replication uses WAL to record the logical changes independent from specific architecture or version. The subscriber first receives a copy of a replicated database from the publisher. Normally, changes will be published on the publisher database. These changes should be replicated in the subscriber database to ensure that the two databases remain in sync. The subscriber node pulls the changes as they happen on the publisher database in real-time. This way, the subscriber and the publisher databases remain in sync. 

Read more about the PostgreSQL logical replication here.

The Architecture of PostgreSQL Logical Replication

The PostgreSQL logical replication follows the publish/subscribe model. The publisher node creates a publication, which is also called a change set or a replication set. The publication is simply a set of changes from one or more tables. The subscriber node creates a subscription, with the capability to subscribe to either one or more publications. Now that you know the basics of logical replication, how is the logical replication in PostgreSQL done? That’s what you will learn in the next section. 

PostgreSQL Logical Replication Process

The logical replication in PostgreSQL follows the steps given below:

Step 1: A snapshot of the publishing database is copied to the subscriber. This step is also called the table synchronization phase. To reduce the amount of time spent in this phase, you can spawn multiple table synchronization workers. However, you can only have one synchronization worker for each table. 

 Step 2: After the copy is completed, subsequent changes made in the publisher node will be sent to the subscriber node. The changes will be sent as they happen in real-time. The changes will be applied in a commit order to ensure there is transactional consistency. 

Method 1: PostgreSQL logical replication using manual method

Basic Syntax

To perform logical replication in PostgreSQL, you’ve to change the wal_level = logical parameter. The parameter tells the server to store additional information in WAL for converting binary changes into logical ones. To change the value of this parameter, you have to open the postgresql.conf file. 

On Linux, this file can be found in the following directory:

/etc/postgresql/version/main

On Windows, the file can be found in the following directory:

C:Program FilesPostgreSQLversiondata

After opening the file, go to the WRITE-AHEAD LOG settings. 

Uncomment the parameter wal_level and set it to the following:

wal_level = logical

You can then restart the PostgreSQL service. 

On Windows, you can open the Services dialog and restart the service from there. 

On Linux, you can stop and then start the service by running the following sequence of commands:

sudo service postgresql stop

sudo service postgresql start

To check whether the change has taken effect, run the following command on PostgreSQL terminal:

SHOW wal_level;

It should return the following:

Once you get this, logical replication has been turned on. 

After that, you can create a publication by running the following command:

CREATE PUBLICATION my_publication FOR ALL TABLES;

If you don’t want to publish all tables, you can specify the tables that you need to publish. You can also choose to limit the changes that will be published. 

However, for DELETE and UPDATE operations to be replicated, the “published” table should have a REPLICA IDENTITY, which can be a primary key. This makes it possible for the modified rows to be identified in the subscriber. INSERT operations can be done without a replica identity. After the publications have been created, you can go ahead to create subscriptions in the subscriber node. 

You can use the following command for this:

CREATE SUBSCRIPTION my_subscription CONNECTION '... 
<connection string> ...' PUBLICATION my_publication;

The command will add a subscription named my_subscription for the current database that will begin to receive logical changes from the publication named my_publication. Once you run the command, a logical replication worker will be spawned, and this will receive logical changes coming from the publisher. On the side of the publisher, a walsender process will be spawned, which will be reading the WAL step-by-step, decode changes, and send them to the correct subscriber. 

With all the above knowledge, you are set to get into the practical stuff. Now, you will see a real example of how to set up logical replication in PostgreSQL. So, let’s start.

A Real-life Example

You will be setting up logical replication on a table. You will employ a very simple trick for this. Remember that to perform logical replication, you should have the publisher and the subscriber. In this case, you will create both in the same host. However, you will create them in different ports. 

First, you should create the publication. Let’s create it on the port 5432. 

Here are the steps to do this:

Step 1: First, create a table then insert a row into it. Run the following command:

create table table1(x int primary key, y int); 

The above command creates a table named table1 with two integer columns, x and y. Column x is the primary key for the table. The table is ready. 

Step 2: Let’s insert a row into it by running this command:

insert into table1 values(10, 11);  

We have inserted a single row into the table named table1. 10 will be inserted into column x while 11 will be inserted into column y. 

Step 3: It’s now time to set up publication on the table. Here is the command for this:

create publication my_publication for table table1;   

The command will create a publication on table1 and give it the name my_publication. The publication is now ready. It’s time for you to create the subscription. You will do it on the port 5431. Following are the steps for this:

Step 1: Recreate the table on the subscriber node. Just run the following command:

create table table1(x int primary key, y int); 

Step 2: Next, we can create the subscription and give it the name my_subscription. 

Just run the following command:

CREATE SUBSCRIPTION my_subscription 
CONNECTION 'host=localhost port=5432 dbname=postgres' PUBLICATION my_publication;

Step 3: Since we had inserted one row into the table on the publisher side, we can check whether it has been replicated on the subscriber. Run the following command:

SELECT * FROM table1;  

It returns the following:

So, the row has been replicated in the subscriber. 

Step 4: We should now check whether the changes are being replicated or not. We will simply insert a new row into the publisher and see whether this will be updated in the subscriber. So, run the following command on the publisher:

insert into table1 values(20, 25);  

Now, run the following command on the subscriber:

SELECT * FROM table1; 

You should get the following:

So, the changes were replicated!

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Limitations in PostgreSQL Logical Replication

Let’s discuss some limitations in the process of logical replication in PostgreSQL:

1. It doesn’t replicate the schema or DDL.
2. It doesn’t replicate sequences.
3. Tables must have a primary key or unique key to participate in the PostgreSQL logical replication process.
4. It doesn’t replicate truncate.
5. Bi-directional replication is not supported.
6. Large objects are not replicated using PostgreSQL logical replication.

Know more about the limitations of PostgreSQL logical replication here.

Conclusion

In this article, you have done the following:

• You configured PostgreSQL to perform logical replication. 
• You built the publisher and subscriber nodes. 
• You demonstrated how to implement the publisher and the subscriber nodes on the same host. 
• You demonstrated how to keep the publisher and the subscriber nodes in sync.

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Share your experience of setting up PostgreSQL logical replication in the comment section below.