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Alexander KukushkinandGitHub
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Patroni is caching the cluster view in the DCS object because not all operations require the most up-to-date values. The cached version is valid for TTL seconds. So far it worked quite well, the only known problem was that the `last_leader_operation` for some DCS implementations was not very up-to-date: * Etcd: since the `/optime/leader` key is updated right after the `/leader` key, usually all replicas get the value from the previous HA loop. Therefore the value is somewhere between `loop_wait` and `loop_wait*2` old. We improve it by using the 10ms artificial sleep after receiving watch notification from `compareAndSwap` operation on the leader key. It usually gives enough time for the primary to update the `/optime/leader`. On average that makes the cached version `loop_wait/2` old. * ZooKeeper: Patroni itself is not so much interested in most up-to-date values of member and leader/optime ZNodes. In case of the leader race it just reads everything from ZooKeeper, but during normal operation it is relying on cache. In order to see the recent value on replicas they are doing watch on the `leader/optime` Znode and will re-read it after it was updated by the primary. On average that makes the cached version `loop_wait/2` old. * Kubernetes: last_leader_operation is stored in the same object as the leader key itself and therefore update is atomic and we always see the latest version. That makes the cached version `loop_wait/2` old on avg. * Consul: HA loops on the primary and replicas are not synchronized, therefore at the moment when we read the cluster state from the Consul KV we see the last_leader_operation value that is between 0 and loop_wait old. On average that makes the cached version `loop_wait` old. Unfortunately we can't make it much better without performing periodic updates from Consul, which might have negative side effects. Since the `optime/leader` is only updated at most once per HA loop cycle, the value stored in the DCS is usually `loop_wait/2` old on avg. For majority of DCS implementations we could promise that the cached version in Patroni will match the value in DCS most of the time, therefore there is no need to make additional requests. The only exception is Consul, but probably we could just document it, so when someone relying on last_leader_operation value to check the replication lag can correspondingly adjust thresholds. Will help to implement #1599
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Patroni: A Template for PostgreSQL HA with ZooKeeper, etcd or Consul
--------------------------------------------------------------------
You can find a version of this documentation that is searchable and also easier to navigate at `patroni.readthedocs.io <https://patroni.readthedocs.io>`__.
There are many ways to run high availability with PostgreSQL; for a list, see the `PostgreSQL Documentation <https://wiki.postgresql.org/wiki/Replication,_Clustering,_and_Connection_Pooling>`__.
Patroni is a template for you to create your own customized, high-availability solution using Python and - for maximum accessibility - a distributed configuration store like `ZooKeeper <https://zookeeper.apache.org/>`__, `etcd <https://github.com/coreos/etcd>`__, `Consul <https://github.com/hashicorp/consul>`__ or `Kubernetes <https://kubernetes.io>`__. Database engineers, DBAs, DevOps engineers, and SREs who are looking to quickly deploy HA PostgreSQL in the datacenter-or anywhere else-will hopefully find it useful.
We call Patroni a "template" because it is far from being a one-size-fits-all or plug-and-play replication system. It will have its own caveats. Use wisely.
**Note to Kubernetes users**: Patroni can run natively on top of Kubernetes. Take a look at the `Kubernetes <https://github.com/zalando/patroni/blob/master/docs/kubernetes.rst>`__ chapter of the Patroni documentation.
.. contents::
:local:
:depth: 1
:backlinks: none
=================
How Patroni Works
=================
Patroni originated as a fork of `Governor <https://github.com/compose/governor>`__, the project from Compose. It includes plenty of new features.
For an example of a Docker-based deployment with Patroni, see `Spilo <https://github.com/zalando/spilo>`__, currently in use at Zalando.
For additional background info, see:
* `Elephants on Automatic: HA Clustered PostgreSQL with Helm <https://www.youtube.com/watch?v=CftcVhFMGSY>`_, talk by Josh Berkus and Oleksii Kliukin at KubeCon Berlin 2017
* `PostgreSQL HA with Kubernetes and Patroni <https://www.youtube.com/watch?v=iruaCgeG7qs>`__, talk by Josh Berkus at KubeCon 2016 (video)
* `Feb. 2016 Zalando Tech blog post <https://tech.zalando.de/blog/zalandos-patroni-a-template-for-high-availability-postgresql/>`__
==================
Development Status
==================
Patroni is in active development and accepts contributions. See our `Contributing <https://github.com/zalando/patroni/blob/master/docs/CONTRIBUTING.rst>`__ section below for more details.
We report new releases information `here <https://github.com/zalando/patroni/releases>`__.
=========
Community
=========
There are two places to connect with the Patroni community: `on github <https://github.com/zalando/patroni>`__, via Issues and PRs, and on channel #patroni in the `PostgreSQL Slack <https://postgres-slack.herokuapp.com/>`__. If you're using Patroni, or just interested, please join us.
===================================
Technical Requirements/Installation
===================================
**Pre-requirements for Mac OS**
To install requirements on a Mac, run the following:
::
brew install postgresql etcd haproxy libyaml python
**Psycopg2**
Starting from `psycopg2-2.8 <http://initd.org/psycopg/articles/2019/04/04/psycopg-28-released/>`__ the binary version of psycopg2 will no longer be installed by default. Installing it from the source code requires C compiler and postgres+python dev packages.
Since in the python world it is not possible to specify dependency as ``psycopg2 OR psycopg2-binary`` you will have to decide how to install it.
There are a few options available:
1. Use the package manager from your distro
::
sudo apt-get install python-psycopg2 # install python2 psycopg2 module on Debian/Ubuntu
sudo apt-get install python3-psycopg2 # install python3 psycopg2 module on Debian/Ubuntu
sudo yum install python-psycopg2 # install python2 psycopg2 on RedHat/Fedora/CentOS
2. Install psycopg2 from the binary package
::
pip install psycopg2-binary
3. Install psycopg2 from source
::
pip install psycopg2>=2.5.4
**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
=======================
To get started, do the following from different terminals:
::
> etcd --data-dir=data/etcd --enable-v2=true
> ./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 <http://www.haproxy.org/>`__ 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 <https://github.com/zalando/patroni/blob/master/docs/SETTINGS.rst>`__ for comprehensive information about settings for etcd, consul, and ZooKeeper. And for an example, see `postgres0.yml <https://github.com/zalando/patroni/blob/master/postgres0.yml>`__.
=========================
Environment Configuration
=========================
Go `here <https://github.com/zalando/patroni/blob/master/docs/ENVIRONMENT.rst>`__ 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 <https://github.com/zalando/patroni/blob/master/docs/replication_modes.rst>`__ 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.
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