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Patroni: A Template for PostgreSQL HA with ZooKeeper or etcd

Patroni was previously known as Governor.

There are many ways to run high availability with PostgreSQL. Here, we present a template for you to create your own customized, high-availability solution using Python and — for maximum accessibility — a distributed configuration store like ZooKeeper or etcd.

Getting Started

To get started, do the following from different terminals:

::

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

From there, you will see a high-availability cluster start up. Test different settings in the YAML files to see how its behavior changes. Kill some of the components to see how the system behaves.

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

We provide a 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

How Patroni Works

For a diagram of the high availability decision loop, review this PDF: postgres-ha.pdf <https://github.com/zalando/patroni/blob/master/postgres-ha.pdf>__

YAML Configuration

For an example file, see postgres0.yml. Regarding settings:

• ttl: the TTL to acquire the leader lock. Think of it as the length of time before initiation of the automatic failover process.

• loop_wait: the number of seconds the loop will sleep

• restapi:

  • listen: IP address + port that Patroni will listen to, to provide health-check information for haproxy.
  • connect_address: IP address + port through which restapi is accessible.
  • auth: (optional) 'username:password' to protect dangerous REST API endpoints.
  • certfile: (optional) Specifies a file with the certificate in the PEM format. If the certfile is not specified or is left empty, the API server will work without SSL.
  • keyfile: (optional) Specifies a file with the secret key in the PEM format.

• etcd:

  • scope: the relative path used on etcd's HTTP API for this deployment; makes it possible to run multiple HA deployments from a single etcd.
  • ttl: the TTL to acquire the leader lock. Think of it as the length of time before initiation of the automatic failover process.
  • host: the host:port for the etcd endpoint.

• zookeeper:

  • scope: the relative path used on etcd's HTTP API for this deployment; makes it possible to run multiple HA deployments from a single etcd.
  • session_timeout: the TTL to acquire the leader lock. Think of it as the length of time before initiation of the automatic failover process.
  • reconnect_timeout: how long we should try to reconnect to ZooKeeper after a connection loss. After this timeout, assume that you no longer have a lock and restart in read-only mode.
  • hosts: list of ZooKeeper cluster members in format: ['host1:port1', 'host2:port2', 'etc...']
  • exhibitor: if you are running a ZooKeeper cluster under the Exhibitor supervisory, the following section might interest you:
    • poll_interval: how often the list of ZooKeeper and Exhibitor nodes should be updated from Exhibitor
    • port: Exhibitor port.
    • hosts: initial list of Exhibitor (ZooKeeper) nodes in format: ['host1', 'host2', 'etc...' ]. This list updates automatically whenever the Exhibitor (ZooKeeper) cluster topology changes.

• postgresql:

  • name: the name of the Postgres host. Must be unique for the cluster.

  • listen: IP address + port that Postgres listens to; must be accessible from other nodes in the cluster, if you're using streaming replication. Multiple comma-separated addresses are permitted, as long as the port component is appended after to the last one with a colon, i.e. listen: 127.0.0.1,127.0.0.2:5432. The first address from this list will be used by Patroni to establish local connections to the PostgreSQL node.

  • connect_address: IP address + port through which Postgres is accessible from other nodes and applications.

  • data_dir: file path to initialize and store Postgres data files.

  • maximum_lag_on_failover: the maximum bytes a follower may lag.

  • use_slots: whether or not to use replication_slots. Must be False for PostgreSQL 9.3. You should comment out max_replication_slots before it becomes ineligible for leader status.

  • initdb: List options to be passed on to initdb

    • encoding: default encoding for new databases
    • locale: default locale for new databases
    • data-checksums # When pg_rewind is needed on 9.3, this needs to be enabled

  • pg_hba: list of lines which should be added to pg_hba.conf.

    • - host all all 0.0.0.0/0 md5.

  • replication:

    • username: replication username; user will be created during initialization.
    • password: replication password; user will be created during initialization.
    • network: network setting for replication in pg_hba.conf.

  • callbacks callback scripts to run on certain actions. Patroni will pass the action, role and cluster name. See scripts/aws.py as an example on how to write them.

    • on_start: a script to run when the cluster starts.
    • on_stop: a script to run when the cluster stops.
    • on_restart: a script to run when the cluster restarts.
    • on_reload: a script to run when configuration reload is triggered.
    • on_role_change: a script to run when the cluster is being promoted or demoted.

  • superuser:

    • password: password for the Postgres user, set during initialization.

  • admin:

    • username: admin username; user is created during initialization. It will have CREATEDB and CREATEROLE privileges.
    • password: admin password; user is created during initialization.

  • recovery_conf: additional configuration settings written to recovery.conf when configuring follower.

    • parameters: list of configuration settings for Postgres. Many of these are required for replication to work.

  • create_replica_methods: an ordered list of the create methods for turning a patroni node into a new replica. "basebackup" is the default method; other methods are assumed to refer to scripts, each of which is configured as its own config item.

  • replica_method for each create_replica_method other than basebackup, you would add a configuration section of the same name. At a minimum, this should include "command" with a full path to the actual script to be executed. Other configuration parameters will be passed along to the script in the form "parameter=value".

Replication Choices

Patroni uses Postgres' streaming replication. By default, this replication is asynchronous. For more information, see the Postgres documentation on streaming replication <http://www.postgresql.org/docs/current/static/warm-standby.html#STREAMING-REPLICATION>__.

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 follower. This setting should be increased or decreased based on business requirements.

When asynchronous replication is not optimal for your use case, investigate how Postgres's synchronous replication <http://www.postgresql.org/docs/current/static/warm-standby.html#SYNCHRONOUS-REPLICATION>__ works. Synchronous replication ensures consistency across a cluster by confirming that writes are written to a secondary before returning to the connecting client with a success. The cost of synchronous replication: reduced throughput on writes. This throughput will be entirely based on network performance. In hosted datacenter environments (like AWS, Rackspace, or any network you do not control), synchrous replication significantly increases the variability of write performance. If followers become inaccessible from the leader, the leader effectively becomes readonly.

To enable a simple synchronous replication test, add the follow lines to the parameters section of your YAML configuration files:

.. code:: YAML

    synchronous_commit: "on"
    synchronous_standby_names: "*"

When using synchronous replication, use at least three Postgres data nodes to ensure write availability if one host fails.

Choosing your replication schema is dependent on your business considerations. Investigate both async and sync replication, as well as other HA solutions, to determine which solution is best for you.

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 undesireable.

Requirements on a Mac

Run the following on a Mac to install requirements:

::

brew install postgresql etcd haproxy libyaml python
pip install psycopg2 pyyaml

Notice

There are many different ways to do HA with PostgreSQL: See the PostgreSQL documentation <https://wiki.postgresql.org/wiki/Replication,_Clustering,_and_Connection_Pooling>__ for a complete list.

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.

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