| --- |
| title: Policies |
| layout: website-normal |
| |
| --- |
| |
| Policies perform the active management enabled by Brooklyn. |
| They can subscribe to entity sensors and be triggered by them (or they can run periodically, |
| or be triggered by external systems). |
| |
| <!--- |
| TODO, clarify below, members of what? |
| --> |
| Policies can add subscriptions to sensors on any entity. Normally a policy will subscribe to its |
| associated entity, to the child entities, and/or to the members of a "group" entity. |
| |
| Common uses of a policy include the following: |
| |
| * perform calculations, |
| * look up other values, |
| * invoke effectors (management policies) or, |
| * cause the entity associated with the policy to emit sensor values (enricher policies). |
| |
| Entities can have zero or more ``Policy`` instances attached to them. |
| |
| |
| Off-the-Shelf Policies |
| ---------------------- |
| |
| Policies are highly reusable as their inputs, thresholds and targets are customizable. |
| |
| ### Management Policies |
| |
| - AutoScaler Policy |
| |
| Increases or decreases the size of a Resizable entity based on an aggregate sensor value, the current size of the entity, and customized high/low watermarks. |
| |
| An AutoScaler policy can take any sensor as a metric, have its watermarks tuned live, and target any resizable entity - be it an application server managing how many instances it handles, or a tier managing global capacity. |
| |
| e.g. if the average request per second across a cluster of Tomcat servers goes over the high watermark, it will resize the cluster to bring the average back to within the watermarks. |
| |
| <!--- |
| TODO - list some |
| TODO - describe how they can be customised (briefly mention sensors) |
| --> |
| |
| |
| ### Enrichers |
| |
| * Delta |
| |
| Converts absolute sensor values into a delta. |
| |
| |
| * Time-weighted Delta |
| |
| Converts absolute sensor values into a delta/second. |
| |
| * Rolling Mean |
| |
| Converts the last *N* sensor values into a mean. |
| |
| * Rolling Time-window Mean |
| |
| Converts the last *N* seconds of sensor values into a weighted mean. |
| |
| * Custom Aggregating |
| |
| Aggregates multiple sensor values (usually across a tier, esp. a cluster) and performs a supplied aggregation method to them to return an aggregate figure, e.g. sum, mean, median, etc. |
| |
| |
| Next: Writing a Policy |
| --------------------------- |
| |
| To write a policy, see the section on [Writing a Policy](policy.html). |
