excerpt from http://www.postgresql.org/about/

PostgreSQL is a powerful, open source object-relational database system. It has more than 15 years of active development and a proven architecture that has earned it a strong reputation for reliability, data integrity, and correctness. It is fully ACID compliant, has full support for foreign keys, joins, views, triggers, and stored procedures (in multiple languages). It includes most SQL:2008 data types, including INTEGER, NUMERIC, BOOLEAN, CHAR, VARCHAR, DATE, INTERVAL, and TIMESTAMP. It also supports storage of binary large objects, including pictures, sounds, or video. It has native programming interfaces for C/C++, Java, .Net, Perl, Python, Ruby, Tcl, ODBC, among others, and exceptional documentation.

An enterprise class database, PostgreSQL boasts sophisticated features such as Multi-Version Concurrency Control (MVCC), point in time recovery, tablespaces, asynchronous replication, nested transactions (savepoints), online/hot backups, a sophisticated query planner/optimizer, and write ahead logging for fault tolerance. It supports international character sets, multibyte character encodings, Unicode, and it is locale-aware for sorting, case-sensitivity, and formatting. It is highly scalable both in the sheer quantity of data it can manage and in the number of concurrent users it can accommodate. There are active PostgreSQL systems in production environments that manage in excess of 4 terabytes of data.

This charm supports several deployment models:

• A single service containing one unit. This provides a 'standalone' environment.

• A service containing multiple units. One unit will be a 'master', and every other unit is a 'hot standby'. The charm sets up and maintains replication for you, using standard PostgreSQL streaming replication.

To setup a single 'standalone' service::

juju deploy postgresql pg-a

To replicate this 'standalone' database to a 'hot standby', turning the existing unit into a 'master'::

juju add-unit pg-a

To deploy a new service containing a 'master' and two 'hot standbys'::

juju deploy -n 3 postgresql pg-b

You can remove units as normal. If the master unit is removed, failover occurs and the most up to date 'hot standby' is promoted to 'master'. The 'db-relation-changed' and 'db-admin-relation-changed' hooks are fired, letting clients adjust::

juju remove-unit pg-b/0

To setup a client using a PostgreSQL database, in this case a vanilla Django installation listening on port 8080::

juju deploy postgresql
juju deploy python-django
juju deploy gunicorn
juju add-relation python-django postgresql:db
juju add-relation python-django gunicorn
juju expose python-django

• Do not attempt to relate client charms to a PostgreSQL service containing multiple units unless you know the charm supports a replicated service.

• You cannot host multiple units in a single juju container. This is problematic as some PostgreSQL features, such as tablespaces, use user specified absolute paths.

At a minimum, you just need to join a the db relation, and a user and database will be created for you. For more complex environments, you can provide the database name allowing multiple services to share the same database. A client may also wish to defer its setup until the unit name is listed in allowed-units, to avoid attempting to connect to a database before it has been authorized.

The db-admin relation may be used similarly to the db relation. The automatically generated user for db-admin relations is a PostgreSQL superuser.

• database: Optional. The name of the database to use. The postgresql service will create it if necessary.

• host: the host to contact
• database: a regular database
• port: the port PostgreSQL is listening on
• user: a regular user authorized to read the database
• password: the password for user
• state: 'standalone', 'master' or 'hot standby'.
• allowed-units: space separated list of allowed clients (unit name). You should check this to determine if you can connect to the database yet.

• host: the host to contact
• port: the port PostgreSQL is listening on
• user: a created super user
• password: the password for user
• state: 'standalone', 'master' or 'hot standby'
• allowed-units: space separated list of allowed clients (unit name). You should check this to determine if you can connect to the database yet.