Theoretically, the architecture of a dynamic system is considered as stable when there are no change in progress. This concept is useful to determine when the system as complete its startup sequence, but also important ofter any deployment action that may have change the underlying architecture.

Chameleon provides a service to wait for stability, but also let you add you own checks.

Stability Service

The org.ow2.chameleon.core.services.Stability service is exposed by Chameleon itself, and let you wait for
stability. The waitForStability method waits for stability and returns whether the stability has been reached. Be aware that this method blocks the caller thread until the stability is reached or until a timeout is reached. The getStabilityResult also waits for stability, but gives you more details about the different stability check.

The isStable method checks if the system is stable. This check may block the caller threads, but for a reasonnable amount of time.

Stability Computation

By default, 3 checks are made in this order:

• Bundle Stability - it waits for all bundles to reach their final state (ACTIVE or RESOLVED for fragments) (priority:0)
• Service Stability - it waits until the number of published service is stable (check made on a time window) (priority:1)
• iPOJO Stability - it waits until iPOJO has processed all bundles (meaning that factories and instances are created) (priority:2)

Stability Computation Extension

It is possible to extend the check made to detect stability by exposing the org.ow2.chameleon.core.services.StabilityChecker service. The getName version defines the name of the checker (human-friendly). The getPriority method sets the check priority. This priority is used to determine the check order. You should not return an integer inferior to 3 (as 0, 1 and 2 are used by the core checks). Lower priorities are executed first. Finally, the check method computes the stability.

The check method has to be blocking (it can rely on the grace method from the org.ow2.chameleon.core.services.AbstractStabilityChecker class), and check whether the stability has been reached. In addition, it must have a reasonnable timeout. Once reached, the check must return StabilityResult.unstable(reson, cause). If the stability has been reached, the check must return StabilityResult.stable().

Extension needs to use the grace method and getDefaultNumberOfAttempts to block the caller thread and to know when they should declare the system as unstable.