Contexts are always named to start with a '@' symbol, except for in the filename of the file in which they are stored.
Contexts in Aegir are stored as a simple array of data in a file within the backend of Aegir. However this array of data is accessed and modified with a set of objects and accessors. Here is an example of what a context file looks like:
$aliases['hostmaster'] = array (
'context_type' => 'site',
'platform' => '@platform_hostmaster',
'server' => '@server_master',
'db_server' => '@server_localhost',
'uri' => 'aegir.example.com',
'root' => '/var/aegir/hostmaster-0.4-beta2',
'site_path' => '/var/aegir/hostmaster-0.4-beta2/sites/aegir.example.com',
'site_enabled' => true,
'language' => 'en',
'client_email' => 'firstname.lastname@example.org',
'redirection' => false,
'profile' => 'hostmaster',
These files are stored in the
/var/aegir/.drush directory on the master Aegir server.
In the above example we can see the properties of the context, like the 'uri' of the site, and the 'profile' that the site is running.
There are also a number of more interesting properties in the example, note the 'db_server' property, which has a value that is another context. We shall see later that these properties are special, and allow developers to easily access functions of the 'db_server' without needing to care which db server they are talking to.
Contexts are used within drush commands as objects, which subclass
provision_context. This allows much more flexibility and cleverness, though can make them very confusing to use sometimes! As a developer you will only need to worry about the context objects, as Aegir handles storing them in the files for you. But it is important to note that each context must be representable in a flat text file (or be prepared to do some serious leg-work) by that I mean you probably don't want to be storing massive amounts of relational data in them, use a database for that!