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Patroni: A Template for PostgreSQL HA with ZooKeeper, etcd or Consul

You can find a searchable version of this documentation at patroni.readthedocs.io <https://patroni.readthedocs.io>__.

Patroni is a Python template for building PostgreSQL high availability (HA) clusters. It supports several distributed configuration stores, including ZooKeeper <https://zookeeper.apache.org/>, etcd <https://github.com/coreos/etcd>, Consul <https://github.com/hashicorp/consul>, and Kubernetes <https://kubernetes.io>.

Supported PostgreSQL versions: 9.3 to 18.

Note to Citus users: Since version 3.0, Patroni integrates with the Citus <https://github.com/citusdata/citus>__ extension. See the Citus support page <https://github.com/patroni/patroni/blob/master/docs/citus.rst>__ for details.

Note to Kubernetes users: Patroni runs natively on Kubernetes. See the Kubernetes guide <https://github.com/patroni/patroni/blob/master/docs/kubernetes.rst>__ for more information.

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How Patroni Works

Patroni (formerly known as Zalando's Patroni) started as a fork of Governor <https://github.com/compose/governor>__ and adds support for modern HA patterns.

For additional background info, see:

• Elephants on Automatic: HA Clustered PostgreSQL with Helm <https://www.youtube.com/watch?v=CftcVhFMGSY>_
• PostgreSQL HA with Kubernetes and Patroni <https://www.youtube.com/watch?v=iruaCgeG7qs>__
• Zalando Tech blog post from Feb. 2016 <https://engineering.zalando.com/posts/2016/02/zalandos-patroni-a-template-for-high-availability-postgresql.html>__

Development Status

Patroni is actively developed and welcomes contributions.

• Contributing guidelines <https://github.com/patroni/patroni/blob/master/docs/contributing_guidelines.rst>__
• Release notes <https://github.com/patroni/patroni/releases>__

Community

Connect with the Patroni community on GitHub or Slack:

• Issues, discussions and PRs in the GitHub repository <https://github.com/patroni/patroni>__
• #patroni channel <https://postgresteam.slack.com/archives/C9XPYG92A>__ on the PostgreSQL Slack <https://pgtreats.info/slack-invite>__

Requirements and Installation

Pre-requirements for macOS ^^^^^^^^^^^^^^^^^^^^^^^^^^^

Install the requirements on macOS with Homebrew:

::

brew install postgresql etcd haproxy libyaml python

Psycopg choices ^^^^^^^^^^^^^^^

Patroni requires a PostgreSQL Python driver. Recent versions of psycopg2 <http://initd.org/psycopg/articles/2019/04/04/psycopg-28-released/>__ no longer install a binary package by default, which means building from source may require a C compiler and development libraries.

Options:

1. Install using the package manager from your Linux distribution:

::

sudo apt-get install python3-psycopg2
sudo yum install python3-psycopg2

2. Install one of the supported Python packages with pip:

• psycopg
• psycopg2
• psycopg2-binary

Installing with pip ^^^^^^^^^^^^^^^^^^

Install Patroni with optional dependency groups:

::

pip install patroni[dependencies]

Available dependency extras:

• etcd or etcd3: python-etcd for Etcd as DCS
• consul: py-consul for Consul as DCS
• zookeeper: kazoo for ZooKeeper as DCS
• exhibitor: kazoo for Exhibitor as DCS
• kubernetes: kubernetes for Kubernetes as DCS
• raft: pysyncobj for the python Raft DCS
• aws: boto3 for AWS callbacks
• systemd: systemd-python for sd_notify integration
• all: all of the above (except psycopg family)
• psycopg3: psycopg[binary]>=3.0.0
• psycopg2: psycopg2>=2.5.4
• psycopg2-binary: psycopg2-binary

For example:

::

pip install patroni[psycopg3,etcd3,aws]

Note: external tools used by bootstrap or replica creation scripts (for example WAL-G) must be installed separately.

