# Introduction

Patroni originated as a fork of Governor, the project from Compose. It includes plenty of new features.

For an example of a Docker-based deployment with Patroni, see Spilo, currently in use at Zalando.

For additional background info, see:

- PostgreSQL HA with Kubernetes and Patroni, talk by Josh Berkus at KubeCon 2016 (video)
- Feb. 2016 Zalando Tech blog post

## Development Status

Patroni is in active development and accepts contributions. See our Contributing section below for more details.

We report new releases information here.

## Technical Requirements/Installation

Pre-requirements for Mac OS

To install requirements on a Mac, run the following:

    brew install postgresql etcd haproxy libyaml python

General installation for pip

Patroni can be installed with pip:

    pip install patroni[dependencies]

where dependencies can be either empty, or consist of one or more of the following:

etcd
  python-etcd module in order to use Etcd as DCS
consul
  python-consul module in order to use Consul as DCS
zookeeper
  kazoo module in order to use Zookeeper as DCS
exhibitor
  kazoo module in order to use Exhibitor as DCS (same dependencies as for Zookeeper)
kubernetes
  kubernetes module in order to use Kubernetes as DCS in Patroni
aws
  boto in order to use AWS callbacks

For example, the command in order to install Patroni together with dependencies for Etcd as a DCS and AWS callbacks is:

    pip install patroni[etcd,aws]

Note that external tools to call in the replica creation or custom bootstap scripts (i.e. WAL-E) should be installed independently of Patroni.

## Running and Configuring

The following section assumes Patroni repository as being cloned from https://github.com/zalando/patroni. Namely, you will need example configuration files postgres0.yml and postgres1.yml. If you installed Patroni with pip, you can obtain those files from the git repository and replace ./patroni.py below with patroni command.

To get started, do the following from different terminals:

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

You will then see a high-availability cluster start up. Test different settings in the YAML files to see how the cluster’s behavior changes. Kill some of the components to see how the system behaves.

Add more `postgres*.yml` files to create an even larger cluster.

Patroni provides an HAProxy configuration, which will give your application a single endpoint for connecting to the cluster’s leader. To configure, run:

    > haproxy -f haproxy.cfg

    > psql --host 127.0.0.1 --port 5000 postgres

## YAML Configuration

Go here for comprehensive information about settings for etcd, consul, and ZooKeeper. And for an example, see postgres0.yml.

## Environment Configuration

Go here for comprehensive information about configuring(overriding) settings via environment variables.

## Replication Choices

Patroni uses Postgres’ streaming replication, which is asynchronous by default. Patroni’s asynchronous replication configuration allows for `maximum_lag_on_failover` settings. This setting ensures failover will not occur if a follower is more than a certain number of bytes behind the leader. This setting should be increased or decreased based on business requirements. It’s also possible to use synchronous replication for better durability guarantees. See replication modes documentation for details.

## Applications Should Not Use Superusers

When connecting from an application, always use a non-superuser. Patroni requires access to the database to function properly. By using a superuser from an application, you can potentially use the entire connection pool, including the connections reserved for superusers, with the `superuser_reserved_connections` setting. If Patroni cannot access the Primary because the connection pool is full, behavior will be undesirable.