zgres 0.4

Basic Design

All inter-server communication occurs via ZooKeeper. No SSH or network connections of any kind.

There are 2 different daemons which make up the system: “deadman” and the “sync” daemons. On the nodes which actually use the PostgreSQL cluster, the “deadman” daemon is run to control the PostgreSQL process and optionally the sync daemon to update PostgreSQL configuration files (e.g. pg_hba.conf). On application servers or proxies the “sync” daemon is run.

Terminology

• database cluster or cluster: http://www.postgresql.org/docs/9.4/static/creating-cluster.html

• znode: a ZooKeeper data node in the ZooKeeper database

• group, cluster group, database group: A group of master/replica providing one logical database, made up of one or more database clusters.

ZooKeeper Directory Layout

In ZooKeeper, we have one directory which contains the information on all the nodes in multiple master-replica database groups. It is a directory with 4 sub-directories types:

/state/{DATABASE_GROUP_NAME}_{IPADDR}:{PORT}

This contains all the information about a database cluster nodes, healthy or not. It is updated frequently with data such as the WAL log replay status. It is an ephemeral node and will dissapear if the connection to zookeeper is lost. Ephemeral, created/maintained by the “deadman” daemon.

/conn/{DATABASE_GROUP_NAME}_{IPADDR}:{PORT}

This znode contains a subset of information from the state- node. It is the static connection information/metadata about a single healthy (i.e. connectable) cluster. If the node is not “healthy”, this entry will not exist. The information in this znode is not vollatile and is gaurenteed not to change over the lifespan of the znode. Ephemeral, created/maintained by the “deadman” daemon.

/lock/{DATABASE_GROUP_NAME}

This contains the IPADDR:PORT of the current master for the database group. Connection info should be looked up in the “_conn_” node (if it exists). Created/maintained by the “deadman” daemon on the current

/static/{DATABASE_GROUP_NAME}-db-id

Contains the database identifier of the database group.

Most of the above znodes contain a JSON encoded dictionary/object/hashmap.

Sync Daemon

This daemon runs on any node which wishes to connect to a database group and maintains the local machine’s configuration files. For example, it can rewrite a pgbouncer configuration if the master of one database group fails over. It can also start a new pgbouncer daemon if a new database group is created. Another example is dynamically changing HAProxy weight according to the node location (e.g. availability zone) or replication lag.

Actually applying the configuration changes is the job of plugins, the sync daemon will not apply any changes by itself. Plugins can be specified in 2 ways:

• Using setuptools entry points to subscribe in-process to the changes. This allows subscribers to subscribe to events from either state-, master- or healthy- znodes.

• Provide an executable which will be called with the path to a JSON encoded file containing the information from the healthy- and master- znodes. This is provided by the zgres-apply package which plugs into zgres-sync using the previous plugin method. This plugin does not recieve state- events for performance reasons.

These plugins MUST be idempotent, they will be called repeatedly with the same data.

Deadman Daemon

This daemon controls one PostgreSQL database cluster and registers it in zookeeper (creating/maintaining the state-, conn- and master- znodes). It must run on the same machine as the database cluster.

It is responsible for promoting or shutting down it’s postgresql database cluster.

Currently, remastering and starting PostgreSQL should be handled outside before deadman is started.

Plugins for the deadman daemon should be able to do 2 things:

• Provide extra metadata (i.e. availability-zone or replication lag)

• Veto the cluster being up (aside from a builtin SELECT 1)

Challenges

• multiple replicas: How to fail over to the “best” replica.

• multiple replicas: How to connect replicas to new master.

• Would it be a good idea to know where the basebackups are and initialize new replicas on startup?

• PLUGINS PLUGINS PLUGINS:

  • Provide a lot of plugins builtin, allow them to be seen and enabled via the “config” in zookeeper? EEEK: dynamic reconfiguration of daemons?

  • What happens if a few nodes don’t have some plugins?

  • Configuration on a “cluster group” level

The Good

• Relatively simple configuration. No ssh config.

• Fully distributed.

Implementation Thoughts

• Implement daemons in python, log to stdout and stderr. Have them be run via systemd with configured to restart on fail. Fail noisily!

Dependencies

• systemd

• kazoo - ZooKeeper client

• psycopg2 - connections to PostgreSQL

Acknowledgment

Zgres is heavily influenced by HandyRep (https://github.com/pgexperts/handyrep) and Patroni (https://github.com/zalando/patroni). Many thanks to the developers of those for some great ideas.