Dependent Configuration

Under the covers Brooklyn has a sophisticated sensor event and subscription model, but conveniences around this model make it very simple to express cross-entity dependencies. Consider the example where Tomcat instances need to know the URL of a database (or a set of URLs to connect to a Monterey processing fabric, or other entities)

setConfiguration(UsesJava.JAVA_OPTIONS, ImmutableMap.of("mysql.url", 
        attributeWhenReady(mysql, MySqlNode.MY_SQL_URL) ))

The attributeWhenReady(Entity, Sensor) call (a static method on the class DependentConfiguration) causes the configuration value to be set when that given entity's attribute is ready. In the example, attributeWhenReady() causes the JVM system property mysql.url to be set to the value of the MySqlNode.MY_SQL_URL sensor from mysql when that value is ready. As soon as the database URL is announced by the MySql entity, the configuration value will be available to the Tomcat cluster.

By default "ready" means being set (non-null) and, if appropriate, non-empty (for collections and strings) or non-zero (for numbers). Formally the interpretation of ready is that of "Groovy truth" defined by an asBoolean() method on the class and in the Groovy language extensions.

You can customize "readiness" by supplying a Predicate (Google common) or Closure (Groovy) in a third parameter. This evaluates candidate values reported by the sensor until one is found to be true. For example, passing { it.size()>=3 } as the readiness argument would require at least three management plane URLs.

More information on this can be found in the javadoc for DependentConfiguration, along with a few other methods such as valueWhenAttributeReady which allow post-processing of an attribute value.

Note that if the value of CONFIG_KEY passed to Entity.getConfig is a Closure or Task (such as returned by attributeWhenReady), the first access of Entity.getConfig(CONFIG_KEY) will block until the task completes. Typically this does the right thing, blocking when necessary to generate the right start-up sequence without the developer having to think through the order, but it can take some getting used to. Be careful not to request config information until really necessary (or to use non-blocking "raw" mechanisms), and in complicated situations be ready to attend to circular dependencies. Trying to resolve a circular dependency leads to a deadlock in those activities. The presence of such deadlocks can be seen by viewing the web-console's 'activities' tab of the (hung) entities – it will show what the executing activities are waiting for. Automated detection/alerting of deadlocks is currently not supported in Brooklyn. The management console gives useful information for understanding what is happening and resolving the cycle.

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