PostgresML is fully integrated into the Postgres replication system and requires very little special consideration. Setting up a PostgreSQL replica may seem to be a daunting task, but it's actually a pretty straight forward step-by-step process.


PostgreSQL replication is composed of three (3) parts: a primary, a replica, and a Write-Ahead Log archive. Each is independently configured and operated, providing a high degree of reliability in the architecture.


The primary serves all queries, including writes and reads. In a replicated configuration, every single write made to the primary is replicated to the replicas and to the Write-Ahead Log archive.


A replica serves only read queries. Setting up additional replicas helps to horizontally scale the read capacity of a database cluster. Adding more replicas to the system can be done dynamically as demand on the system increases, and removed, as the number of clients and queries decreases.

Postgres supports three (3) kinds of replication: streaming, logical, and log-shipping. Streaming replication sends data changes as they are written to the database files, ensuring that replicas are almost byte-for-byte identical to the primary. Logical replication sends the queries interpreted by the primary, e.g. SELECT/ UPDATE / DELETE, to the replica where they are re-executed in the same order. Log-shipping replicas download the Write-Ahead Log from the archive and replay it at their own pace.

Each replication type has its own pros and cons. In this guide, we'll focus on setting up the more commonly used streaming replication, because it allows for very high throughput and reliable replication of all data changes.

Write-Ahead Log archive

The Write-Ahead Log archive, or WAL for short, is a safe place where the primary can upload every single data change that occurs as part of normal operations. The WAL files can be downloaded in case of disaster recovery or, in our case, for replicas to stay up-to-date with what's happening on the primary. Typically, the WAL archive is stored on a separate machine, network-attached storage or, more commonly these days, in an object storage system like S3 or CloudFlare's R2.


PostgreSQL replication requires third-party software to operate smoothly. At PostgresML, we're big fans of the pgBackRest project, and we'll be using it in this guide. In order to install it and some other dependencies, add the PostgreSQL APT repository to your sources:

sudo apt install -y postgresql-common
sudo /usr/share/postgresql-common/pgdg/
sudo apt update

You can now install pgBackRest:

sudo apt install -y pgbackrest

Configure the primary

The primary needs to be configured to allow replication. By default, replication is disabled in PostgreSQL. To enable replication, change the following settings in /etc/postgresql/14/main/postgresql.conf:

archive_mode = on
wal_level = replica
archive_command = 'pgbackrest --stanza=main archive-push %p'

Postgres requires that a user with replication permissions is used for replicas to connect to the primary. To create this user, login as a superuser and run:


Once the user is created, it has to be allowed to connect to the database from another machine. Postgres configures this type of access in /etc/postgresql/14/main/pg_hba.conf:

Open that file and append this to the end:

host replication replication_user scram-sha-256

This configures Postgres to allow the replication_user to connect from anywhere ( and authenticate using the now default SCRAM-SHA-256 algorithm.

Restart PostgreSQL for all these settings to take effect:

sudo service postgresql restart

Create a WAL archive

In this guide, we'll be using an S3 bucket for the WAL archive. S3 is a very reliable and affordable place to store WAL and backups. We've used it in the past to transfer, store and replicate petabytes of data.

Create an S3 bucket

You can create an S3 bucket in the AWS Console or by using the AWS CLI:

aws s3api create-bucket \
--bucket postgresml-tutorial-wal-archive \

By default, S3 buckets are protected against public access, which is important for safeguarding database files.

Configure pgBackRest

pgBackRest can be configured by editing the /etc/pgbackrest.conf file. This file should be readable by the postgres user and nobody else, since it'll contain some important information.

Using the S3 bucket we created above, we can configure pgBackRest to use it for the WAL archive:


Once configured, we can create the archive:

sudo -u postgres pgbackrest stanza-create --stanza main

You can validate the archive created successfully by listing the files using the AWS CLI:

aws s3 ls s3://postgresml-tutorial-wal-archive/wal-archive/main/
PRE archive/
PRE backup/

Create a replica

A PostgreSQL replica should run on a different system than the primary. The two machines have to be able to communicate via the network in order for Postgres to send changes made to the primary over to the replica. If you're using a firewall, ensure the two machines can communicate on port 5432 using TCP and are able to make outbound TCP connections to S3.

