Link

Link allows you to specify the connection of different instances to setup a distributed app.

Supposing you have a WordPress server to point to a DB server, you can specify the following in wp-config.php:
define('DB_HOST', '@{db_server.PrivateIpAddress}');, which will be rendered to: define('DB_HOST', '10.0.0.4'); upon execution.

As shown here, Link is similar to Chef Databag or Salt Pillar. However it is far clearer, therefore easier to understand and subsequently use.

There are also some interesting and unique uses:

VisualOps has a wait state, which could be used for the coordination among different instances: wait @{db_server.state.3},which will blocks the calling agent until db_server finishes the 3rd state.

If you are usi ng AutoScaling, you can use @{asg_group.PrivateIpAddress}, which will be replace by ip1,ip2,ip3, ...

Link also supports Jinja2. Combined with AutoScaling, it has the abiity to be extremely powerful:

Which will be:

server.1=10.0.0.4:2888:3888
server.2=10.0.0.5:2888:3888
server.3=10.0.0.6:2888:3888

Further explanation can be found here.