Running and Configuring

A minimal cluster can be started from different terminals:

::

> etcd --data-dir=data/etcd --enable-v2=true
> ./patroni.py postgres0.yml
> ./patroni.py postgres1.yml

Then verify cluster behavior and experiment with the YAML configuration files.

Add more postgres*.yml files to scale the cluster.

Memory issue on Python 3.11+ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If you run Patroni on a system with strict memory limits, for example with vm.overcommit_memory=2 (recommended for PostgreSQL), and use Python 3.11 or newer, you may observe unexpected behavior:

• Patroni appears healthy
• PostgreSQL continues to run
• Patroni REST API becomes unresponsive
• the operating system reports that Patroni is listening on the REST API port
• Patroni logs look normal; however, following messages may appear once: Exception ignored in thread started by: <object repr() failed>, MemoryError
• kernel logs may contain messages such as not enough memory for the allocation

This is caused by a Python 3.11+ issue <https://github.com/python/cpython/issues/140746>__. Under strict memory conditions, starting a new thread may hang indefinitely when there is not enough free memory.

Recommended solution """"""""""""""""""""

Recent Patroni releases (4.1.1+, 4.0.8+) reduce the impact of this issue by starting all required threads early in startup before memory pressure builds.

Additional recommendations (Linux, glibc) """""""""""""""""""""""""""""""""""""""""

When running with vm.overcommit_memory=2 (recommended for PostgreSQL), we also recommend starting Patroni with the following environment variables configured:

• MALLOC_ARENA_MAX=1 - reduces the amount of virtual memory allocated by glibc for multi-threaded applications
• PG_MALLOC_ARENA_MAX= - resets the value of MALLOC_ARENA_MAX for PostgreSQL processes started by Patroni.

In addition, you may tune the following Patroni configuration parameters:

• thread_stack_size - stack size used for threads started by Patroni. Lowering this value reduces memory usage of the Patroni process. The default value set by Patroni is 512kB. Increase thread_stack_size if Patroni experience stack-related crashes; otherwise the default value is sufficient.
• thread_pool_size - size of the thread pool used by Patroni for asynchronous tasks and REST API communication with other members during leader race or failsafe checks. The default value is 5, which is sufficient for three-node clusters.
• restapi.thread_pool_size - size of the thread pool used to process REST API requests. The default value is 5, allowing up to five parallel REST API requests. Note that requests involving SQL queries are effectively serialized because a single database connection is used, so increasing this value typically provides no benefit.

HAProxy support ^^^^^^^^^^^^^^^

Patroni includes an HAProxy <http://www.haproxy.org/>__ configuration for a single application endpoint. Start it with:

::

> haproxy -f haproxy.cfg

Then connect with:

::

> psql --host 127.0.0.1 --port 5000 postgres

Configuration References

YAML configuration ^^^^^^^^^^^^^^^^^^

For complete YAML options, see docs/dynamic_configuration.rst <https://github.com/patroni/patroni/blob/master/docs/dynamic_configuration.rst>__ and the example file postgres0.yml <https://github.com/patroni/patroni/blob/master/postgres0.yml>__.

Environment configuration ^^^^^^^^^^^^^^^^^^^^^^^^^

For environment variable configuration, see docs/ENVIRONMENT.rst <https://github.com/patroni/patroni/blob/master/docs/ENVIRONMENT.rst>__.

Replication choices ^^^^^^^^^^^^^^^^^^^

Patroni uses PostgreSQL streaming replication. It supports:

• asynchronous replication with maximum_lag_on_failover
• synchronous replication for stronger durability guarantees

See the replication modes documentation <https://github.com/patroni/patroni/blob/master/docs/replication_modes.rst>__ for details.

Application connections ^^^^^^^^^^^^^^^^^^^^^^^

Applications should connect with a non-superuser. Using a superuser can consume reserved connections for Patroni and cause undesirable behavior if the leader becomes unavailable.

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