Install dependencies

Before configuring the replica, we need to make sure it's running the same software the primary is. Before proceeding, follow the Self-hosting guide to install PostgresML on the system. Once completed, install pgBackRest and configure it the same way we did above for the primary. The replica has to be able to access the files stored in the WAL archive.

Replicating data

A streaming replica is byte-for-byte identical to the primary, so in order to create one, we first need to copy all the database files stored on the primary over to the replica. Postgres provides a very handy command line tool for this called pg_basebackup.

On Ubuntu 22.04, the PostgreSQL 14 Debian package automatically creates a new Postgres data directory and cluster configuration. Since the replica has to have the same data as the primary, first thing we need to do is to delete that automatically created data directory and replace it with the one stored on the primary.

To do so, first, stop the PostgreSQL server:

sudo service postgresql stop

Once stopped, delete the data directory:

sudo rm -r /var/lib/postgresql/14/main

Finally, copy the data directory from the primary:

PGPASSWORD= pg_basebackup \
-h \
-p 5432 \
-U replication_user \
-D /var/lib/postgresql/14/main

Depending on how big your database is, this will take a few seconds to a few hours. Once complete, don't start Postgres just yet. We need to set a few configuration options first.

Configuring the replica

In order to start replicating from the primary, the replica needs to be able to connect to it. To do so, edit the configuration file /etc/postgresql/14/main/postgresql.conf and add the following settings:

primary_conninfo = 'host= port=5432 user=replication_user password='
restore_command = 'pgbackrest --stanza=main archive-get %f "%p"'

Enable standby mode

By default, if Postgres is started as a replica, it will download all the WAL it can find from the archive, apply the data changes and promote itself to the primary role. To avoid this and keep the Postgres replica running as a read replica, we need to configure it to run in standby mode. To do so, place a file called standby.signal into the data directory, like so:

sudo -u postgres touch /var/lib/postgresql/14/main/standby.signal

Start the replica

The replica is ready to start:

sudo service postgresql start

If everything is configured correctly, the database server will start and output something like this in the log file, located in /var/log/postgresql/postgresql-14-main.log:

LOG: redo starts at 0/5A000060
LOG: consistent recovery state reached at 0/5A000110
LOG: database system is ready to accept read-only connections
LOG: started streaming WAL from primary at 0/5A000000 on timeline 1

If you connect to the server with psql, you can validate it's running in read-only mode:

postgres=# SELECT pg_is_in_recovery();

Adding more replicas

Adding more replicas to the system is identical to adding just one replica. A Postgres primary can support up to 16 replicas, which is more than enough to serve millions of queries per second and provide high availability for enterprise-grade deployments of PostgresML.

PostgresML considerations

PostgresML uses regular Postgres tables to store the data required for its operation. This data is automatically sent to replicas as part of streaming replication, so a Postgres replica is a normal PostgresML inference server.

PostgresML integrates with HuggingFace which stores its cache in /var/lib/postgresql/.cache folder. That folder is not replicated because it lives outside the Postgres WAL system. When you try to use pgml.embed() or pgml.transform() on the replica for the first time, it will need to download the models from the HuggingFace repository. Once downloaded, it will continue to operate just like the primary.

The cache can be preemptively transferred to the replica in order to minimize cold start time. Any file transfer tool can be used, e.g. rsync or scp. The HuggingFace cache is identical on all machines in a PostgresML cluster.


If you're using a pooler to load balance your traffic and have followed our Pooler installation guide, you can add your newly created replica to the pooler configuration. With PgCat, it's as easy as:

servers = [
["", 5432, "primary"],
["", 5432, "replica"],

Reload the config by running kill -SIGHUP $(pgrep pgcat) and you can now connect to the pooler and automatically load balance your queries against both the primary and the replica.